foukal (sloppy): 229 papers, Hirsch index 30, Hirsch ratio 0.131. adsauthor "Foukal, P." abstract sloppy @-------------------------------------------------------------------- Title: Dimming of the 17th Century Sun Authors: Foukal, Peter; Ortiz, Ada; Schnerr, Roald Bibliographic Code: 2011ApJ...733L..38F, eprint = 1103.5442 Abstract Reconstructions of total solar irradiance (TSI) rely mainly on linear relations between TSI variation and indices of facular area. When these are extrapolated to the prolonged 15th-17th century Spörer and Maunder solar activity minima, the estimated solar dimming is insufficient to explain the mid-millennial climate cooling of the Little Ice Age. We draw attention here to evidence that the relation departs from linearity at the lowest activity levels. Imaging photometry and radiometry indicate an increased TSI contribution per unit area from small network faculae by a factor of 2-4 compared with larger faculae in and around active regions. Even partial removal of this more TSI-effective network at prolonged minima could enable climatically significant solar dimming, yet be consistent with the weakened but persistent 11 yr cycle observed in Be 10 during the Maunder Minimum. The mechanism we suggest would not alter previous findings that increased solar radiative forcing is insufficient to account for 20th century global warming. @-------------------------------------------------------------------- Title: Dimming of the 17th Century Sun Authors: Foukal, Peter V.; Ortiz, A.; Schnerr, R. Bibliographic Code: 2011SPD....42.0702F Abstract Reconstructions of total solar irradiance (TSI) rely mainly on linear relations between TSI variation and indices of facular area. When these are extrapolated to the prolonged 15^th - 17^th century Spörer and Maunder solar activity minima, the estimated solar dimming is insufficient to explain the mid- millennial climate cooling of the Little Ice Age. We draw attention here to evidence that the relation departs from linearity at the lowest activity levels. Imaging photometry and radiometry indicate an increased TSI contribution per unit area from small network faculae by a factor of 2-4 compared to larger faculae in and around active regions. Even partial removal of this more TSI - effective network at prolonged minima could enable climatically significant solar dimming, yet be consistent with the weakened but persistent 11- yr cycle observed in Be 10 during the Maunder Minimum. The mechanism we suggest would not alter previous findings that increased solar radiative forcing is insufficient to account for 20^th century global warming. This work was supported at Heliophysics, Inc. by NASA grants NNX09AP96G and NNX10AC09G. @-------------------------------------------------------------------- Title: Dimming of the 17th Century Sun Authors: Foukal, Peter V.; Ortiz, A.; Schnerr, R. Bibliographic Code: 2011AAS...21822423F Abstract Reconstructions of total solar irradiance (TSI) rely mainly on linear relations between TSI variation and indices of facular area. When these are extrapolated to the prolonged 15^th - 17^th century Spörer and Maunder solar activity minima, the estimated solar dimming is insufficient to explain the mid- millennial climate cooling of the Little Ice Age. We draw attention here to evidence that the relation departs from linearity at the lowest activity levels. Imaging photometry and radiometry indicate an increased TSI contribution per unit area from small network faculae by a factor of 2-4 compared to larger faculae in and around active regions. Even partial removal of this more TSI - effective network at prolonged minima could enable climatically significant solar dimming, yet be consistent with the weakened but persistent 11- yr cycle observed in Be 10 during the Maunder Minimum. The mechanism we suggest would not alter previous findings that increased solar radiative forcing is insufficient to account for 20^th century global warming. @-------------------------------------------------------------------- Title: Comment on ``A homogeneous database of sunspot areas covering more than 130 years'' by L. A. Balmaceda et al. Authors: Foukal, Peter Bibliographic Code: 2010JGRA..11509102F Abstract Abstract Available from http://www.agu.org @-------------------------------------------------------------------- Title: What Irradiance Studies Tell Us about Solar/Stellar Convection and Magnetism Authors: Foukal, Peter V. Bibliographic Code: 2010AAS...21621901F Abstract Despite their enormous thermal inertia, many late - type stars exhibit luminosity fluctuations caused by changing photospheric magnetic structures. These fluctuations exist only because of the high heat diffusivity of stellar convection. Were it lower, the dark spots would be surrounded by intense bright rings, as Gene Parker pointed out in 1974. These rings would cancel the spot - induced luminosity dips. Conversely, dark rings around the bright faculae would cancel their positive luminosity contribution. Photometric measurements of this heat diffusivity place independent constraints on solar magnetic diffusivities - a key parameter in dynamo models. Irradiance studies also suggest that the structure of emerging magnetic fields shifts toward lower spatial frequencies with increasing activity. This finding could provide new information on the field source function in dynamo models. Differential and near - IR imaging photometry reveal the decreased temperature gradient of facular magnetic flux tubes and the sunspot- like darkness of their deepest observable layers. Both of these features support current mhd flux tube models. Bolometric imaging measures the wide- band contribution to total irradiance variation, of spot and facular magnetic flux tubes. The remarkably constant solar limb - darkening measured over the past 33 years constrains fluctuations in quiet photospheric temperature gradient and thus, in global convective efficiency over the past three solar cycles. Reconstruction of irradiance variation over past millennia relies on radio- isotope studies. These provide many interesting insights, but they assume that C14 and Be10 are formed only by solar modulation of the galactic cosmic ray flux. This assumption would break down if solar activity and particle fluxes much exceeded levels experienced in cycle 19. Such a "hyperactive'' Sun would vary more in its radiative outputs, be dimmer in total irradiance, although brighter in the EUV and X rays. Work is supported by NASA grant NNX09AP96G @-------------------------------------------------------------------- Title: John Allen Eddy Authors: Foukal, Peter Bibliographic Code: 2010PhT....63a..60F Abstract @-------------------------------------------------------------------- Title: Recent Anomalous TSI Decrease Not Due To Low Polar Facula and Network Areas: Time to Broaden Our View of Solar Luminosity Variation? Authors: Foukal, Peter V.; Bernasconi, P.; Frohlich, C. Bibliographic Code: 2009SPD....40.1113F Abstract Total solar irradiance (TSI) values measured during the present activity minimum by the VIRGO, ACRIM, and TIM radiometers are significantly ( 0.018% +/- 0.006 % rms) lower than reported during the last minimum in 1996 (1). This decrease represents 1/4 the amplitude of 11 - yr TSI variation. Differences in spots, faculae and active network cannot account for this anomalous decrease. A sufficient difference in the TSI contribution from quiet network also seems unlikely, since the solar microwave flux index, F10.7, has dipped only 4 % below its 1996 minimum. This is an order of magnitude less than required to explain the TSI decrease by a decline in network area. The remaining explanation in terms of photospheric magnetic structures, might lie in a decrease in the area of polar faculae, whose cycle amplitude is presently at a minimum for this century. We evaluate their TSI contribution using area and contrast measurements with the Solar Bolometric Imager (SBI), together with polar facula counts (2). We find that their TSI contribution between the present and 1996 activity minima, is below 0.002%. This is again, an order of magnitude below the observed TSI decrease. We conclude that the anomalous TSI decrease is unlikely to be caused by photospheric magnetic changes. This suggests that solar luminosity may be able to change significantly over decadal time scales through an as- yet- unidentified, relatively shallow mechanism that avoids the 10*5 year thermal relaxation time of the solar convection zone. This work was supported at Heliophysics, Inc by NSF grant ATM 0718305, and at APL by NASA grant NNG 05WC07G References: 1. Frohlich, C. 2008, AGU Fall Meeting, Abstract \# SH21C-05. 2. Sheeley, N. 2008, Ap.J. , 680, 1553. @-------------------------------------------------------------------- Title: A Century of Solar Ca ii Measurements and Their Implication for Solar UV Driving of Climate Authors: Foukal, Peter; Bertello, Luca; Livingston, William C.; Pevtsov, Alexei A.; Singh, Jagdev; Tlatov, Andrey G.; Ulrich, Roger K. Bibliographic Code: 2009SoPh..255..229F Abstract Spectroheliograms and disk-integrated flux monitoring in the strong resonance line of Ca ii (K line) provide the longest record of chromospheric magnetic plages. We compare recent reductions of the Ca ii K spectroheliograms obtained since 1907 at the Kodaikanal, Mt. Wilson, and US National Solar Observatories. Certain differences between the individual plage indices appear to be caused mainly by differences in the spectral passbands used. Our main finding is that the indices show remarkably consistent behavior on the multidecadal time scales of greatest interest to global warming studies. The reconstruction of solar ultraviolet flux variation from these indices differs significantly from the 20th-century global temperature record. This difference is consistent with other findings that, although solar UV irradiance variation may affect climate through influence on precipitation and storm tracks, its significance in global temperature remains elusive. @-------------------------------------------------------------------- Title: Solar Network Bolometric Properties at Minimum of Activity Observed by the Solar Bolometric Imager Authors: Bernasconi, P. N.; Foukal, P. V. Bibliographic Code: 2008AGUFMSH23A1625B Abstract On September 13 2007, the Solar Bolometric Imager (SBI) observed the Sun in wide band spectrally integrated for 16 hours while suspended from a balloon at ~120,000 feet altitude above New Mexico. SBI represents a totally new approach in finding the sources of the solar irradiance variation. Its detector is an array of 320x240 thermal IR elements whose spectral sensitivity has been extended and flattened by a layer of gold-black deposited on its IR sensitive surface. The combination of bolometric array and telescope, a 30- cm Dall-Kirkham with uncoated primary and secondary Pyrex mirrors, provide an image of the Sun with constant spectral response between ~ 280 and 2600 nm, over a field of view of 960 x 720 arcsec with a pixel size of 3 arcsec. The September 13, 2007 flight provided bolometric (integrated light) maps of the photosphere when the Sun was near a minimum of activity. At the time of the flight no active regions were present giving us the opportunity to measure with high accuracy the bolometric contrast of the weak solar magnetic network from Sun center to the limb. The network was easily detectable by SBI near the limb. We measured an average bolometric contrast of ~ 0.8 to 1.0 %, which is slightly above the 5-minute oscillation brightness signal (the most prominent solar induced noise source for us). We were also able to detect the bolometric brightness signature of network near Sun center by averaging 720 bolometric images taken close to Sun center over a period of 1 hour. The resulting RMS noise was < 0.02% and most of the 5-minute oscillation brightness was removed in the average. This enabled us to measure an average network bolometric contrast at Sun center of 0.25% with a spread of about ± 0.05%. Ours is the first bolometric measurement (constant spectral sensitivity from 280 to 2600 nm) of the center-to-limb contrast of magnetic network. Our observations demonstrate that SBI can accurately measure the bolometric contrast of even quiet network across the solar disk. These measurements will enable a more precise estimate of the TSI contribution from changes of the enhanced magnetic network, which consist of larger elements than the quiet network. This will enable us to determine whether other low level brightness sources besides faculae and spots contribute to TSI and evaluate their possible long term influence in TSI change and climate. @-------------------------------------------------------------------- Title: Barium strontium titanate (BST) pyroelectric detector for bolometric solar imaging Authors: Noble, M.; Bernasconi, P.; Francomacaro, A.; Eaton, H.; Carkhuff, B.; Foukal, P. Bibliographic Code: 2008SPIE.7055E...6N Abstract The Solar Bolometric Imager (SBI) is an imaging solar telescope assembly that employs a novel single-detector broadband bolometric measurement technique. An uncooled thermal IR imaging detector is coated with a thin gold-black film that absorbs over 98% of the solar spectrum. The absorbed energy is then re-radiated in the thermal IR and sampled by the detector array. This technique [4] provides an evenly weighted integrated responsivity that spans the majority of the solar spectrum (0.2-2.5mum). We present here performance results from the follow-on gold-black deposition process investigation, radiation testing results, spacecraft instrument design and some of the prototype detector/imaging system's flight performance and calibration data from our 2007 Ft. Sumner balloon flight that demonstrates the instrument met or exceeded all of its specification. @-------------------------------------------------------------------- Title: Do Photospheric Brightness Structures Outside Magnetic Flux Tubes Contribute to Solar Luminosity Variation? Authors: Bernasconi, P. N.; Foukal, P. V. Bibliographic Code: 2008AGUSMSP53B..07B Abstract Variations in total solar irradiance (TSI) correlate well with changes in projected area of photospheric magnetic flux tubes associated with dark sunspots and bright faculae in active regions and network. This correlation does not, however, rule out possible TSI contributions from photospheric brightness inhomogeneities located outside flux tubes, and spatially correlated with them. Previous reconstructions of TSI report agreement with radiometry that seems to rule out significant "extra-flux tube" contributions. We show that these reconstructions are more sensitive to the facular contrasts used than has been generally recognized. Measurements with the Solar Bolometric Imager (SBI) provide the first reliable support for the relatively high, wide-band, disc-center contrasts required to produce 10% rms agreement. Longer-term bolometric imaging will be required to determine whether the small but systematic TSI residuals we see here are caused by remaining errors in spot and facular areas and contrasts, or by extra-flux tube brightness structures such as bright rings around sunspots, or "convective stirring" around active regions. @-------------------------------------------------------------------- Title: Did Solar UV Flux Variation Contribute to 20th Century Global Warming? Authors: Foukal, P.; Chulsky, G.; Weisenstein, D. Bibliographic Code: 2008AGUSMSP53B..06F Abstract Solar UV irradiance below 240 nm (here Fuv) varies mainly in proportion to area changes of bright photospheric faculae, so its time behavior differs from that of longer UV wavelengths, and of the total solar irradiance (TSI), both of which are also determined by dark sunspots. We showed previously (ref.1) that the time series of Fuv and TSI differed significantly during the 20th century, and that Fuv variation accounted for less than 20 percent of the variance in global temperature, Tg, over 1915-1999. The time series of facular areas used in our reconstructions has now been found to agree ( r = 0.97) with results from an independent reduction of the same Mount Wilson Observatory spectroheliograms (ref.2)included in our study. Although the 170 to 240 nm Fuv range (responsible for molecular oxygen dissociation) correlates relatively weakly with Tg, the time behavior of the UV irradiance between 240 and 320 nm (responsible for ozone dissociation) closely resembles that of TSI and thus of Tg. Therefore, our results might be reconciled with possible Tg driving by solar UV irradiance variation (e.g. ref. 3) if the time behavior of ozone concentration were determined more by its dissociation rate, rather than by that of oxygen. However, our tests with the AER 2- D stratospheric chemistry model (ref.4) indicate that the direct impact on ozone is almost entirely due to wavelengths below 240 nm. Thus, the low correlation we find between Fuv and Tg seems to argue against solar UV driving of 20th century global warming. This conclusion could be sensitive to possible feedbacks between ozone and stratospheric temperature, circulation or water vapor, which are not included in our modeling. It appears, however, to be consistent with the recent assessment (ref.5) that, while solar UV flux variation affects important tropospheric circulations, a consistent effect on Tg remains to be identified. 1 P. Foukal, GRL 29,2089,doi:10:1029/2002GL015474 2.L. Bertello et al., abstract at AGU Joint Assembly(2008) 3.D.Shindell et al., Science, 294, 2149 (2001) 4.D. Weisenstein et al.JGR 109.D18310,doi:10:1029/2004 5.J. Haigh et al., J.Climate, 18,3672 (2005) @-------------------------------------------------------------------- Title: Preliminary Results Of the 2007 Flight of the Solar Bolometric Imager at Solar Minimum Authors: Bernasconi, P. N.; Foukal, P. V.; Eaton, H. H.; Noble, M. Bibliographic Code: 2008AGUSMSP41B..05B Abstract On September 13 2007, the Solar Bolometric Imager (SBI) successfully observed the Sun for several hours while suspended from a balloon in the stratosphere above New Mexico. The SBI represents a totally new approach in finding the sources of the solar irradiance variation. The SBI detector is an array of 320x240 thermal IR elements whose spectral absorptance has been extended and flattened by a deposited layer of gold-black. The telescope is a 30-cm Dall-Kirkham with uncoated primary and secondary Pyrex mirrors. The combination of telescope and bolometric array provide an image of the Sun with a constant spectral response between ~ 280 and 2600 nm, over a field of view of 960 x 720 arcsec with a pixel size of 3 arcsec. This is the second successful flight of SBI, following a successful one on September 2003 which produced the first measurements in broad band of the center-to-limb variation of bolometric facular contrast (a flight attempt from Antarctica in 2006 was aborted). This latest flight provided bolometric (integrated light) maps of the solar photosphere during a time of minimum of solar activity. The SBI imagery will enable us to evaluate the photometric contribution of weak magnetic structures (e.g. network) more accurately than has been achievable with spectrally selective imaging over restricted wavebands. It will also enable us to investigate the presence, if any, of other thermal structures unrelated to magnetic activity, such as e.g. giant cells and pole-to-equator temperature gradients. During the 16 hour flight the SBI gathered several thousand bolometric images that are now being processed to produce full-disk maps of spatial variation in total solar output at solar minimum. The SBI flight is also providing important engineering data to validate the space worthiness of the novel gold-blackened thermal array detectors. In this paper we will briefly describe the characteristics of the SBI, its in-flight performance, and we will present the first results of the analysis of the bolometric images. @-------------------------------------------------------------------- Title: Do Photospheric Brightness Structures Outside Magnetic Flux Tubes Contribute to Solar Luminosity Variation? Authors: Foukal, Peter; Bernasconi, Pietro N. Bibliographic Code: 2008SoPh..248....1F Abstract Variations in total solar irradiance (TSI) correlate well with changes in projected area of photospheric magnetic flux tubes associated with dark sunspots and bright faculae in active regions and network. This correlation does not, however, rule out possible TSI contributions from photospheric brightness inhomogeneities located outside flux tubes and spatially correlated with them. Previous reconstructions of TSI report agreement with radiometry that seems to rule out significant ``extra-flux-tube'' contributions. We show that these reconstructions are more sensitive to the facular contrasts used than has been generally recognized. Measurements with the Solar Bolometric Imager (SBI) provide the first reliable support for the relatively high, wide-band, disk-center contrasts required to produce 10% rms agreement. Longer term bolometric imaging will be required to determine whether the small but systematic TSI residuals we see here are caused by remaining errors in spot and facular areas and contrasts or by extra-flux-tube brightness structures such as bright rings around sunspots or ``convective stirring'' around active regions. @-------------------------------------------------------------------- Title: Variations on Sun's role in climate change Authors: Foukal, Peter; Schmidt, Diedrich; Klaassen, Wim; Gulledge, Jay; Socci, Anthony D.; Smith, W. H.; Smith, J. R.; Cohen, Roger W.; Scafetta, Nicola; West, Bruce J. Bibliographic Code: 2008PhT....61j..10F Abstract @-------------------------------------------------------------------- Title: Do Photospheric Brightness Structures Outside Magnetic Flux Tubes Contribute to Solar Luminosity Variation? Authors: Bernasconi, P. N.; Foukal, P. Bibliographic Code: 2007AGUFMGC31B0345B Abstract Variations in total solar irradiance (TSI) correlate well with changes in projected area of photospheric magnetic flux tubes associated with spots, faculae and network. This correlation does not, however, rule out possible TSI contributions from photospheric brightness inhomogeneities located outside flux tubes, and spatially correlated with them. Previous reconstructions report 10% amplitude agreement with radiometry that seems to rule out significant extra-flux tube contributions. We show that, while these reconstructions are insensitive to behavior of near- limb facular contrast, their sensitivity to contrasts on the disc is relatively high. Given this sensitivity, previously used observational and theoretical approximations to wide-band facular contrast are too uncertain to support claims of 10% reconstruction accuracy. Recent measurements with the Solar Bolometric Imager (SBI) provide the first observational support for the relatively high wide-band, disc-center contrasts required to produce 10% rms agreement. Longer-term bolometric imaging to measure areas and bolometric contrasts homogeneously will be required to determine whether the systematic TSI residuals we see are caused mainly by uncertainties in sunspot contrasts, or by extra-flux tube brightness structures due to bright spot rings or convective stirring. @-------------------------------------------------------------------- Title: Did the Sun's Prairie Ever Stop Burning? Authors: Foukal, P.; Eddy, J. Bibliographic Code: 2007SoPh..245..247F Abstract The presence of the red flash at total solar eclipses requires the existence of an extended chromosphere and therefore of a photospheric magnetic network that gives rise to spicules. We draw attention to the earliest historical reports of a red flash at the 1706 and 1715 eclipses, which therefore imply a substantial, widespread photospheric field during at least the last decade of the Maunder Minimum. Our finding is consistent with reports of a persistent photospheric field throughout the Maunder Minimum from analyses of ¹⁰Be radioisotope evidence. We note, however, that the last decade may not be representative of conditions throughout the roughly 1645 1715 extent of that prolonged activity minimum. @-------------------------------------------------------------------- Title: Variations in solar luminosity and their effect on the Earth's climate Authors: Foukal, P.; Fröhlich, C.; Spruit, H.; Wigley, T. M. L. Bibliographic Code: 2006Natur.443..161F Abstract Variations in the Sun's total energy output (luminosity) are caused by changing dark (sunspot) and bright structures on the solar disk during the 11-year sunspot cycle. The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years. In this Review, we show that detailed analysis of these small output variations has greatly advanced our understanding of solar luminosity change, and this new understanding indicates that brightening of the Sun is unlikely to have had a significant influence on global warming since the seventeenth century. Additional climate forcing by changes in the Sun's output of ultraviolet light, and of magnetized plasmas, cannot be ruled out. The suggested mechanisms are, however, too complex to evaluate meaningfully at present. @-------------------------------------------------------------------- Title: Ethics Concerns Draw Many Questions, Some Answers Authors: Foukal, Peter Bibliographic Code: 2005PhT....58g..12F Abstract @-------------------------------------------------------------------- Title: Can Changing Sunspot and Facular Areas Reproduce the Amplitude of Total Irradiance Variations?(Look,Mom; No Free Parameters!) Authors: Foukal, P.; Bernasconi, P. N.; Walton, S. R. Bibliographic Code: 2005AGUSMSH22B..02F Abstract Empirical models of total solar irradiance variation demonstrate a high correlation between observed irradiance fluctuations and the changing areas of spots and faculae. However, the contrast of these structures (especially the faculae) in integrated light is still uncertain. Consequently, the agreement in amplitude of the measured and modeled irradiance time series remains poorly known. Recently, the first measurements of facular contrast in broad - band integrated light were obtained using the balloon -borne Solar Bolometric Imager (Foukal et al., Ap.J. Letts 611,57,2004). These measurements, obtained over approximately the same wavelength range accepted by radiometers such as VIRGO or ACRIM, enable the first reconstruction of the total irradiance expected from spots and faculae, with no free parameters. We compare this reconstruction with the radiometric record to determine whether other contributions besides the darkness of spots and brightness of faculae are required to explain solar irradiance variation, at least over rotational time scales. @-------------------------------------------------------------------- Title: Finding the sources of irradiance variation at sunspot minimum. Authors: Bernasconi, P. N.; Foukal, P.; Rust, D. M.; LaBonte, B. J. Bibliographic Code: 2005MmSAI..76..907B Abstract In 2006-2007 the Solar Bolometric Imager (SBI) will operate in the polar stratosphere where near-space conditions can be attained for 10 to 30 days. The instrument will provide bolometric (wavelength-integrated light) and color temperature images of the Sun. At the upcoming sunspot minimum, SBI observations will be able to detect subtle sources of solar irradiance variation with the least confusion by signals from the magnetic fields. This is the best observational approach to characterizing potential causes of the long-term irradiance variations. Possible predicted sources of secular variability include torsional waves and meridional flow variations. SBI uses a 30-cm diameter F/12 Dall-Kirkham telescope with uncoated mirrors, and neutral density filters to provide broadband (bolometric) sensitivity that varies only by ±7% over the wavelengths from 0.31 mu m to 2.6 mu m. Inferred solar irradiance variations will be compared with space based full-disk radiometric measurements. @-------------------------------------------------------------------- Title: Comment on ``Variations of Total Solar Irradiance Produced by Structural Changes in the Solar Interior'' Authors: Foukal, P.; Spruit, H. Bibliographic Code: 2004EOSTr..85..524F Abstract In a recent Eos article, Sofia [2004] argues for the influence on irradiance variation of global changes in the Sun's structure associated with its magnetic dynamo. These changes would act in addition to the relatively well understood modulation by dark sunspots and bright faculae at the surface. His assessment of the present observational evidence for such a global change agrees with our earlier conclusion that it is not widely convincing at the present time [Foukal, 2003]. But Sofia's article also claims (1) that the numerical results obtained by him and his collaborators at Yale disagree with and correct earlier work, and (2) that a hydrostatic approximation is not adequate for variations on the 11-year solar cycle timescale. These surprising claims are based on the results of recently published hydrostatic models [e.g., Sofia and Li, 2004] using the same mixing length approximation for convective heat transport used in earlier work [Spruit, 1982, 1991; Gilliland, 1988]. @-------------------------------------------------------------------- Title: Finding the Sources of Irradiance Variation at Sunspot Minimum Authors: Rust, D. M.; Bernasconi, P. N.; Foukal, P. V.; Labonte, B. J. Bibliographic Code: 2004AGUFMSH51E..02R Abstract In 2006-2007 the Solar Bolometric Imager (SBI) and the Multi-Spectral Imager (MSI) will operate in the polar stratosphere where near-space conditions can be attained for 10 to 20 days. The instruments will provide bolometric (wavelength-integrated light) and color temperature images of the Sun. At the upcoming sunspot minimum, SBI observations will be able to detect subtle sources of solar irradiance variation with the least confusion by signals from the magnetic fields. This is the best observational approach to characterizing potential causes of the long-term irradiance variations. Possible predicted sources of secular variability include torsional waves and meridional flow variations. SBI uses a 30-cm diameter F/12 Dall-Kirkham telescope with uncoated mirrors, and neutral density filters to provide broadband (bolometric) sensitivity that varies only by ±7 percent over the wavelengths from 0.28 microns to 2.6 microns. The MSI is a CCD-based imager that will provide diagnostics of solar magnetic and thermal structures while SBI assesses their radiance. Sunspots, faculae and magnetic network will be identified from the MSI images. Sonic filtering of the MSI images will isolate the oscillatory signal. That signal will be used to remove oscillations from SBI averages to reduce the solar noise. Inferred solar irradiance variations will be compared with SORCE/TIM and ACRIMSAT measurements. The images and data products will be openly available via the Web. @-------------------------------------------------------------------- Title: Solar Irradiance Variation on Centennial to Millennial Time Scales Authors: Foukal, P. Bibliographic Code: 2004AGUFM.U41B..04F Abstract Solar irradiance variation observed over the 11 yr sunspot cycle is caused by the changing areas of dark and bright magnetic structures (sunspots, faculae)on the solar disc, but its barely 0.1 % amplitude is insufficient to drive existing climate models. Irradiance reconstructions incorporating an additional slowly varying component of sufficient amplitude to drive such models have been widely used in recent climate studies. But these reconstructions were based on results from photometry of Sun like stars which have now been largely retracted. This changed evidence challenges our understanding how solar luminosity variation could drive climate. Variation of UV flux may play a role, but its correlation with global temperature seems low, at least in the 20th century. The Sun's enormous thermal inertia restricts sources of luminosity variation on centennial to millennial time scales, to relatively superficial layers. This constraint diminishes the likelihood that deeper lying structural changes associated with e.g. the solar dynamo play a significant role. Still, some newly discovered aspects of solar magnetic behavior suggest how luminosity variation on these time scales might conceivably occur with the sign and amplitude implied by the correlations between solar activity and climate. More accurate solar and stellar observations and modeling will be required to investigate such mechanisms at the frontier of our understanding of the Sun. @-------------------------------------------------------------------- Title: Broadband Measurements of Facular Photometric Contrast Using the Solar Bolometric Imager Authors: Foukal, Peter; Bernasconi, Pietro; Eaton, Harry; Rust, David Bibliographic Code: 2004ApJ...611L..57F Abstract We present the first photometric measurements of solar faculae in broadband light. Our measurements were made during the recent flight of the Solar Bolometric Imager (SBI), a 30 cm balloon-borne telescope that imaged the Sun with a spectrally constant response between about 0.31 and 2.6 mum. Our curve of facular contrast versus limb distance agrees well with values obtained by the blackbody correction of monochromatic measurements. This decreases uncertainty in the facular irradiance contribution, which limits searches for other possible mechanisms of solar luminosity variation, besides changes of photospheric magnetism. @-------------------------------------------------------------------- Title: Solar Astrophysics, 2nd, Revised Edition Authors: Foukal, Peter V. Bibliographic Code: 2004soas.book.....F Abstract @-------------------------------------------------------------------- Title: The solar bolometric imager Authors: Bernasconi, P. N.; Eaton, H. A. C.; Foukal, P.; Rust, D. M. Bibliographic Code: 2004AdSpR..33.1746B Abstract The balloon-borne Solar Bolometric Imager (SBI) will provide the first bolometric (integrated light) maps of the solar photosphere. It will evaluate the photometric contribution of magnetic structures more accurately than has been possible with spectrally selective imaging over restricted wavebands. More accurate removal of the magnetic feature contribution will enable us to determine if solar irradiance variation mechanisms exist other than the effects of photospheric magnetism. The SBI detector is an array of 320×240 ferro-electric thermal IR elements whose spectral absorptance has been extended and flattened by a deposited layer of gold-black. The telescope itself is a 30-cm Dall-Kirkham design with uncoated primary and secondary pyrex mirrors. The combination of telescope and bolometric array provides an image of the Sun with a flat spectral response between 0.28 and 2.6 mum, over a field of view of 917×687 arcsec, and a pixel size of 2.8 arcsec. After a successful set of ground-based tests, the instrument is being readied for a one-day stratospheric balloon flight that will take place in September 2003. The observing platform will be the gondola previously used for the Flare Genesis Experiment (FGE), retrofitted to house and control the SBI telescope and detector. The balloon flight will enable SBI to image over essentially the full spectral range accepted by non-imaging space-borne radiometers such as ACRIM, making the data sets complementary. The SBI flight will also provide important engineering data to validate the space worthiness of the novel gold-blackened thermal array detectors, and verify the thermal performance of the SBI's uncoated optics in a vacuum environment. @-------------------------------------------------------------------- Title: First Results Of The Solar Bolometric Imager Authors: Bernasconi, P. N.; Foukal, P.; Eaton, H. H.; Rust, D. M. Bibliographic Code: 2003AGUFMSH32A1101B Abstract On September 1 2003, the Solar Bolometric Imager (SBI) successfully observed the Sun for several hours while suspended from a balloon in the stratosphere above New Mexico. The SBI represents a totally new approach in finding the sources of the solar irradiance variation. The mission provided the first bolometric (integrated light) maps of the solar photosphere, that will allow to evaluate the photometric contribution of magnetic structures more accurately than has been achievable with spectrally selective imaging over restricted wavebands. The more accurate removal of the magnetic features contribution will enable us to determine if solar irradiance variation mechanisms exist other than the effects of photospheric magnetism. The SBI detector was an array of 320 x 240 thermal IR elements whose spectral absorptance has been extended and flattened by a deposited layer of gold-black. The telescope was a 30-cm Dall-Kirkham with uncoated primary and secondary pyrex mirrors. The combination of telescope and bolometric array provided an image of the Sun with a flat spectral response between 0.28 and 2.6 microns, over a field of view of 917 x 687 arcsec with a pixel size of 2.8 arcsec. The observing platform was the gondola previously used for the Flare Genesis Experiment (FGE), retrofitted to house and control the SBI telescope and detector. During the 9 hours of flight the SBI gathered several thousand bolometric images that are now being processed to produce the first maps of the total solar irradiance. The SBI flight is also providing important engineering data to validate the space worthiness of the novel gold-blackened thermal array detectors, and to verify the thermal performance of the SBI's uncoated optics in a vacuum environment. In this paper we will briefly describe the characteristics of the SBI, its in flight performance, and we will present the first results of the analysis of the bolometric images. This work was funded by NASA under grant\# NAG5-10998. @-------------------------------------------------------------------- Title: The Solar Bolometric Imager: Characteristics and Performance. Authors: Bernasconi, P. N.; Foukal, P.; Rust, D. M. Bibliographic Code: 2003SPD....34.2002B Abstract The Solar Bolometric Imager (SBI) is an innovative solar telescope capable of recording the first bolometric (integrated light) maps of the photosphere. It will enable evaluation of the photometric contribution of magnetic structures more accurately than has been achievable with spectrally selective imaging. The SBI has an angular resolution of 5", sufficient to distinguish sunspots, faculae and enhanced network. These photospheric magnetic structures are known to be linked closely to irradiance variations. Accurate removal of irradiance variations linked to the magnetic features will enable us to determine if other solar irradiance variation mechanisms exist. The SBI detector is an array of 320 x 240 ferro-electric thermal IR elements whose spectral absorptance has been extended and flattened by a deposited layer of gold-black. The telescope is a 30-cm Dall-Kirkham design with uncoated primary and secondary pyrex mirrors. The combination of telescope and bolometric array provides an image of the sun with a flat spectral response between 0.28 microns and 2.6 microns, over a field of view of 917" x 687", and a pixel size of 2.8". After completion of ground tests, the balloon-borne instrument will make a one-day stratospheric flight in September 2003. Observing from an altitude of over 30 km, the SBI will image the sun over nearly the full spectral range accepted by non-imaging satellite-borne radiometers such as ACRIM, making the data sets complementary. The SBI flight will also provide important engineering data to validate the space worthiness of the novel gold-blackened thermal array detectors, and to verify the thermal performance of the SBI's optics in a vacuum environment. Here we will describe the SBI in more detail and present the results of various instrument performance tests, including solar observations from the ground, in preparation for the balloon flight. This work is funded by NASA under grant NAG5-10998. @-------------------------------------------------------------------- Title: A Cryogenic Pyrheliometer for More Accurate Solar Irradiance Measurements Authors: Foukal, P. Bibliographic Code: 2003SPD....34.2001F Abstract Space-borne pyrheliometry over the past two solar cycles has demonstrated the variability of total solar irradiance in response to photospheric magnetic structures such as sunspots or faculae, over the 11-yr activity cycle.But the reproducibility of the measurements remains marginal to detect or rule out possible trends in irradiance below the 0.05-0.1% variation over the 11-yr cycle, but conceivably dominant over multi-decadal time scales of greatest relevance to climate. In metrology laboratories,conventional radiometers similar to those presently flown by NASA and ESA have been superseded in the past ten years by cryogenic radiometers of ten times higher absolute accuracy and long term reproducibility.But their helium cooling makes them difficult to use in space. Recently, advances in superconducting transition thermometry at NIST, and in high-temperature superconducting materials,have presented the opportunity to reach cryogenic radiometer performance at LN2 temperatures attainable with space qualified single stage cryocoolers. We report here on our results with a prototype SCT-based radiometer, developed to investigate this opportunity to improve the accuracy of space borne pyrheliometry.We show that the sensitivity achieved is an order of magnitude better than with conventional radiometers, although the noise threshold falls short of values attainable with LHe cooling.The measured non-equivalence errors, and results of monochromatic intercomparisons against trap detectors, are both consistent with absolute accuracy at the 0.01% level, thus comparable to LHe cooled radiometers. Improved thermal and mechanical design will be required to reduce slow drifts, to test this accuracy conclusively. @-------------------------------------------------------------------- Title: Does a slowly varying component of solar irradiance exist? Authors: Foukal, P. Bibliographic Code: 2003EAEJA.....4696F Abstract Fluctuations of total irradiance, S, caused by sunspots and faculae, are too small (˜0.1%) to force current climate models. Recent reconstructions of S incorporate an additional, slowly varying component, which follows the smoothed spot or group number, or the cycle period. But evidence for this larger (0.2--0.5%) variation, originally based on photometry of sun-like stars, is weakened by recent findings that true solar analog stars are difficult to find. Also, disappearance of the chromospheric network in the early 20th century, implied by these reconstructions, is not observed in archival solar images. I will show some new evidence for an additional component, suggested by the high correlation found between global temperature and the (small-amplitude) variation of S reconstructed using recently digitized facular areas between 1915--1999.The UV irradiance variation modeled from the same data exhibits much lower correlation with global warming, so it is less likely to account for a solar activity signature in 20th century climate. This indirect evidence suggests a mechanism that might amplify the small modulation of S caused specifically by the net effect of bright faculae and dark spots. We now recognize that amplification on multi-decadal time scales could only operate in near- photospheric layers, given the enormous thermal inertia of the deeper solar atmosphere. Also, increasingly tight constraints on such a mechanism are placed by solar photometry and radiometry, by spectro-photometric monitoring of the quiet photospheric effective temperature, and by absence of detectable solar diameter variations. The shrinking set of mechanisms that might satisfy these constraints deserves closer study, and I will discuss some interesting new observational diagnostics, such as bolometric imaging of the photosphere, planned to search for such processes. But at present, little direct evidence exists for a large-amplitude, multi-decadal variation of S (or of UV flux). A clearer recognition of this fact would benefit climate research by stimulating study of possible climate sensitivity to the smaller variations in S and UV irradiance that have been observed over the past two solar cycles, and to other potential solar drivers such as heliospheric modulation of cosmic rays. @-------------------------------------------------------------------- Title: Reply: Evaluation of Climate Sensitivity to Solar Influences Is an Important Goal Authors: Foukal, Peter Bibliographic Code: 2003EOSTr..84..532F Abstract The detection of an 11-year global temperature signal by Douglass and Clader, and in other studies cited by David Douglass in his letter, is an important achievement. However, these studies assume that the driver is the measured 11-year variation in total solar irradiance. They do not attempt to estimate the possible contributions of the equally well-measured 11-year variations in solar ultraviolet flux, and in solar modulation of galactic cosmic rays. Both of these variable solar influences are under study as possible drivers of 11-year global temperature variation [e.g., Haigh, 1996; Svensmark and Friis-Christensen, 1997]. These suggested mechanisms operate differently from the direct coupling of total irradiance to climate. So it may be premature to claim that the sensitivity to total irradiance has been measured. Also, to the extent that the sign of possible climate influences from solar UV [e.g., Shindell et al.; 1999] and plasma output variations remains model-dependent, it seems uncertain in what sense the reported sensitivities represent limits. @-------------------------------------------------------------------- Title: Can Slow Variations in Solar Luminosity Provide Missing Link Between the Sun and Climate? Authors: Foukal, Peter Bibliographic Code: 2003EOSTr..84..205F Abstract Recent evidence from ocean and ice cores suggests that a significant fraction of the variability in Northern Hemisphere climate since the last Ice Age correlates with solar activity [Bond et al., 2001]. This finding extends previous evidence connecting solar activity and climate during the past millennium [Eddy, 1976, Lean et al., 1995]. The simplest mechanism relies on increases of wavelength-integrated output of solar heat and light (total irradiance, S) accompanying increases in solar activity. But recent findings cast doubt on earlier evidence for a sufficiently large variation of S. At the same time, advances in instrumentation give promise of answering this question, to support timely decisions on global warming. In this article, we assess the status of the topic and suggest some new initiatives. @-------------------------------------------------------------------- Title: A comparison of variable solar total and ultraviolet irradiance outputs in the 20th century Authors: Foukal, Peter Bibliographic Code: 2002GeoRL..29w...4F Abstract Differences in time- variation between total and ultraviolet solar irradiance could help in separating their influence on climate. We present the first models based on area measurements of magnetic plages from CaK spectroheliograms obtained between 1915-1999. Correlation of our time series of UV irradiance with global temperature, T, accounts for only 20% of the global temperature variance during the 20th century. Correlation of our total irradiance time series with T accounts statistically for 80% of the variance in global temperature over that period, although the irradiance variation amplitude is insufficient to influence global warming in present-day climate models. This interesting difference has been obscured in past modelling by additional components introduced to represent secular variations, which are no longer supported by current observational evidence. Future irradiance models emphasizing the more securely- based contributions of photospheric magnetic structures seem to provide better prospects for improved physical understanding of sun-climate links. @-------------------------------------------------------------------- Title: Investigation of the Sources of Irradiance Variation on the Sun (ISIS) Authors: LaBonte, B. J.; Bernasconi, P. N.; Rust, D.; Foukal, P.; Hudson, H.; Spruit, H. Bibliographic Code: 2002AAS...200.5608L Abstract There is a persistent correlation of the longterm climate change and solar irradiance. ISIS is designed to understand the physical basis of this correlation. ISIS combines an innovative bolometric imager and a multiband CCD imager. The bolometric imager has uniform response from 200 nm to 3000 nm, spatial resolution < 5 arcseconds, and precision of < 0.1% in a one minute integration. The multiband imager records ultraviolet irradiance variation in the band from 200 to 350 nm, measures photospheric temperature structure, and provides chromospheric structure in Ca II K and H-alpha, with spatial resolution <1.0 arcsecond. Designed for flight on the Solar Dynamics Observatory, ISIS will provide the comprehensive photometric measurements needed to characterize the irradiance variation from identifiable structures and challenge theoretical models of convection and the solar dynamo. @-------------------------------------------------------------------- Title: The Solar Bolometric Imager Authors: Bernasconi, P. N.; Foukal, P.; Rust, D. M. Bibliographic Code: 2002AAS...200.5605B Abstract The Solar Bolometric Imager (SBI) is an innovative solar telescope capable of recording images in essentially total photospheric light, with an angular resolution of 5", sufficient to distinguish sunspots, faculae and enhanced network. These are the photospheric magnetic structures so far linked most closely to irradiance variation. The balloon-borne SBI will provide the first bolometric maps of the photosphere, to evaluate the photometric contribution of magnetic structures more accurately than has been achievable so far, using spectrally selective imaging over restricted wavebands. More accurate removal of the magnetic feature contribution will enable us to determine whether other solar irradiance mechanisms exist besides the effects of photospheric magnetism. The SBI detector is an array of 320 X 240 ferro-electric thermal IR elements whose spectral absorptance has been extended and flattened by a deposited layer of gold-black. The telescope itself is a 30-cm Dall-Kirkham design with uncoated primary and secondary pyrex mirrors. The combination of telescope and bolometric array provides an image of the solar irradiance with a flat spectral response between 0.28 um and 2.6 um, over a field of view of 15.2' X 11.4', and a pixel size of 2.8". After a successful set of ground-based tests, the instrument is being readied for a one-day stratospheric balloon flight that will take place in September 2003. The observing platform will be the gondola previously used for the Flare Genesis Experiment project (FGE), retrofitted to house and control the SBI telescope and detector. The balloon flight will enable SBI to image over essentially the full spectral range accepted by non-imaging space borne radiometers such as ACRIM, making the data sets complementary. The SBI flight will also provide important engineering data to validate the space worthiness of the novel gold-blackened thermal array detectors, and verify the thermal performance of the SBI's uncoated optics in a vacuum environment. This work was funded by NASA under grant NAG5-10998. @-------------------------------------------------------------------- Title: A Comparison of Variable Total and Ultraviolet Solar Irradiance Inputs to 20 th Century Global Warming Authors: Foukal, P. V. Bibliographic Code: 2002AAS...200.2802F Abstract Analysis of spaceborne radiometry has shown that the total solar irradiance variation over the past two activity cycles was approximately proportional to the weighted difference between areas of dark spots and bright faculae and enhanced network. Empirical models of ultraviolet irradiance variation indicate that its behavior is dominated by changes in area of the bright component alone, whose photometric contrast increases at shorter wavelength.This difference in time behavior of total and UV irradiances could help to discriminate between their relative importance in forcing of global warming. Our recent digitization of archival Ca K images from Mt Wilson and NSO provides the first direct measurement of variations in area of the bright component, extending between 1915 and 1999 (previous models have relied on the sunspot number or other proxies to estimate the bright - component contribution). We use these more direct measurements to derive the time behavior of solar total and UV irradiance variation, over this period .We find that they are significantly different;the total irradiance variation accounts for over 80 percent of the variance in global temperature during this period, while the ultraviolet irradiance variation accounts for only about 20 percent. The amplitude of total irradiance variation in our model is smaller than required to influence global warming,in current climate models.Also, the impact of sulfate aerosol variations on the extended cooling between the 1940's and 1970's must be better understood before the significance of correlations between 20 th century global warming, and any solar activity index can be properly assessed. Despite these caveats, the lower correlation we find between global temperature and UV,compared to total, irradiance requires consideration in the search for physical mechanisms linking solar activity and climate. This work was supported in part under NASA grant NAG5-7607 to CRI, Inc., and NAG5-10998 to the Applied Physics Laboratory, Johns Hopkins University. @-------------------------------------------------------------------- Title: The solar bolometric imager Authors: Rust, D.; Bernasconi, P.; Foukal, P. Bibliographic Code: 2002cosp...34E1200R Abstract The balloon-borne Solar Bolometric Imager (SBI) will provide the first bolometric (integrated light) maps of the photosphere, to evaluate the photometric contribution of magnetic structures more accurately than has been achievable with spectrally selective imaging over restricted wavebands. More accurate removal of the magnetic feature contribution will enable us to determine if solar irradiance variation mechanisms exist other than the effects of photospheric magnetism. The SBI detector is an array of 320 x 240 ferro -electric thermal IR elements whose spectral absorptance has been extended and flattened by a deposited layer of gold- black. The telescope itself is a 30-cm Dall-Kirkham design with uncoated primary and secondary pyrex mirrors. The combination of telescope and bolometric array provides an image of the sun with a flat spectral response between 0.28 microns and 2.6 microns, over a field of view of 15.2 x 11.4 min, and a pixel size of 2.8 arcsec. After a successful set of ground-based tests, the instrument is being readied for a one-day stratospheric balloon flight that will take place in September 2003. The observing platform will be the gondola previously used for the Flare Genesis Experiment (FGE), retrofitted to house and control the SBI telescope and detector. The balloon flight will enable SBI to image over essentially the full spectral range accepted by non-imaging space-borne radiometers such as ACRIM, making the data sets complementary. The SBI flight will also provide important engineering data to validate the space worthiness of the novel gold-blackened thermal array detectors, and verify the thermal performance of the SBI's uncoated optics in a vacuum environment. This work was funded by NASA under grant NAG5-10998. @-------------------------------------------------------------------- Title: Radiometric and Photometric Tests of Solar Luminosity Variation Mechanisms Authors: Foukal, P. Bibliographic Code: 2001AGUSM..SP31B04F Abstract A variety of diagnostics has been developed to discriminate among competing physical explanations of solar luminosity variation, and to use them for new insight into magneto-convection on the sun and similar stars. The shape of spot -induced irradiance dips correlates well with changes in spot projected area, but not with growth or decay rate. This argues against storage of the spot's missing heat flux in the spot magnetic field.Efficient heat flow blocking and storage in the HCZ seems to provide the simplest mechanism.The weakness of sunspot bright rings and thermal shadows is consistent with eddy thermal diffusivities calculated from models of the HCZ. The contribution of faculae and network is still too uncertain to decide whether they account for all of the remaining variance in luminosity, after spot dimming is removed.But the correlation of irradiance variations is much higher with the difference of compensating spot and facular contributions than with total magnetic flux. This argues against "magnetic stirring" as an important factor in luminosity variation. The darkness of small flux tubes in continuum near disc center, especially near 1.63 microns,and their center - to - limb contrast variation, seems to favor their interpretation as photospheric heat leaks.This is supported by the lowered facular temperature gradient measured using two -color photometric imaging. Photometric searches for large - scale photospheric temperature inhomogeneities have yielded useful upper limits.Possible global variations in effective temperature, studied through monitoring of photospheric limb-darkening, and of temperature - sensitive Fraunhofer lines, have not revealed any convincing variations.But the percentage sensitivity to solar irradiance change is limited to about 0.2 per year, and might be improved with helioseismological techniques. The decrease in facular-to-spot area ratio observed at high solar activity levels suggests a simple explanation for the increased photometric variability of younger sun - like stars in terms of photospheric magnetism. The apparent absence of detectable solar luminosity variations outside modulation due to photospheric magnetism poses an interesting new constraint on stellar convection theory. Ongoing advances in cryogenic radiometry, thermal imaging,and helioseismology are all likely to contribute to the search for possible more subtle luminosity variations below the threshold of present measurements. @-------------------------------------------------------------------- Title: A measurement of the quiet network contribution to solar irradiance variation Authors: Foukal, Peter; Milano, Leo Bibliographic Code: 2001GeoRL..28..883F Abstract A large increase in quiet network area since the 17th century Maunder Minimum has been suggested as a mechanism for increasing solar irradiance sufficiently to drive global warming. We show that this mechanism requires essentially complete disappearance of network proceeding back in time to the beginning of the 20th century. This disappearance is ruled out by the many Ca K spectroheliograms taken since the discovery of the network in the early 1890's. Furthermore, network area measurements we have carried out on Ca K spectroheliograms digitized from the Mt. Wilson and NSO/Sacramento Peak archives, for the nine solar activity minima between 1914 and 1996, show no evidence of network area variations large enough to produce a significant long-term component of total irradiance variation. A network brightness variation of sufficient magnitude is also unlikely, given the linear dependence of solar microwave flux on area of bright structures. More generally, recent analyses of cycle 21, 22 pyrheliometry, and of broadband stellar photometry, provide little support for any long-term irradiance component. These results do not rule out a secular irradiance increase. But they suggest that high climate sensitivity to the relatively small changes in solar total and UV irradiance that have been observed, provides a more likely explanation of the global temperature-solar activity correlation. @-------------------------------------------------------------------- Title: Total-Light Imager with Flat Spectral Response for Solar Photometric Measurements Authors: Foukal, Peter; Libonate, Scott Bibliographic Code: 2001ApOpt..40.1138F Abstract @-------------------------------------------------------------------- Title: Solar luminosity eludes understanding Authors: Foukal, Peter Bibliographic Code: 2000PhT....53l..11F Abstract @-------------------------------------------------------------------- Title: Total Light Imager with Flat Spectral Response for Solar Photometric Measurements Authors: Libonate, S.; Foukal, P. Bibliographic Code: 2000SPD....3102118L Abstract The Solar Bolometric Imager (SBI) is a novel solar imaging system optimized for studying mechanisms of total irradiance variation. Uncertain broad-band photometric contrasts of spots, and especially faculae and network, currently present the main obstacle to improved modeling of total irradiance fluctuations. After 20 years of effort, accurate contrasts remain elusive because the photometric response functions of conventional camera and telescope systems are highly wavelength dependent, and difficult to remove from measurements of structures having non-black-body radiance distributions. The SBI can provide the required data in a single image because it has the same spectrally `flat' (i.e. constant) photometric response as pyrheliometers such as ACRIM over the wavelength range between approximately 0.26 um and 2.6 um, containing over 96% of the total solar irradiance. The prototype SBI system at CRI utilizes a 50,000-element uncooled thermal imaging array whose spectral absorptance has been flattened by gold-blacking, without significantly degrading its modulation transfer. We use a 30 cm-aperture Dall-Kirkham design with uncoated (i.e. bare glass) primary and secondary mirrors to provide uniform spectral response, and to avoid solar heating and saturation of the imager. The image quality ( 5' resolution over a 13 X 7 arc minute FOV) is very satisfactory for our purpose of accurately discriminating the total irradiance contributions of photospheric magnetic structures, such as spots, faculae and network from other possible solar heat flow inhomogeneities. We are currently redesigning the (commercial) camera electronics to reduce non-linearities and improve calibration accuracy in the telescope. We expect the improved accuracy provided by the SBI to significantly improve the constraints on possible slow changes in solar irradiance that may drive secular climate variations. Balloon flight of the SBI is necessary to avoid the most serious atmospheric transmission variations; useful measurements could be obtained from a short-duration flight, and the full potential of the SBI would be realized with a long-duration underflight of a spaceborne pyrheliometer. This work has been performed at Cambridge Research and Instrumentation (CRI), under NASA research grant NAG5-6979. @-------------------------------------------------------------------- Title: A Measurement of the Quiet Network Contribution to Solar Irradiance Variation Authors: Foukal, P.; Milano, L. Bibliographic Code: 2000SPD....31.0805F Abstract We use recently digitized, archived Ca K images obtained between 1905-present, to measure variations in the area of quiet chromospheric network at the 9 solar activity minima between 1911 and 1996. We find only a marginally significant increase, an order of magnitude smaller than required to provide an increase in total irradiance even comparable in magnitude to the small 0.1% increase observed radiometrically during cycles 21 and 22. Our results cast doubt on the hypothesis that an irradiance decrease caused by disappearance of the magnetic network during the Maunder Minimum could influence global warming since the 17th century. Together with recent findings from stellar photometry and solar radiometry, this result calls into question previous evidence for a significant long-term component of irradiance change beyond the small 11-year modulation measured by space-borne pyrheliometers. @-------------------------------------------------------------------- Title: Advanced absolute radiometers using superconducting transition thermometers Authors: Libonate, S.; Foukal, P. Bibliographic Code: 2000Metro..37..369L Abstract @-------------------------------------------------------------------- Title: Division II: The Sun and heliosphere - Commissions 10: Solar activity. 12: Solar radiation and structure. 49: The interplanetary plasma and the heliosphere. Authors: Foukal, P.; Ai, G.; Verheest, F. Bibliographic Code: 2000IAUTA..24...65F Abstract @-------------------------------------------------------------------- Title: The Solar Bolometric Imager -A New Direction in Solar Irradiance Studies Authors: Libonate, S.; Foukal, P. V. Bibliographic Code: 1999AAS...194.7608L Abstract The Solar Bolometric Imager (SBI) is a novel solar imaging system with spectrally constant photometric response over all wavelengths between the UV and IR, which will provide a new tool for studying mechanisms of total irradiance variation. The SBI utilizes an 80,000 pixel, uncooled thermal IR imaging array whose spectral absorptance has been modified by CRI to provide uniform response over the wavelength range between at least 0.3 um and 2.5 um, containing 95% of the total solar irradiance. We have demonstrated that ferro-electric uncooled arrays can be modified to meet the SBI's spectral uniformity requirements with the deposition of gold blacks, and we have also identified two promising approaches for modifying the spectral absorptance of uncooled microbolometer arrays. A modified 8-bit Raytheon ferro-electric camera is being tested in the lab and on a telescope, while a 12-bit camera that will accommodate either ferro-electric or microbolometer arrays, is under development. The prototype SBI telescope utilizes a Dall-Kirkham design with uncoated (i.e. bare glass) primary and secondary mirrors in order to provide uniform spectral response and reduce the irradiance at the focal plane. Our present research focuses on image quality, photometric precision, stray light, and solar heating in this ground-based, prototype SBI. Ultimately, the SBI will be used to measure and remove temporal variations in solar irradiance due to photospheric magnetic structures, so that the importance of residual variations that may drive secular climate variations associated with global warming, can be determined. Much of the science potential of the SBI could be realized in a balloon experiment while the combination of the SBI and a cavity radiometer would constitute an excellent SMEX experiment to address a key challenge identified in the Sun-Earth Connection Roadmap issued by NASA/OSS. This work is supported by NASA research grant number NAG5-6979. @-------------------------------------------------------------------- Title: Bolometric imager for solar irradiance studies Authors: Foukal, Peter V. Bibliographic Code: 1998SPIE.3442...34F Abstract We are presently developing a solar imager with spectrally uniform photometric response over all wavelengths between the UV and IR. Such a Solar Bolometric Imager (SBI) will be capable of accurately measuring heat flow inhomogeneities at the sun's photosphere and will provide an innovative new tool for identifying mechanisms of long-term solar luminosity variation. Our work builds on recent advances in uncooled, relatively high-definition thermal arrays. We have shown that the spectral absorptance of these arrays can be modified by deposition of gold blacks, to provide spectrally uniform response over at least the wavelength range between about 0.3(mu) and 2.5(mu) containing over 95 percent of the total solar irradiance. Our ongoing work is intended to show that quantitative photometry of the solar disc can be performed with such a modified array. We are constructing a breadboard SBI for immediate use with an 8-bit ferro- electric camera, developing a 12-bit camera to make full use of the ferro-electric array's capabilities, and optimizing our process of gold-blacking the TI arrays. Much of the science potential of the SBI could be realized in a balloon experiment. The combination of the SBI and a cavity radiometer would also constitute an excellent SMEX experiment to address a key challenge identified in the Sun- Earth Connection Roadmap recently issued by NASA/OSS. @-------------------------------------------------------------------- Title: What Determines the Relative Areas of Spots and Faculae on Sun-like Stars? Authors: Foukal, Peter Bibliographic Code: 1998ApJ...500..958F Abstract We analyze newly digitized Ca K plage area data extending back to 1915, and also the white-light facular area data beginning in 1874, to investigate further our earlier finding that the area ratio of faculae to spots decreases at increasing activity levels. We find that this ratio decreases in plage as well as facular data, so it cannot be an artifact of the visibility function of limb faculae. The decrease is also accentuated in daily data, compared to annual means; we explain this as a consequence of the different dependences of facular, plage, and spot lifetimes upon their emergent magnetic flux. From this we show that subphotospheric field properties are more likely to determine this ratio, rather than photospheric field diffusion rates. Systematic, cycle-to-cycle variations in its value suggest an origin in fluctuations of the field generation mechanism; specifically, a mechanism that produces a positive correlation between magnetic flux generation efficiency, and relative power in the spatial spectrum at low frequencies. Our results also suggest that main-sequence stars about 50% more magnetically active than the present Sun might exhibit ratio values an order of magnitude lower than current solar values. This evidence strengthens our earlier argument that a rapid shift toward dark photospheric structures in both active regions and network provides the most likely explanation of the recently reported sharp increase of photometric variability in late-type stars somewhat more active than the Sun. @-------------------------------------------------------------------- Title: Solar Irradiance Variations and Climate Authors: Foukal, Peter Bibliographic Code: 1998fsam.conf..103F Abstract @-------------------------------------------------------------------- Title: Extension of the F10.7 index to 1905 using Mt. Wilson Ca K spectroheliograms Authors: Foukal, Peter Bibliographic Code: 1998GeoRL..25.2909F Abstract The F10.7 index provides a daily record of solar microwave emissions, which vary in rough proportion to the projected area of bright magnetic structures called plages and network, and also sunspots, on the sun's disk. The daily observations used to form the index only began in 1947. Recently, we digitized the archive of daily Ca K spectroheliograms obtained at Mt. Wilson Observatory between 1905-1984, and measured the area variations of plages and enhanced network, on these photographic plates. We calibrated these variations against the F10.7 index between 1947-1984, so we are able to construct a full-disk proxy of F10.7 extending back to 1905. The behavior of this extended index indicates that UV irradiance levels achieved near the peaks of sunspot cycles 15, 16, and 17 between 1915-1945, were 25-40% higher than would be estimated from behavior of the Zurich sunspot number, R_z. @-------------------------------------------------------------------- Title: Chirality, Helicity, and Joy's Law Authors: Foukal, P. Bibliographic Code: 1998ASPC..150..446F Abstract @-------------------------------------------------------------------- Title: Plasma Electric Field Measurements as a Diagnostic of Neutral Sheets in Prominences Authors: Foukal, P. Bibliographic Code: 1998ASPC..150..119F Abstract @-------------------------------------------------------------------- Title: Spectrapolarimetry of the 15-9 Transition of HI as a Diagnostic of Plasma Electric Fields Authors: Foukal, P. Bibliographic Code: 1996AAS...188.3620F Abstract An IR observation of a solar prominence by Brault and Noyes (1982) showed the surprising intensity of high HI transitions such as 7-6, 9-7, 10-7, 11-8, 12-8 and 15-9, in the 8-12 micron atmospheric window. The 15-9 transition at 11.539 microns is of particular interest as a diagnostic of plasma electric fields (and also of electron density) because of its very large calculated Stark splitting (Casini and Foukal, 1994). We present preliminary results of our spectrapolarimetric measurements on the 15-9 line in prominences using the FTS at the McMath telescope at Kitt Peak. Our observed line profiles agree with the structure calculated for this line in a Holtsmark electric field at the plasma density of the prominence, taking into consideration Zeeman effect in the prominence magnetic field B=10G. We discuss how further spectrapolarimetry of this line could significantly increase the measurement sensitivity of the wave-related and d.c. macroscopic electric fields in the sun and in laboratory plasmas. This work is supported by the Solar-Terrestrial Program of the National Science Foundation under grant ATM-9301832. @-------------------------------------------------------------------- Title: Calculated profiles of H I lines of interest for solar plasma electric field measurements Authors: Casini, Roberto; Foukal, Peter Bibliographic Code: 1996SoPh..163...65C Abstract We present calculated Stark-polarized line profiles for a number of H i lines observed in the visible and infrared emission spectrum of solar prominences and other limb activity. For use in measurements of possible electric fields in these structures, we also calculate curves giving the difference in line width between the 1/2 (I ± Q) profiles as a function of electric-field intensity. Our calculations take into account magnetic fields in these structures, and incorporate typical observed values of Doppler broadening. These calculations explicitly consider the H i fine structure neglected in previous work, and thus are more accurate in the range of low to intermediate electric-field intensity likely to be encountered in solar plasmas (E < 10³ V cm^-1). Our results enable us to compare behavior when E and B are parallel, or perpendicular. We draw particular attention to the high electric-field sensitivity of the transitions between high levels such as 12 8 and 15 9 in H i, observed in prominences at wavelengths around 11mu. Their sensitivity is roughly an order of magnitude larger than that of the high Paschen-series lines used in solar plasma electric field studies so far. @-------------------------------------------------------------------- Title: The behavior of solar magnetic plages measured from Mt. Wilson observations between 1915-1984 Authors: Foukal, P. Bibliographic Code: 1996GeoRL..23.2169F Abstract We describe the digitization and reduction of the daily solar chromospheric spectroheliograms obtained in the K-line of CaII between 1915-1984 at Mt. Wilson Observatory. Our results provide the first reliable information on the behavior of solar magnetic plage area prior to 1947, thus extending by almost 70% the length of the primary data base needed to model past total, UV and EUV solar irradiances. Comparison with other solar activity indices confirms the remarkably linear relation between plage areas, and sunspot number and area, found in the post-1947 data. Our analysis also shows a markedly non-linear relation between the areas of chromospheric plage, and of photospheric white-light faculae. We provide an explanation of this surprising nonlinearity between measurements of two magnetic structures that are known to be closely related in the sun's atmosphere. @-------------------------------------------------------------------- Title: Daily Digital Mt Wilson CaK Images and Plage Areas 1915-1985 Authors: Foukal, P.; Harman, M.; Risacher, S.; Yang, R. Bibliographic Code: 1995SPD....26..513F Abstract @-------------------------------------------------------------------- Title: Testing MHD models of prominences and flares with observations of solar plasma electric fields Authors: Foukal, Peter V.; Behr, Bradford B. Bibliographic Code: 1995SoPh..156..293F Abstract We present measurements of electric fields in quiescent prominences and in a small flare surge, obtained with CRI electrograph at the NSO/SP 40 cm coronagraph, in 1993 and 1994. Our results on the 9 brightest quiescent prominences enable us to place r.m.s. upper limits of E_t less than 2 - 5 V/cm on the component of E transverse to the line of sight. We show that these upper limits may be difficult to reconcile with non-ideal MHD models of quiescent prominences formed in extended neutral sheets, whethere or not the tearing mode instability is present. They do, however, seem consistent with ideal MHD models of prominence support. We point out also that these upper limits are within a factor 4 of the minimum value of anistropic electric field that exists due to motional Stark effect in any thermal plasma permeated by a directed magnetic field. Our data on the flare surge suggest and electric field of intensity E approximately 35 V/cm, oriented approximately parallel to the inferred magnetic field. This detection of E_parallel needs to be verified in other flares. But we note that a detectable E_parallel would not be expected in the current interruption flare mechanism, if only a single double layer is present. We show further that the observed relatively narrow, approximately-Gaussian, and only slightly Doppler-shifted Paschen lines, seem inconsistent with the multiple double layers invoked in other models based on the current interruption mechanism. Our detection of E_parallel does seem consistent with reconnection (including tearing-mode) models of flares, provided the field-aligned electrical conductivity is anomalous over substantial volumes of the plasma circuit joining the reconnecting domain to the photosphere. @-------------------------------------------------------------------- Title: Solar irradiance variability and luminosity changes Authors: Foukal, P. Bibliographic Code: 1995HiA....10..294F Abstract @-------------------------------------------------------------------- Title: Reply to the comment on "Stellar luminosity variations and global warming", by R. R. Radick. Authors: Foukal, P. Bibliographic Code: 1994Sci...266.1073F Abstract @-------------------------------------------------------------------- Title: Stellar Variability and Global Warming Authors: Radick, Richard R.; Foukal, Peter Bibliographic Code: 1994Sci...266.1072R Abstract @-------------------------------------------------------------------- Title: Stellar Luminosity Variations and Global Warming Authors: Foukal, Peter Bibliographic Code: 1994Sci...264..238F Abstract Recent studies indicate that variation in the sun's luminosity is less than that observed in many other stars of similar magnetic activity. Current findings also indicate that in more active stars, the attenuation by faculae of sunspot luminosity modulation is less effective than in the sun at present. The sun could thus become photometrically more variable (and dimmer) if its magnetic activity exceeded present levels. But the levels of solar activity required for this to occur are not observed in carbon-14 and beryllium-10 records over the past several millennia, which indicates that such an increase in amplitude of surface magnetism-driven variations in solar luminosity is unlikely in the present epoch. @-------------------------------------------------------------------- Title: Measurements of electric fields in coronal magnetic structures. Authors: Foukal, P. V.; Behr, B. B. Bibliographic Code: 1994scs..conf..177F Abstract The authors review the use of Stark effect to measure d.c. and wave-related electric fields predicted in models of coronal and flare heating, prominence support, and related phenomena. They describe the Mk II limb electrograph now in routine operation at NSO/Sac Peak. @-------------------------------------------------------------------- Title: Properties of Faculae from Observations Near the Opacity Minimum Authors: Foukal, P.; Moran, T. Bibliographic Code: 1994IAUS..154...23F Abstract @-------------------------------------------------------------------- Title: Study of solar irradiance variations holds key to climate questions Authors: Foukal, P. Bibliographic Code: 1994EOSTr..75..377F Abstract @-------------------------------------------------------------------- Title: The Curious Case of the Greenwich Faculae Authors: Foukal, Peter Bibliographic Code: 1993SoPh..148..219F Abstract We analyze the record of facular areas compiled by the Royal Greenwich Observatory (RGO) from daily white-light observations between 1874 and 1976. Curiously, the relative amplitudes of the three largest sunspot cycles 17, 18, and 19 in this record are reversed when they are ranked by facular area. We show that this negative correlation arises from a general decrease of the ratioA _F/A _S, of facular to sunspot area, with increasingA _S. Within a given cycle,A _F/A _Sdecreases in active regions of largeA _S, butA _F/A _Sis also lower at allA _S, in cycles of higher peak amplitude inA _S. This decrease ofA _F/A _Sin large spot groups is consistent with its decrease in younger, more active solar-mass stars, and it may explain why stars only slightly more magnetically active than the Sun tend to exhibit much greater variability in broad-band photometry. We suggest that the physical explanation is an increased spatial filling factor of magnetic flux, favoring formation of sunspots over faculae. We also explain why the decrease inA _F/A_Sis not seen in the disc-integrated Ca K plage areas, nor in theF10.7 microwave index, both of which exhibit remarkable linearity when plotted against smoothed sunspot area. This explanation suggests how complementary data on faculae and plages from RGO and Mt. Wilson could be used to improve empirical models of total irradiance variation, extending back to 1874. @-------------------------------------------------------------------- Title: Liquid crystal tunable light filters for surveillance and remote sensing applications Authors: Foukal, Peter; Miller, Peter J.; Hoyt, Clifford C. Bibliographic Code: 1993SPIE.1952..168F Abstract In this paper we put forward some conceptual designs for liquid crystal tunable filters (LCTF's) that offer improved wavelength flexibility, tuning speed, power consumption and reliability, over the mechanical filter wheels presently baselined for the High Resolution Earth Processing Imager (HEPI) and Advanced Lightning Mapper (ALM) geosynchronous remote sensing experiments. We also point out advantages that accrue from the extremely wide acceptance angle (F 1) achievable with birefringent filters. Thermal vacuum testing and radiation damage analysis will be required to investigate the space hardening of these new filters. @-------------------------------------------------------------------- Title: Measurement of Plasma Electric Fields in Prominences Authors: Foukal, P.; Behr, B. Bibliographic Code: 1993BAAS...25Q1206F Abstract @-------------------------------------------------------------------- Title: The RISE Precision Solar Photometric Telescope Project Authors: Kuhn, J. R.; Foukal, P. V. Bibliographic Code: 1993BAAS...25.1184K Abstract @-------------------------------------------------------------------- Title: Application of Cryogenic Electrical Substitution Radiometers in the Calibration of Solar-Terrestrial Remote Sensing Instruments Authors: Foukal, P. V.; Jauniskis, L. Bibliographic Code: 1993Metro..30..279F Abstract We describe two recent applications of cryogenic Electrical Substitution Radiometers (ESRs), which can provide significantly improved accuracy and sensitivity in the calibration of solar-terrestrial remote sensing instruments. In the first, a portable UV filter radiometer was calibrated at 257 nm using a cryogenic ESR and an intensity-stabilized laser. This filter radiometer was then used to measure the UV irradiance at 257 nm of deuterium lamps and of the SURF II synchrotron. Our results indicate that the accuracy of these measurements is three to eight times better than presently achieved with calibrations of these source standards, at this wavelength. Improvement of UV calibrations is critical to the success of NASA remote sensing of ozone, and of related solar UV irradiance variations. In the second application, a fast cryogenic ESR was developed for use at TRW as a spectrally flat transfer standard, to provide radiometric calibrations of the CERES spectroradiometer with accuracies of better than 1% in the short-wave band and 0,5% in the long-wave and total bands. The CERES instrument is designed to provide improved measurements of the Earth's radiation budget for climate studies. @-------------------------------------------------------------------- Title: Characterization of an absolute cryogenic radiometer as a standard detector for radiant-power measurements Authors: Datla, R. U.; Stock, K.; Parr, A. C.; Hoyt, C. C.; Miller, P. J.; Foukal, P. V. Bibliographic Code: 1992ApOpt..31.7219D Abstract @-------------------------------------------------------------------- Title: A photometric study of faculae and sunspots between 1.2 and 1.6 micron Authors: Moran, T.; Foukal, P.; Rabin, D. Bibliographic Code: 1992SoPh..142...35M Abstract The interpretation of dark magnetic faculae observed in previous imaging of the solar photosphere at 1.63 micron is further investigated. It is shown that their contrast at 1.63 micron increases with magnetic flux beyond a threshold value of about 2 x 10 exp 18 Mx and blends smoothly with the contrast vs flux relation measured at this wavelength for larger structures of sunspot size. The contrast of the dark faculae observed near the disk center at 1.63 micron is approximately 4 percent. The present observations at 1.23 micron, which probe slightly higher photospheric levels, do not show these dark faculae. It is inferred that magnetic flux tubes of diameter as small as 500 km significantly inhibit convective heat flow to the photosphere, much as do sunspot flux tubes of much larger diameter. It is suggested that, in even smaller flux tubes, the inhibition becomes readily less significant. It is shown that the sunspot-size dependence of umbral IR contrast vs wavelength that is observed can be explained in terms of instrumental blurring. @-------------------------------------------------------------------- Title: Wavelength tunable liquid crystal imaging filters for remote sensing from geosynchronous platforms Authors: Foukal, Peter Bibliographic Code: 1992crii.rept.....F Abstract Recent advances in liquid crystal technology have enabled us to construct tunable birefringent filters with bandwidths between approximately 0.1 nm and 50 nm. The center wavelength of these filters can be selected electronically, in a few tens of milliseconds, with no moving parts. These liquid crystal tunable filters (LCTF's), together with existing CCD detectors, make possible a new generation of lightweight, rugged, high-resolution imaging spectrophotometers. Such instruments would be particularly interesting for remote sensing applications from geosynchronous platforms. Important advantages exist in the aperture, absence of image shift, power consumption, size, weight, and absence of high drive frequencies, compared to current instruments used or considered for multispectral scene analysis. In the present work, we have reviewed spectral requirements of planned NASA geosynchronous remote sensing missions and identified several applications of the liquid crystal tunable filter technology. We have modeled the LCTF performance in the visible and near-infrared, and carried out a literature study on space-hardening of the filter components, to evaluate the suitability of LCTF's for geosynchronous missions. We have also compared the power consumption, weight, size, reliability, and optical performance of an imaging spectrophotometer using a LCTF monochromator, to other instruments that have been put forward for remote sensing from geosynchronous platforms. We put forward some conceptual designs for LCTF's that seem to offer important reliability, over the mechanical filter wheels presently baselined for the HEPI and ALM experiments. The extremely wide acceptance angle achievable with LCTF's could also avoid the present need for large-aperture interference filters in the ALM (and LIS) experiments. Thermal vacuum testing and radiation damage analysis is required to investigate the space hardening of these new filters for geosynchronous flight. @-------------------------------------------------------------------- Title: Absolute calibration of an ultraviolet spectrometer using a stabilized laser and a cryogenic cavity radiometer Authors: Jauniskis, L.; Foukal, P.; Kochling, H. Bibliographic Code: 1992ApOpt..31.5838J Abstract We carry out the calibration of an ultraviolet spectrometer by using a cryogenic electrical-substitution radiometer and intensity-stabilized laser sources. A comparison of the error budgets for the laser-based calibration described here and for a calibration using a type-FEL tungsten spectral-irradiance standard indicates that this technique could provide an improvement of a factor of about three in the uncertainty of the spectrometer calibration, resulting in an absolute accuracy (standard deviation of three) of about 1 percent at 257 nm. The technique described here might significantly improve the accuracy of calibrations on NASA ozone-monitoring and solar ultraviolet-monitoring spectrophotometers when used to complement present procedures that employ lamps and the SURF II synchrotron ultraviolet radiation facility at the National Institute of Standards and Technology. @-------------------------------------------------------------------- Title: Darkness can illuminate Authors: Foukal, Peter Bibliographic Code: 1992Natur.358..285F Abstract @-------------------------------------------------------------------- Title: The Case of the Missing Faculae Authors: Foukal, P. V. Bibliographic Code: 1992AAS...180.0703F Abstract @-------------------------------------------------------------------- Title: Book-Review - Solar Astrophysics Authors: Foukal, P. V. Bibliographic Code: 1992S&T....83..173F Abstract @-------------------------------------------------------------------- Title: Solar Luminosity Variation Authors: Foukal, Peter Bibliographic Code: 1992ASPC...27..439F Abstract @-------------------------------------------------------------------- Title: Do changes in the photospheric magnetic network cause the 11 year variation of total solar irradiance? Authors: Foukal, P.; Harvey, K.; Hill, F. Bibliographic Code: 1991ApJ...383L..89F Abstract Changes in the area of the photospheric magnetic network over the sunspot cycle have been put forward as the 'missing component' required to explain the 11-yr variation of total solar irradiance observed by space-borne radiometers. It is shown that this explanation is consistent with recent measurements of the photometric contrast of magnetic faculae and with the present measurement of the network area change during cycle 21. @-------------------------------------------------------------------- Title: An electrograph for measurement of macroscopic electric fields in prominences and flares Authors: Moran, T.; Foukal, P. Bibliographic Code: 1991SoPh..135..179M Abstract An 'electrograph' instrument designed for measurement of macroscopic electric fields in solar plasmas is described, using the polarization dependence of line width in Stark-broadened hydrogen Paschen emission lines. Observations of quiescent prominences and limb chromosphere with the electrograph at the NSO/Sac Peak Evans Coronal Facility provide upper limits of 5-10 V/cm for transverse macroscopic electric fields in these structures, averaged over an area of about 5 x 7 arcsec. Random thermal motions of hydrogen ions across magnetic field lines generate a quasi-static electric field, which should be distinguishable from pressure broadening in the intensely magnetized chromosphere over a sunspot, given an electrograph sensitivity a factor 2-3 better than that achieved here. Future electrograph measurements of limb flares, post-flare loops, and eruptive prominences, even at 5-V/cm sensitivity, could provide a useful new test of reconnection and discharge effects in such dynamic structures. @-------------------------------------------------------------------- Title: Cryogenic radiometers and intensity-stabilized lasers for Eos radiometric calibrations Authors: Foukal, P.; Hoyt, C.; Jauniskis, L. Bibliographic Code: 1991SPIE.1493...72F Abstract Liquid helium-cooled electrical substitution radiometers (ESRs) provide irradiance standards with demonstrated absolute accuracy at the 0.01 percent level, spectrally flat response between the UV and IR, and sensitivity down to 0.1 nW/sq cm. We describe an automated system developed for NASA - Goddard Space Flight Center, consisting of a cryogenic ESR illuminated by servocontrolled laser beams. This system is designed to provide calibration of single-element and array detectors over the spectral range between 257nm in the UV to 10.6 microns in the IR. We also describe a cryogenic ESR optimized for black body calibrations that has been installed at NIST, and another that is under construction for calibrations of the CERES scanners planned for Eos. @-------------------------------------------------------------------- Title: Book Review: Solar astrophysics / Wiley Interscience, 1990 Book Authors: Foukal, P. V. Bibliographic Code: 1991SoPh..133..405F Abstract @-------------------------------------------------------------------- Title: Electric fields in the solar atmosphere - A review Authors: Foukal, P.; Hinata, S. Bibliographic Code: 1991SoPh..132..307F Abstract A review is presented of remote sensing techniques which measure the electric field component transverse to the line of sight and achieve a sensitivity range of 5-10 V/cm. Three models are shown to predict quasistatic, macroscopic values of E(parallel), the electric field component parallel to the magnetic vector, beyond the sensitivity range considered. These processes are: the discharge model of flares; the models of return currents related to flare particle beams; and the models of neutral sheets related to two-ribbon flares and postflare loops. Time dependent electric fields related to MHD and plasma waves, and to plasma disturbance, may allow the detection of both E components (parallel and perpendicular). The uncertainty relating to the emission measures, time scales, volumes, and plasma conditions of these flares and electrified plasma volumes is emphasized. It is pointed out, however, that important information can be obtained by observing electric fields at existing sensitivity levels. By measuring these E-fields, the understanding of flares and related dynamic events can be improved. @-------------------------------------------------------------------- Title: Book-Review - Solar Astrophysics Authors: Foukal, P. V. Bibliographic Code: 1991JRASC..85..119F Abstract @-------------------------------------------------------------------- Title: On the Role of the Photospheric Magnetic Network in the 11-Year Variation of Total Solar Irradiance Authors: Harvey, K. L.; Foukal, P. V. Bibliographic Code: 1991BAAS...23.1068H Abstract @-------------------------------------------------------------------- Title: Objectives and Scope of RISE Authors: Foukal, P. Bibliographic Code: 1991BAAS...23.1040F Abstract @-------------------------------------------------------------------- Title: Book Review: Solar astrophysics / Wiley Interscience, 1990 Book Authors: Foukal, P. V. Review Author: Harrison, R. A. Bibliographic Code: 1991Obs...111...45H Abstract @-------------------------------------------------------------------- Title: Measurement of polarization-dependent Stark broadening as a diagnostic of electric fields in the solar atmosphere. Authors: Moran, T.; Foukal, P. Bibliographic Code: 1991sopo.work..393M Abstract Electric fields play a key role in models of energy dissipation and charged particle acceleration in flares, as well as other dynamic solar phenomena. An instrument designed to observe the transverse (to the line of sight) component of such electric fields has been constructed and installed at the 40 cm coronagraph at Sacramento Peak. This "electrograph" measures the halfwidth of Stark-broadened hydrogen emission lines as a function of polarization in the sky plane. Thus, the instrument resembles a transverse magnetograph except that it operates on Stark-broadened hydrogen emission lines in coronal structures rather than on Zeeman-broadened absorption lines on the disk. The authors describe the principles behind the instrument, its design, and some first data. @-------------------------------------------------------------------- Title: Cryogenic Radiometers for Remote Sensing and Pyrheliometry Authors: Foukal, P. V.; Hoyt, C. C.; Miller, P. J. Bibliographic Code: 1991Metro..28..255F Abstract Liquid-helium cooling of electrical substitution radiometers (ESRs) offers significant improvements in the time constant and sensitivity of these spectrally flat receivers. These cryogenic improvements have been used to good advantage in the design and prototype construction of fast ESR receivers (and receiver arrays) with sufficient sensitivity for NASA Earth remote-sensing applications. We describe some of our results in this area and suggest how the advantages offered by cryogenic ESRs might be useful in achieving the very long-term reproducibility that will be required in 21st-century pyrheliometry. @-------------------------------------------------------------------- Title: Cryogenic Radiometers and their Application to Metrology Authors: Hoyt, C. C.; Foukal, P. V. Bibliographic Code: 1991Metro..28..163H Abstract Two types of electrical substitution radiometers have been developed at Cambridge Research and Instrumentation (CRI) for applications in metrology. We describe a table-top instrument for calibrating detectors using stabilized laser radiation, a high sensitivity instrument for measuring cryogenic black-body radiation, and an automated system for calibrating photodiodes and thermopiles with several laser lines and a cryogenic radiometer as a reference standard. We also discuss performance characteristics of installed instruments. @-------------------------------------------------------------------- Title: Solar irradiance variability from modern measurements Authors: Froehlich, C.; Foukal, P. V.; Hickey, J. R.; Hudson, H. S.; Willson, R. C. Bibliographic Code: 1991suti.conf...11F Abstract Direct measurements from satellites of the solar 'constant' (the total irradiance at mean sun-earth distance) during more than ten years show variations over time scales from minutes to years and decades. At high frequencies, solar oscillations contribute to the variance. The most important influences are related to solar activity: during the passage of active regions on the solar disk (sunspots and faculae) changes of a few 0.1 percent lasting for several days are observed. The effects of spots can be well reproduced by the projected sunspot index, whereas the influence of faculae have to be modeled from proxy data like the Ca-K plage index or the He I index. Long-term trends are detected which are connected to the 11-yr solar activity cycle. @-------------------------------------------------------------------- Title: Solar astronomy Authors: Rosner, Robert; Noyes, Robert; Antiochos, Spiro K.; Canfield, Richard C.; Chupp, Edward L.; Deming, Drake; Doschek, George A.; Dulk, George A.; Foukal, Peter V.; Gilliland, Ronald L. Bibliographic Code: 1991aap..reptR....R Abstract An overview is given of modern solar physics. Topics covered include the solar interior, the solar surface, the solar atmosphere, the Large Earth-based Solar Telescope (LEST), the Orbiting Solar Laboratory, the High Energy Solar Physics mission, the Space Exploration Initiative, solar-terrestrial physics, and adaptive optics. Policy and related programmatic recommendations are given for university research and education, facilitating solar research, and integrated support for solar research. @-------------------------------------------------------------------- Title: Cryogenic cavity radiometers as detectors and calibration standards for remote sensing Authors: Foukal, P.; Hoyt, C.; Koechling, H.; Miller, P. Bibliographic Code: 1990SPIE.1299...92F Abstract The large decrease in specific heat of pure metals achieved at liquid helium temperatures can be used to greatly reduce the time constant and increase the sensitivity of spectrally nonselective electrical substitution radiometers used for satellite remote sensing o fhte earth's radiations. Single-element and array receivers with a natural time constant below 25 msec and a sensitivity below 10 nW have been constructed and servocontrolled to measure radiations from below 300 nm to beyond 40 microns, to better than 1 percent absolute accuracy. Flight of such an experiment on the Atlas pallet would provide an important proof of concept for future cryogenic radiometry at high accuracy, sensitivity and spatial resolution from the Space Station, from polar platforms and possibly from geosynchronous platforms, as flight qualified 2 K cryocoolers are developed in the next decade. The use of cryogenic radiometers in radiometric calibrations at NIST and elsewhere, and their possible applications for ground-based calibrations of the Eos remote sensing instrument are described. @-------------------------------------------------------------------- Title: Solar cycle modulation of total irradiance: an empirical model from 1874 to 1988 Authors: Lean, J.; Foukal, P. Bibliographic Code: 1990NASCP3086..197L Abstract Evidence acquired during the past decade indicates that over time scales of the solar cycle, enhanced emission from bright solar faculae cause significant variations in the sun's total irradiance even though, on shorter time scales, the most pronounced variations are those resulting from the passage of dark sunspots across the solar disc. An empirical model which accounts for the competing effects of dark sunspots and bright faculae has been developed from the available radiometry in cycle 21, and extended back to the beginning of solar cycle 12. According to this model, the largest 11-year modulation of total irradiance during the C20th occurred in the most recent cycle 21. @-------------------------------------------------------------------- Title: The program on radiative inputs of the sun to the Earth (rise) Authors: Foukal, Peter V. Bibliographic Code: 1990NASCP3086..116F Abstract General concerns about changes in the Earth's climate and in the ozone layer have increased the importance of measuring and understanding variations in the sun's radiative outputs. These outputs appear, for instance, at the top of the list of global change forcing agents in the recent FY-90 document on the U.S. Global Change Program. Significant advances have been made over the past decade in radiometry of the total solar irradiance. Photometry of light variations in stars similar to the sun, but much younger, is providing new insights into the sun's variations in luminosity and UV radiation at previous epochs of interest to paleoclimate studies. Measurement of the sun's 11-year output variability in the ultraviolet and extreme ultraviolet still possess a challenge of great importance. All of these topics are addressed by the program on Radiative Inputs of the Sun to Earth (RISE). RISE is a 5-year program of observations, data analysis, and theory, that has been defined at two workshops held in Boulder, Colorado in November 1987, and in Tucson, Arizona in October 1989. These meetings involved about 60 solar and atmospheric physicists, and stellar astronomers. A proceedings of the 1987 workshop was issued in early 1988. A substantial component of the program, consisting of ground-based observations, data analysis, and theory, is directed at the National Science Foundation (NSF) and the program is seeking a FY-92 funding start at the NSF. RISE also makes recommendations on measurements required from NASA and NOAA satellites. @-------------------------------------------------------------------- Title: Die veränderliche Sonne. Authors: Foukal, P. V. Bibliographic Code: 1990SpWis...4...66F Abstract @-------------------------------------------------------------------- Title: Periodicity and Aperiodicity in Solar Magnetic Activity: Discussion Authors: Foukal, P.; Weiss, N. O. Bibliographic Code: 1990RSPTA.330..625F Abstract @-------------------------------------------------------------------- Title: Solar Luminosity Variations over Timescales of Days to the past Few Solar Cycles Authors: Foukal, P. Bibliographic Code: 1990RSPTA.330..591F Abstract Dips in the total irradiance of up to 0.2% and lasting typically 10 days are now well known to be caused by the transit of dark sunspots across the photospheric disc. The large bright magnetic faculae usually associated with spots cause irradiance increases of comparable magnitude although the form of their signal is more subtle. Radiometry from five satellites beginning in late 1978 indicates a minimum in irradiance at the epoch of lowest magnetic activity between solar cycles 21 and 22. Analysis of these radiometric measurements indicates that this irradiance decline between about 1981 and 1986 was caused mainly by decay in the excess radiation of bright faculae in the magnetic network outside of active regions. Empirical models of irradiance modulation extending back to 1874 indicate that the Sun is typically about 0.05% brighter at activity maximum than at minimum. @-------------------------------------------------------------------- Title: Astronomical Determinations of the Solar Variability: Discussion Authors: Stanford, J. S.; Ribes, Elizabeth; Foukal, P. Bibliographic Code: 1990RSPTA.330..497S Abstract @-------------------------------------------------------------------- Title: Infrared imaging of faculae at the deepest photospheric layers Authors: Foukal, P.; Little, R.; Graves, J.; Rabin, D.; Lynch, D. Bibliographic Code: 1990ApJ...353..712F Abstract The NOAO 58 x 62 InSb array and the National Solar Observatory McMath telescope are used to image the deepest photospheric layers of three active regions at the 1.63-micron opacity minimum. The faculae are darker than the photosphere, with a measured contrast of at least 2 percent at positions on the disk with mu = 0.75-1.0. Near the limb, they are brighter than the photosphere, as in the visible. At mu = 0.5-0.75, they are difficult to detect at 1.63 micron. The observation that faculae and their immediate surroundings exhibit a clear deficit of brightness temperature near disk center at 1.63 micron seems to rule out the hillock model put forward to explain their center-to-limb contrast variation. @-------------------------------------------------------------------- Title: An Electrograph for the Measurement of Electric Fields in Coronal Structures Authors: Moran, T.; Foukal, P. Bibliographic Code: 1990BAAS...22..794M Abstract @-------------------------------------------------------------------- Title: Cryogenic absolute radiometers as laboratory irradiance standards, remote sensing detectors, and pyroheliometers Authors: Foukal, Peter V.; Hoyt, C.; Kochling, H.; Miller, P. Bibliographic Code: 1990ApOpt..29..988F Abstract The dramatic improvement in heat diffusivity of pure Cu at liquid-He temperatures makes possible very important advances in the absolute accuracy, reproducibility, sensitivity, and time constant of cryogenic electrical substitution radiometers (ESRs), relative to conventional ESRs. The design and characterization of a table-top cryogenic ESR now available for detector calibration work to the 0.01-percent level of absolute accuracy under laser illumination is discussed. A sensitive cryogenic ESR recently delivered to the NIST for radiometric calibrations of black bodies is also described, along with the design and testing of a very fast cryogenic ESR developed for NASA remote-sensing studies of the earth's radiation budget. @-------------------------------------------------------------------- Title: The variable sun Authors: Foukal, Peter V. Bibliographic Code: 1990SciAm.262...34F Abstract The variations in the sun's magnetic field, in its total light output, and in its output of charged particles are discussed. Present understanding of the causes of these variations is reviewed. The effects of these variations on the earth are addressed. @-------------------------------------------------------------------- Title: The variable sun. Authors: Foukal, P. V. Bibliographic Code: 1990SciAm.262...26F Abstract The sun's steady warmth and brightness are illusory; the sun's output of radiation and particles varies. Systematic observations are beginning to unveil the causes of these changes and their effects on the earth. @-------------------------------------------------------------------- Title: An empirical model of total solar irradiance variation between 1874 and 1988 Authors: Foukal, P.; Lean, J. Bibliographic Code: 1990Sci...247..556F Abstract An empirical model of variations in the total solar irradiance caused by observed changes in photospheric magnetic activity between 1874 and 1988 is presented. The model provides a remarkably good representation of the irradiance variations observed by satellite-borne radiometers between 1980 and 1988. It suggests that the mean total irradiance has been rising steadily since about 1945, with the largest peak so far at about 1980 and another large peak expected during the current solar cycle 22. But it is doubtful whether even this rise can contribute significantly to global warming, unless the temperature increase of about 0.02 C that it produces in current energy balance models seriously underestimates the sensitivity of climate to solar irradiance changes. @-------------------------------------------------------------------- Title: Solar astrophysics Authors: Foukal, Peter Bibliographic Code: 1990soas.book.....F Abstract The present work discusses advancements in such aspects of solar astrophysics as radiative transfer in the solar atmosphere, solar spectroscopic techniques, the dynamics of solar plasmas, the solar photosphere, and the sun's internal structure and energy generation. Also treated are solar rotation and advection, observations of solar photospheric activity and magnetism, the solar chromosphere and corona, solar prominences and flares, the dynamics of the solar magnetic field, the solar wind and heliosphere, and the variability of the sun. @-------------------------------------------------------------------- Title: Solar luminosity variations over timescales of days to the past few solar cycles. Authors: Foukal, P. Bibliographic Code: 1990ecvs.conf..591F Abstract Dips in the total irradiance of up to 0.2% and lasting typically 10 days are now well known to be caused by the transit of dark sunspots across the photospheric disc. The large bright magnetic faculae usually associated with spots cause irradiance increases of comparable magnitude although the form of their signal is more subtle. Radiometry from five satellites beginning in late 1978 indicates a minimum in irradiance at the epoch of lowest magnetic actvity between solar cycles 21 and 22. Analysis of these radiometric measurements indicates that this irradiance decline between about 1981 and 1986 was caused mainly by decay in the excess radiation of bright faculae in the magnetic network outside of active regions. Empirical models of irradiance modulation extending back to 1874 indicate that the Sun is typically about 0.05% brighter at activity maximum than at minimum. @-------------------------------------------------------------------- Title: Comparison between a side-viewing cryogenic radiometer and self-calibrated silicon photodiodes Authors: Hoyt, Clifford C.; Miller, Peter J.; Foukal, Peter; Zalewski, Edward F. Bibliographic Code: 1989SPIE.1109..236H Abstract @-------------------------------------------------------------------- Title: Comment on 'Asymmetry and variations of solar limb darkening along the diameter defined by diurnal motion in April 1981' by Neckel and Labs (1987) Authors: Foukal, P. Bibliographic Code: 1989SoPh..120..249F Abstract In a recent paper (Neckel and Labs, 1987a) a strong claim is made for detection of surprisingly large variations in photospheric limb darkening over time-scales of minutes to hours. Some of this evidence relies on re-interpretation of our measurements carried out at Kitt Peak between 1980-1982 (Petro et al., 1984). The purpose of this Comment is to draw attention to information we have published which suggests that the variations noted by Neckel and Labs are more likely to be of instrumental than solar origin. @-------------------------------------------------------------------- Title: The Solar Radiative Output Variation Program Authors: Foukal, P. V. Bibliographic Code: 1989BAAS...21..832F Abstract @-------------------------------------------------------------------- Title: Infrared Imaging of Faculae at the Deepest Photospheric Layers Authors: Foukal, P.; Little, R.; Graves, B.; Rabin, D.; Lynch, D. Bibliographic Code: 1989BAAS...21..828F Abstract @-------------------------------------------------------------------- Title: Infrared imaging of sunspots and faculae at the photospheric opacity minimum Authors: Foukal, P.; Little, R.; Mooney, J. Bibliographic Code: 1989ApJ...336L..33F Abstract Continuum observations at 1.63-microns using a PtSi IR CCD camera have yielded images of the deepest observable layers in an active region. When these are compared with spectroheliograms in visible continuum and in the Ca K wing, it becomes evident that faculae are seen near disk center as low-contrast, dark structures; also, the ratio of umbral intensity for small relative to large spots is substantially lower at 1.63 microns than in the red continuum. These findings suggest that, at the deepest observable layers, faculae as well as spots contain plasma which is cooler than the surrounding photosphere at equal optical depth; they can be explained straightforwardly if flux tubes of all diameters exhibit convective heat flow along their axes. @-------------------------------------------------------------------- Title: A Model of Solar Luminosity Modulation by Magnetic Activity between 1954 and 1984 Authors: Lean, J.; Foukal, P. Bibliographic Code: 1988Sci...240..906L Abstract A simple model based on the changes in excess radiation from bright magnetic faculae and on changes in reduced radiation from dark spots is remarkably successful in matching the slow variations of total solar irradiance measured simultaneously by the ERB and ACRIM satellite radiometers between 1981 and 1984. This model was extended back to 1954 to reconstruct the modulation of irradiance by magnetic activity during the past three 11-year solar cycles. The model predicts that the sun is consistently brighter at activity maximum than at minimum. The 0.07 percent brightening at the peak of the last cycle in 1980 was more pronounced than the brightenings found for either of the two previous cycles, even though cycle 19, which peaked around 1957, had the largest sunspot number amplitude in the history of reliable sunspot records. @-------------------------------------------------------------------- Title: Magnetic modulation of solar luminosity by photospheric activity Authors: Foukal, P.; Lean, J. Bibliographic Code: 1988ApJ...328..347F Abstract The authors study the behavior of slow changes in solar irradiance, S, using measurements obtained with radiometers on the SMM and Nimbus 7 spacecraft. The analysis of the 1978 - 1984 ACRIM and ERB radiometry reveals low-amplitude (0.04% - 0.07%) variations in S on time scales of 4 - 9 months that are well correlated between these two data bases. The variations correlate very well with changes in facular radiations. It is also shown that the slow downtrend in S seen since 1981 by the ACRIM and ERB arises mainly from a decreasing irradiance contribution of bright photospheric magnetic elements outside the large faculae included in the daily CaK plage index. The finding that this network contribution is unbalanced over several years shows that photospheric activity has a net influence on solar luminosity, besides the more nearly balanced contributions of the spots and the large faculae. @-------------------------------------------------------------------- Title: Paschen-line Stark-broadening as an electric field diagnostic in erupting prominences Authors: Foukal, P.; Little, R.; Gilliam, L. Bibliographic Code: 1988SoPh..114...65F Abstract We analyse the Stark broadening of the Balmer and Paschen lines emitted in two bright eruptive prominences, to determine the total electric field in these structures. We show that the Paschen lines provide a significantly more sensitive and accurate electric field indicator than the Balmer lines used previously in such studies. In the two eruptive events analysed here, the total electric fields agree to within 5 10 V cm^-1 with the pressure-broadening fields expected from local densities of the cool plasma, measured simultaneously and co-spatially by a line-ratio diagnostic. We conclude that in such structures the upper limit to any widespread macroscopic fields is roughly 10 V cm^-1 or less. This is in agreement with the motional electric field that might be associated with reconnection at the observed rate of the prominences' outward motion of about 135 km s^-1. @-------------------------------------------------------------------- Title: Infrared Imaging of Magnetic Faculae at the Photospheric Opacity Minimum Authors: Foukal, P.; Little, R.; Mooney, J. Bibliographic Code: 1988BAAS...20..689F Abstract @-------------------------------------------------------------------- Title: Introduction Authors: Foukal, P. Bibliographic Code: 1988srov.procD...1F Abstract @-------------------------------------------------------------------- Title: A Model of Solar Luminosity Modulation by Magnetic Activity Between 1954-1984 Authors: Foukal, P.; Lean, J. Bibliographic Code: 1988srov.proc..323F Abstract @-------------------------------------------------------------------- Title: Solar Radiative Output Variation Authors: Foukal, Peter Bibliographic Code: 1988srov.proc.....F Abstract @-------------------------------------------------------------------- Title: Solar radiative output variations Authors: Foukal, Peter V. Bibliographic Code: 1988EOSTr..69.1598F Abstract A National Science Foundation sponsored workshop on Solar Radiative Output Variations was held at the National Center for Atmospheric Research, Boulder, Colo., November 9-11, 1987. The aims were to review the status of this topic, identify research areas most likely to prove fruitful in the next 5-10 years, and make recommendations on desirable initiatives, particularly in ground-based observations and theory.Fifty three scientists participated in 2½ days of review and contributed papers, and in the organizational meetings of six working groups. The sessions covered subjects ranging from ``what atmospheric and climate physicists need to know about the solar radiative output and its variations,'' through direct and proxy observations of EUV, UV, and total irradiance variability of the Sun and similar stars, to the physical interpretation of such variations in the present and distant past. @-------------------------------------------------------------------- Title: Theoretical interpretation of total solar irradiance variations Authors: Foukal, P. Bibliographic Code: 1988AdSpR...8...43F Abstract Analysis of the radiometric record of the total solar irradiance obtained over the past ten years has revealed characteristic variations caused by sunspots and faculae, the magnetic network, and also the sun's internal oscillations and photospheric convection. The present physical understanding of the variations caused by spots and faculae in active regions is discussed and the results are applied to interpretation of recently discovered solar irradiance changes over time scales of several months to most of an 11-year activity cycle. @-------------------------------------------------------------------- Title: Measurement of macroscopic electric fields in solar plasma structures Authors: Foukal, Peter Bibliographic Code: 1987crii.rept.....F Abstract The work performed in this effort has led to several important advances in our investigation of a new technique for remote sensing of plasma electric fields. The atomic physics underlying the technique is placed on a firmer basis and the polarization structure of the high Balmer and Paschen lines behaves as predicted by perturbation theory. Balmer- and Paschen-line observations of erupting prominences are obtained at Sacramento Peak Observatory and show that the marked increase in sensitivity of the technique expected with the Paschen lines can be realized. Understanding of the interpretation of the observed hydrogen-line broadening is improved, and shown that the sensitivity of the technique, when applied to structures emitting intense hydrogen lines, can exceed 10 volts/cm. This sensitivity is more than adequate to test predictions of transverse electric fields intensities and orientations, in models of flares and other active phenomena. The goal is to construct an instrument optimized for solar plasma electric field measurements, to be installed at the SPO Big Dome. Such an electrograph would make important advances toward understanding the mechanisms and prediction of solar eruptions such as flares and active filaments. @-------------------------------------------------------------------- Title: Active region flows Authors: Foukal, Peter Bibliographic Code: 1987NASCP2483...15F Abstract A wide range of observations has shown that active region phenomena in the photospheric, chromospheric and coronal temperature regimes are dynamical in nature. At the photosphere, recent observations of full line profiles place an upper limit of about + or - 20/msec on any downflows at supergranule cell edges. Observations of the full Stokes 5 profiles in the network show no evidence for downflows in magnetic flux tubes. In the area of chromospheric dynamics, several models were put forward recently to reproduce the observed behavior of spicules. However, it is pointed out that these adiabatic models do not include the powerful radiative dissipation which tend to damp out the large amplitude disturbances that produce the spicular acceleration in the models. In the corona, loop flows along field lines clearly transport mass and energy at rates important for the dynamics of these structures. However, advances in understanding the heating and mass balance of the loop structures seem to require new kinds of observations. Some results are presented using a remote sensing diagnostic of the intensity and orientation of macroscopic plasma electric fields predicted by models of reconnective heating and also wave heating. @-------------------------------------------------------------------- Title: Flare Electric Fields Measured by Stark Effect in Paschen and Balmer Lines Authors: Foukal, P.; Little, R.; Gilliam, L. Bibliographic Code: 1987BAAS...19..950F Abstract @-------------------------------------------------------------------- Title: The Performance of a Schottky Barrier and HgCdTe Infrared Arrays in Monochromatic Light Authors: Graves, B.; Foukal, P.; Rieke, M.; Fowler, A. Bibliographic Code: 1987BAAS...19..928G Abstract @-------------------------------------------------------------------- Title: Magnetic Modulation of Solar Luminosity by Photospheric Activity Authors: Foukal, P.; Lean, J. Bibliographic Code: 1987BAAS...19..924F Abstract @-------------------------------------------------------------------- Title: A photometric search for solar giant convection cells Authors: Chiang, W.-H.; Petro, L. D.; Foukal, P. V. Bibliographic Code: 1987SoPh..110..129C Abstract The authors limit the photometric contrast of solar giant convection cells using lambda525.6 nm continuum images obtained on 15 days in May 1985. The r.m.s. of the giant cell intensity pattern must be less than or equal to the observed r.m.s. on spatial scales 80 to 240 Mm which is 0.023% or, equivalently, 0.33K. However, the spatial scale and time-scale dependence of the variance demonstrate that giant cells are not the source of the observed variance. Consequently, a tighter constraint on the r.m.s. of the giant cell pattern may be placed, namely 0.016% or 0.23K. This limit is consistent with temperature perturbations estimated from recent nonlinear simulations of global-scale solar convection. The authors use this limit on the r.m.s. of the giant cell pattern to estimate that the contribution of giant cells to the fluctuation of the solar irradiance on a one-month time-scale is less than 3×10^-5s. @-------------------------------------------------------------------- Title: Physical interpretation of variations in total solar irradiance Authors: Foukal, P. Bibliographic Code: 1987JGR....92..801F Abstract Radiometry from the Solar Maximum Mission and Nimbus 7 satellites has demonstrated that the solar constant varies at a peak-to-peak level of up to 0.2 percent on time scales of weeks. The rotation and evolution of dark spots and bright faculae across the sun's disk accounts for most of that variation. Reasonable explanations have been put forward to explain how the spot-blocked heat flow might be stored, and to explain the source of the intense radiation that gives rise to the increased irradiance produced by the bright magnetic faculae. Time-dependent models of the response of the solar convection zone to small perturbations also indicate that slower variations in total solar irradiance of comparable magnitude are likely. More precise observations of the total solar irradiance and radius over long time scales are required to demonstrate the existence of such climatologically relevant changes, and to test models that would enable the interpretation and prediction of these changes. @-------------------------------------------------------------------- Title: Paschen-line Stark-broadening as an electric field diagnostic in erupting prominences Authors: Foukal, P.; Little, R.; Gilliam, L. Bibliographic Code: 1987SoPh..114...65F Abstract The authors analyse the Stark broadening of the Balmer and Paschen lines emitted in two bright eruptive prominences, to determine the total electric field in these structures. They show that the Paschen lines provide a significantly more sensitive and accurate electric field indicator than the Balmer lines used previously in such studies. In the two eruptive events analysed here, the total electric fields agree to within 5 - 10 V cm^-1 with the pressure-broadening fields expected from local densities of the cool plasma, measured simultaneously and co-spatially by a line ratio diagnostic. The authors conclude that in such structures the upper limit to any widespread macroscopic fields is roughly 10 V cm^-1 or less. This is in agreement with the motional electric field that might be associated with reconnection at the observed rate of the prominences' outward motion of about 135 km s^-1. @-------------------------------------------------------------------- Title: Electric fields and plasma structure in coronal magnetic loops Authors: Foukal, P.; Hoyt, C.; Gilliam, L. Bibliographic Code: 1986ApJ...303..861F Abstract Solar flare loop spectra are analyzed by means of recently developed density diagnostics to determine if the Stark effect observed in the Balmer emission lines is due to pressure broadening or the presence of macroscopic electric fields. The database covers five postflare loops and three active prominences. The diagnostics include the density sensitivity of the intensity ratios of strong Na I D, Mg I b, and the Mg I lines relative to the Sr II lines. A total of 43 spectra were obtained in the 3500-6000 A interval, with most lines falling in the 3600-4500 A region. A prism at the entrance to the instrument permitted extracting additional data on the structure of Stark-broadener Balmer lines. A dc electric field was detected, with an energy level of 100 V/cm, oriented transverse to the loop magnetic field vector. Electron densities of 1-10 trillion/cu cm were estimated for the cool structures within loops and 100 billion/cu cm in active prominences. The densities indicated that a pressure deficit may be present in the loop structures in the corona, suggesting an inward directed gas pressure force in cool loop structures. @-------------------------------------------------------------------- Title: Electric Fields in Coronal Loops Authors: Foukal, P.; Hoyt, C.; Gilliam, L. Bibliographic Code: 1986BAAS...18..708F Abstract @-------------------------------------------------------------------- Title: The influence of faculae on total solar irradiance and luminosity Authors: Foukal, P.; Lean, J. Bibliographic Code: 1986ApJ...302..826F Abstract The authors investigate the facular contribution to the total solar irradiance, using the daily active cavity radiometer irradiance monitor (ACRIM) radiometry for 1980 - 1982 and the Earth Radiation Budget (ERB) radiometry for 1978 - 1982. A cross-correlation analysis of the total irradiance, the UV flux measured by Nimbus 7 and calculated contributions from observed sunspot and facular areas is presented. It is found that the solar irradiance records, after subtraction of the calculated sunspot-blocking contribution, exhibit a short-term modulation that is better explained by faculae than by errors in the sunspot-blocking function. The data for 1980 indicate that the facular contribution was comparable to that of spots over time scales of active region evolution in that year. @-------------------------------------------------------------------- Title: Differential photometry of magnetic faculae Authors: Foukal, P.; Duvall, T., Jr. Bibliographic Code: 1985ApJ...296..739F Abstract Previous photometric observations carried out at the KPNO vacuum telescope showed that faculae become clearly visible near disk center in the difference of two widely separated (green and red) continuum passbands. The authors present new observations extending this technique to the near-infrared, to demonstrate that the main factor determining the facular visibility is the difference in H^- opacity between the continuum passbands. It is shown that the difference signal is directly sensitive to changes in the temperature gradient near tau_0.5 = 1 between quiet photosphere and faculae. The measured facular temperature gradient is compared with that given by empirical facular models based on Fraunhofer line observations, and with theoretical predictions. @-------------------------------------------------------------------- Title: Photospheric limb-darkening signatures of global structure variations Authors: Petro, L. D.; Foukal, P. V.; Kurucz, R. L. Bibliographic Code: 1985SoPh...98...23P Abstract Observations of short-term irradiance variations and consideration of mechanisms of the solar activity cycle suggest the possibility of long-term variation of the solar flux. Since the limb darkening is sensitive to effective temperature and convective efficiency, observations of the solar limb darkening may provide a useful means to detect and study long-term global variations. The limb-darkening responses to impulsive variation (in depth) of the source function, to effective temperature variation, and to convection variations are presented. For the variations considered, the limb-darkening variation is approximately linearly proportional to the associated parameters. The minimum detectable amplitude of those parametric variations is derived as a function of observational noise. Given demonstrated errors of observation, single-parameter sensitivities are 3 K for effective temperature variation and 0.007 for local mixing-length variation for year-to-year changes at 99 percent confidence. @-------------------------------------------------------------------- Title: The influence of faculae on sunspot heat blocking Authors: Chiang, W.-H.; Foukal, P. Bibliographic Code: 1985SoPh...97....9C Abstract The influence of faculae on sunspot heat blockage is studied using a thermal model based on eddy heat diffusion through the convection zone. The facula is represented as a localized area of excess emission surrounding the sunspot, which is represented as a thermal plug. Present computations using a range of reasonable combinations of spot and facular depths show no significant influence of the facula on the long storage times of heat blocked by sunspots. However, the local cooling of surface layers produced by excess facular emission in this model propagates globally within the convection zone in a similar way to the heating produced by a spot. The net effect of spots and faculae on solar luminosity over time scales longer than an active region lifetime should thus be determined by the global sum of sunspot flux deficits and facular excesses. @-------------------------------------------------------------------- Title: Investigation of Photospheric Limb-Darkening Variation Between 1980 and 1985 Authors: Petro, L. D.; Foukal, P. V.; Rosen, W. A.; Pierce, A. K.; Kurucz, R. L. Bibliographic Code: 1985BAAS...17..644P Abstract @-------------------------------------------------------------------- Title: A study of solar photospheric limb-darkening variations Authors: Petro, L. D.; Foukal, P. V.; Rosen, W. A.; Kurucz, R. L.; Pierce, A. K. Bibliographic Code: 1984ApJ...283..426P Abstract The authors have obtained regular observations of photospheric limb darkening, using the McMath Solar Telescope, to study possible slow changes in the global temperature structure, in T_eff, and in the ultraviolet continuum flux from the quiet Sun. This paper reports on the analysis of data obtained on 15 days between 1980 September and 1982 December in a continuum window at lambda4451. There are no variations of global limb darkening exceeding 0.1% at the 99% confidence level. The implications of these measurements for slow changes in solar luminosity, convection zone structure, and ultraviolet flux are discussed. @-------------------------------------------------------------------- Title: A photometric study of heat flow at the solar photosphere [ Erratum: 1984ApJ...286..377F ] Authors: Foukal, P.; Fowler, L. Bibliographic Code: 1984ApJ...281..442F Abstract Results of a general survey of photospheric heat flow on scales between granulation and global scale convection are presented. Photometric rasters in the visible continuum show a prominent mottle pattern whose scale and morphology resemble the CaK chromospheric network, but the pattern's correlation with photospheric magnetic fields is too low and its lifetime of 5-10 minutes is too short to support any physical connection to the supergranulation. The network exhibits a slight excess continuum brightness relative to the nonmagnetic photosphere at mu = 1. An upper limit of 2-3 K is placed on nonaxisymmetric brightness inhomogeneities over scales of 50,000-200,000 km. This limit provides a more direct constraint on models of global scale convection than do existing limits on a pole-equator temperature difference. The absence of a thermal shadow preceding an observed spot group seems to favor the higher eddy thermal conductivity profile of Spruit's (1974) model compared to that of Baker and Temesvary (1966). @-------------------------------------------------------------------- Title: On the interpretation of Fraunhofer line Doppler shifts at supergranule boundaries Authors: Miller, P.; Foukal, P.; Keil, S. Bibliographic Code: 1984SoPh...92...33M Abstract A number of investigations have interpreted the systematic redshift of many Fraunhofer lines at the supergranule boundaries as evidence for downward motion, and thus for overturning of supergranular convection cells. In the present investigation, it is shown that this interpretation is to be questioned. A description is given of the measurement and reduction of full line profiles and bisectors observed in the magnetic network and in cells. It is found that three Fe I lines studied exhibit systematic redshifts of the network wing profiles, relative to those formed in the cells, but the deep line cores are not significantly shifted. Line profile computations indicate that the redshifted wings and unshifted line cores suggest somewhat inhibited convection near the network, rather than a general downflow at supergranule cell edges. @-------------------------------------------------------------------- Title: Photometric studies of heat flow at the photosphere Authors: Foukal, P. Bibliographic Code: 1984siva.work...97F Abstract Continuum photometry is carried out and the results of comparing these observations with models of photospheric heat flow are described. The main results are: (1) a possible detection of weak bright rings around some spot penumbrae; (2) no evidence is found for large scale photospheric brightness inhomogeneities exceeding 2-3 K which places tighter constraints on models of global scale convection; (3) supergranular scale continuum structures observed across the photosphere appear mainly due to random clumping of granules; (4) the one case observed of a sunspot emergence shows no thermal shadow exceeding 1.5 K rms one day prior to umbra appearance; (5) network and faculae are found to show a small excess brightness even at mu = 1, so detection of faculae at mu = 1 by differential photometry indicates a gentler temperature gradient near tau = 1 in the facular (relative to cell) atmosphere; (6) the limb darkening study shows no significant global variations to within 0.1% rms. @-------------------------------------------------------------------- Title: Interpretation of Electric Fields in Coronal Magnetic Loops Authors: Foukal, P.; Landman, D. Bibliographic Code: 1984uxsa.coll...25F Abstract @-------------------------------------------------------------------- Title: Radiometric and photometric studies of Solar luminosity variations Authors: Foukal, P. Bibliographic Code: 1984stp..conf..411F Abstract @-------------------------------------------------------------------- Title: Photometric studies of heat flow at the photosphere. Authors: Foukal, P. Bibliographic Code: 1984NASCP2310...97F Abstract The main subject of this review is the photospheric photometry carried out at KPNO since 1980. Also described are some results obtained from comparison of the photometry with a time-dependent model of heat flow developed at Atmospheric and Environmental Research Inc. for interpretation of sunspot effects on solar luminosity. @-------------------------------------------------------------------- Title: Variation of Solar Limb Darkening Authors: Petro, L. D.; Foukal, P. V.; Rosen, W. A.; Kurucz, R. L.; Pierce, A. K. Bibliographic Code: 1983BAAS...15..951P Abstract @-------------------------------------------------------------------- Title: Solar constant Authors: Foukal, Peter; Hoyt, Douglas Bibliographic Code: 1983Natur.303..372F Abstract @-------------------------------------------------------------------- Title: Sunspot bright rings and the thermal diffusivity of solar convection Authors: Fowler, L. A.; Foukal, P.; Duvall, T., Jr. Bibliographic Code: 1983SoPh...84...33F Abstract Raster-scan observations of 10 sunspots, made in 1980 and 1981 with the 512-channel diode array and vacuum telescope at the Kitt Peak National Observatory, are reported. Data from several 10-min scans of 0.25-A passbands of clean continuum were summed to give an rms noise level of 0.25 percent, corrected by applying a limb-darkening curve, and analyzed to determine the average intensity for each of eight segments of a series of concentric rings around each sunspot. Faculae and pores were identified and discarded in constructing radial intensity profiles. Marginally significant bright symmetric rings (peak amplitude 0.1-0.3 percent) not attributable to residual facular signal or instrumental effects were observed around 6 of 10 sunspots. No evidence of more intense bright rings was found. These findings are discussed in terms of thermal-diffusion models proposed to explain the fate of the radiative flux blocked by sunspots. @-------------------------------------------------------------------- Title: A thermal model of sunspot influence on solar luminosity Authors: Foukal, P.; Fowler, L. A.; Livshits, M. Bibliographic Code: 1983ApJ...267..863F Abstract It is shown that the observed properties of the solar irradiance dips can be easily explained by the conventional thermal blocking model of sunspots extended to include time dependence. The model does not rule out energy transfer between spots and faculae, but it does not require it. The analysis indicates that the heat blocked in proportion to a spot's area and contrast is stored very efficiently in the slightly increased thermal and potential energy of the solar convection zone. The radiative flux blocked during high sunspot activity periods is only radiated away over many subsequent 11-year cycles. It is pointed out that this efficient storage implies a contribution to variation of the solar luminosity and irradiance over the 1-year cycle, at an amplitude that can be computed from the known variation of sunspot areas. @-------------------------------------------------------------------- Title: Interpretation of Fraunhofer-Line Red Shifts at Supergranule Boundaries Authors: Foukal, P.; Miller, P.; Keil, S. Bibliographic Code: 1983BAAS...15..719F Abstract @-------------------------------------------------------------------- Title: Electric fields in coronal magnetic loops Authors: Foukal, P.; Miller, P.; Gilliam, L. Bibliographic Code: 1983SoPh...83...83F Abstract The present search for evidence of Stark effect on Balmer lines formed in coronal structures, through the analysis of spectra taken with a 40-cm coronograph, has uncovered significant broadening from H(10) to the limit of Balmer line visibility at about H(20) in spectra taken near the apex of a bright, post-flare loop prominence. This increasing width is interpreted as Stark broadening. The trend observed in one post-flare loop requires an ion density of 2 x 10 to the 12th/cu cm, if it is to be explained entirely as Stark effect caused by pressure broadening. A comparison of evidence from electron scattering with densities derived from emission features and line intensity ratios is adduced as evidence against a volume filling factor small enough to reconcile the ion density and mean electron density derived in the present study, suggesting that the Stark effect observed in the loops could be caused by macroscopic electric fields rather than pressure broadening. @-------------------------------------------------------------------- Title: A Summary of the Joint Discussion at Patras on Solar Luminosity Variations Authors: Eddy, J. A.; Foukal, P. V. Bibliographic Code: 1983HiA.....6...79E Abstract @-------------------------------------------------------------------- Title: A Photometric Study of Low-Amplitude Temperature Inhomogeneities at the Photosphere Authors: Fowler, L. A.; Foukal, P.; Duvall, T., Jr. Bibliographic Code: 1982BAAS...14..938F Abstract @-------------------------------------------------------------------- Title: Electric Fields in Coronal Magnetic Loops Authors: Foukal, P.; Miller, P.; Gilliam, L. Bibliographic Code: 1982BAAS...14..923F Abstract @-------------------------------------------------------------------- Title: A Study of Variations in Photospheric Limb-Darkening and Solar Luminosity Authors: Rosen, W.; Petro, L.; Foukal, P.; Pierce, K. Bibliographic Code: 1982BAAS...14..922R Abstract @-------------------------------------------------------------------- Title: A Helium-Cooled Absolute Cavity Radiometer for Solar Irradiance Measurement Authors: Foukal, P.; Miller, P. Bibliographic Code: 1982BAAS...14..922F Abstract @-------------------------------------------------------------------- Title: Observations of Sunspot Bright Rings and Comparisons with a Convective Blocking Model Authors: Fowler, L. A.; Foukal, P. Bibliographic Code: 1982BAAS...14..624F Abstract @-------------------------------------------------------------------- Title: Variations in solar luminosity. Authors: Foukal, P. V. Bibliographic Code: 1982SAOSR.392B..17F Abstract @-------------------------------------------------------------------- Title: Variations in photospheric limb darkening as a diagnostic of changes in solar luminosity Authors: Rosen, W. A.; Foukal, P. V.; Kurucz, R. L.; Pierce, A. K. Bibliographic Code: 1982ApJ...253L..89R Abstract The paper reports on photospheric limb-darkening measurements obtained with the McMath Solar Telescope in July, September, and October 1980 as part of a continuing program to investigate possible long-term variations in the photospheric emergent flux. A total of 243 usable full-diameter scans were recorded over seven days in the clean continuum window at 4451-25 A. The limb darkening was found to decrease significantly between September 25 and 26. It is suggested that this decrease was caused by a decrease of the temperature gradient in the upper photosphere in the region above approximately tau(5000) = 0.5. The small increase in effective temperature that might accompany this limb-darkening variation is estimated using a standard radiative equilibrium photospheric model. @-------------------------------------------------------------------- Title: The thermal structure of solar coronal loops and implications for physical models of coronae Authors: Raymond, J. C.; Foukal, P. Bibliographic Code: 1982ApJ...253..323R Abstract EUV spectra of three active region loops observed above the solar limb with the SO55 spectrometer on Skylab are analyzed. It is noted that the lengths, peak temperatures, and pressures of the loops are typical of the X-ray coronal loops to which static models have been applied. It is found that the physical parameters of the coronal loop plasma derived from EUV spectra and raster pictures are not well represented by the static models. Although the loops also contain a significant quantity of cool plasma, no physical reason is found to differentiate them from other active region loops of similar length, pressure, and temperature. Several line ratios in the loop spectrum suggest departures from ionization equilibrium caused by rapid cooling. The source of this cooling material is discussed with reference to several models of loop dynamics. @-------------------------------------------------------------------- Title: Magnetic loops in the sun's atmosphere Authors: Foukal, P. Bibliographic Code: 1981S&T....62..547F Abstract It is noted that the magnetic loops are again the subject of wide attention because they are the basic features created by the sun's magnetic field in the tenuous material of its outer atmosphere. Today the structure of the larger loops can be resolved over a wide range of plasma temperature and density. This offers the possibility of testing the processes which have been proposed to carry wave or electric-current energy from the photosphere to the much hotter chromosphere and corona above. It is noted that evidence exists for large electric fields in solar loops during and after flares. Their presence in general would have a pronounced influence on problems ranging from the motions of ionized material across magnetic fields to effects on the ionization potentials of atomic species in the plasma. @-------------------------------------------------------------------- Title: Detection of a temperature deficit in magnetic faculae at the solar photosphere Authors: Foukal, P.; Duvall, T., Jr.; Gillipsie, B. Bibliographic Code: 1981ApJ...249..394F Abstract A photometric technique is presented which allows the observation of faculae across the entire solar disk. A clean continuum was used and granular noise was reduced by subtracting simultaneous spectroheliograms taken at widely separated points of the photospheric radiation curve. Spectroheliographic observations taken from March 12-14, 1980, using two color-scans in the green at gamma 4980.96 and gamma 5256.31 in the fourth order resulted in observation of clean continua in the red at gamma 6641.85 and gamma 7008.8 in another slot of the magnetograph. Approximately 20 raster scans were obtained and indications were found that dark magnetic facular structures in the Delta signal were cooler than the photosphere at equal optical continuum depths @-------------------------------------------------------------------- Title: Erratum - the CI Opacity and Physical Structure of Cool Very Dense Plasma in the Solar Corona Authors: Foukal, P. Bibliographic Code: 1981ApJ...247..382F Abstract @-------------------------------------------------------------------- Title: The C I opacity and physical structure of cool, very dense plasma in the solar corona [ Erratum: 1982ApJ...261..40.. ] Authors: Foukal, P. Bibliographic Code: 1981ApJ...245..304F Abstract The physical structure of the cool material in the volume of the solar corona is investigated. The observational evidence is summarized in a table giving the brightness contrast in optical and EUV radiations. The state of ionization and the opacity sources are discussed, and a physical model is described that is consistent with the EUV and optical data. It is noted that a comparison of the EUV raster pictures with H-alpha and Ca K photoheliograms indicates that the material is commonly injected from below into relatively low-lying magnetic loops (fibrils) seen near sunspots and plages. @-------------------------------------------------------------------- Title: A Photometric Study of Heat Flow Inhomogeneities at the Solar Photosphere Authors: Foukal, P.; Fowler, L. A.; Duvall, T., Jr. Bibliographic Code: 1981BAAS...13..879F Abstract @-------------------------------------------------------------------- Title: Photospheric Limb Darkening as a Ground-based Diagnostic for Variations in the Solar Effective Temperature Authors: Rosen, W. A.; Foukal, P. V.; Kurucz, R.; Pierce, A. K. Bibliographic Code: 1981BAAS...13..551R Abstract @-------------------------------------------------------------------- Title: Detection of a Temperature Deficit in Magnetic Faculae at the Solar Photosphere Authors: Foukal, P.; Duvall, T., Jr.; Gillespie, B. Bibliographic Code: 1981BAAS...13..551F Abstract @-------------------------------------------------------------------- Title: Propagation of magnetically guided acoustic shocks in the solar chromosphere Authors: Foukal, P.; Smart, M. Bibliographic Code: 1981SoPh...69...15F Abstract The propagation of a train of acoustic shocks guided by diverging magnetic fields through a static model of the solar chromospheric network and transition region is investigated. For initial flux densities of 1,000,000 ergs/sq cm/sec in the lower chromosphere, the local efficiency of acoustic transmission into the corona can be much higher than that calculated for a plane parallel atmosphere. Acoustic energy will tend to be deposited at higher chromospheric levels in diverging magnetic fields, and magnetic guiding may influence the temperature profile of the network and plages. The total flux that can be transmitted into the corona along such diverging fields is severely limited since the magnetic elements occupy a small fractional area of the photosphere and the transmission efficiency is a rapidly decreasing function of initial flux density. Diverging magnetic fields and a varying ratio of specific heats are not likely to allow high frequency shocks to dissipate high enough in a static atmosphere to contribute significantly to the coronal energy balance. @-------------------------------------------------------------------- Title: Solar Luminosity Variation on Directly Observable Time Scales: Observational Evidence and Basic Mechanisms Authors: Foukal, P. V. Bibliographic Code: 1981sucl.conf..275F Abstract @-------------------------------------------------------------------- Title: Sunspots and changes in the global output of the sun Authors: Foukal, P. Bibliographic Code: 1981phss.conf..391F Abstract Solar radiometry data from the SMM and Nimbus-7 spacecraft are examined for evidence that decreases in the solar radiance constant can be accounted for by the presence of sunspot groups. Additionally, a storage mechanism is described within the convective zone, where the energy is held for later release. Short-time scale studies have revealed a 0.4% peak-to-peak solar variation, while an rms variation in the radiance has been set at about 0.05%. The results of numerical modeling of the storage time expected (for time-dependent heat diffusion through the upper convection zone of the sun) to validate the convection zone as the storage medium show that adequate storage times do exist in that region. The implications of the calculations for a possible 11-yr modulation of the solar constant are discussed, as are further studies to verify the model. @-------------------------------------------------------------------- Title: Comments on the thermodynamic structure and dynamics of the cool solar corona over sunspots Authors: Foukal, P. Bibliographic Code: 1981phss.conf..191F Abstract The results of 9 mos of spectrophotometric observations of sunspots with the Harvard S055 spectrophotometer on the ATM on board Skylab are summarized. The corona over the sunspot was found to contain plasma 1-2 orders of magnitude cooler than surrounding active region media. The extended sunspot atmosphere displays open or closed loops reaching from 30,000-40,000 above the photosphere. The plumes and loops have a temperature gradient transverse to the magnetic field, with the coolest portion concentrated around the loop axis. No certain value for the local density of the plasma was derived, particularly for a two-order-of-magnitude increase with the coolest material, which is necessary to define a constant gas pressure transverse to the magnetic field. The plasma in the cooler atmosphere, lacking hydrostatic support, was observed to stream downward at 10-100 km/sec. @-------------------------------------------------------------------- Title: On the secular behavior of seasonal changes in ozone column density Authors: Vernazza, J.; Foukal, P. Bibliographic Code: 1980GeoRL...7..993V Abstract @-------------------------------------------------------------------- Title: Average photospheric poloidal and toroidal magnetic field components near solar minimum Authors: Foukal, P.; Duvall, T. L., Jr. Bibliographic Code: 1980SoPh...67....9F Abstract We discuss the dynamical interpretation of evidence for an azimuthal tilt of the global magnetic field from the radial direction at the photosphere. We point out that the Reynolds stresses of super-granular convective motions might produce the required small tilt of intense flux tubes, without implying an unacceptably large momentum flux across the photospheric surface into the solar wind. Our calculations lead us to conclude that there is little reason, at present, to infer (Duvall et al., 1979) a separate low intensity constituent of the global magnetic field, from the observational evidence for an azimuthal tilt. More precise measurements of the vertical component of supergranular motions would be useful in determining the actual torque exerted by the Reynolds stresses on the magnetic field. @-------------------------------------------------------------------- Title: A study of acoustic heating and forced convection in the solar corona Authors: Foukal, P. V. Bibliographic Code: 1980aeri.rept.....F Abstract The S055 EUV spectra was used to perform emission measure and line intensity ratio analyses of loop plasma conditions in a study on the thermodynamics of magnetic loops in the solar corona. The evidence that loops contain plasma hotter than the background corona, and thus, require enhanced local dissipation of magnetic or mechanical energy is discussed. The S055 EUV raster pictures were used to study physical conditions in cool ultraviolet absorbing clouds in the solar corona, and optical data were used to derive constraints on the dimension, time scales and optical depths in dark opaque clouds not seen in H alpha and CaK as filaments or prominences. Theoretical modelling of propagation of magnetically guided acoustic shocks in the solar chromosphere finds it still unlikely that high frequency acoustic shocks could reach the solar corona. Dynamic modelling of spicules shows that such guided slow mode shocks can explain the acceleration of cool spicular material seen high in the corona. @-------------------------------------------------------------------- Title: Does the Sun's Luminosity Vary? Authors: Foukal, Peter Bibliographic Code: 1980S&T....59..111F Abstract @-------------------------------------------------------------------- Title: Solar luminosity variation on short time scales - Observational evidence and basic mechanisms Authors: Foukal, P. Bibliographic Code: 1980asfr.symp...29F Abstract Temporal variations in total solar luminosity on time scales between 100 and 10 to the 9th sec are discussed. Following a brief historical review of solar constant measurements, observations of solar luminosity and its variations made by ground-level radiometry, radiometry from balloons, aircraft and rockets, continuous radiometry from space probes and satellites, measurements of reflected light from solar system bodies, and measurements of solar line depths and limb darkening are presented which demonstrate solar constant variations of less than 1.2% since 1962, no variation over a period of 30 years in the range 0.34 and 2.4 microns, and an influence of magnetic activity. Specific processes which may account for these variations are then examined, including heat flux perturbations due to local variations in thermal impedance, variations in convective heat transport efficiency, energy storage in magnetic fields, and variations in wave heating at the photosphere. Comparison of solar evidence with evidence of luminosity variations in other late-type stars indicates that magnetic activity can influence luminosity on time scales from days to tens of years. Future prospects for experimental observations from spacecraft and from the ground are indicated. @-------------------------------------------------------------------- Title: A Doppler measurement with low scattered light of the higher rotation rate of sunspot magnetic fields at the photosphere Authors: Foukal, P. Bibliographic Code: 1979ApJ...234..716F Abstract Abstract image available at: http://esoads.eso.org/abs/1979ApJ...234..716F @-------------------------------------------------------------------- Title: The effect of magnetic fields on solar luminosity Authors: Foukal, P.; Vernazza, J. Bibliographic Code: 1979ApJ...234..707F Abstract The paper presents an investigation into the influence of magnetic fields in sunspots and faculae on solar luminosity, using measurements of the solar constant from ground level and from space. Attention is given to an analysis that shows that it is difficult to devise an atmospheric mechanism that would rapidly lower visible and infrared transmission in response to sunspots, increase it in response to faculae, and anticipate the magnetic development of these features by about one day. It is shown that the phase shift of the luminosity variation provides a promising new technique to determine the depth at which the magnetic fields of sunspots and faculae redistribute the flow of convective energy. @-------------------------------------------------------------------- Title: Angular velocity gradients in the solar convection zone Authors: Gilman, P. A.; Foukal, P. V. Bibliographic Code: 1979ApJ...229.1179G Abstract Numerical calculations of Boussinesq nonaxisymmetric convection in a rotating spherical shell are reported which were performed to study how convection in the supergranule layer redistributes angular momentum. It is found that supergranules are at best weakly influenced by rotation and can be largely responsible for the radial gradient of angular velocity observed in the thin supergranule layer below the photosphere. The results indicate that convection in a thin spherical shell weakly influenced by rotation can produce a substantial outward decrease in rotational velocity that approaches the limit predicted for radially moving particles that conserve their angular momentum. This phenomenon is shown to provide a plausible explanation for the observed difference in angular velocity between sunspots and the photospheric plasma. @-------------------------------------------------------------------- Title: Solar Luminosity Variation: Observational Evidence and Basic Mechanisms Authors: Foukal, P. V. Bibliographic Code: 1979BAAS...11Q.421F Abstract @-------------------------------------------------------------------- Title: Solar luminosity variation: observational evidence and basic mechanisms. Authors: Foukal, P. V. Bibliographic Code: 1979BAAS...11..423F Abstract @-------------------------------------------------------------------- Title: Solar luminosity variation on short time scales: Observational evidence and basic mechanisms Authors: Foukal, P. V. Bibliographic Code: 1979LPICo.390...37F Abstract @-------------------------------------------------------------------- Title: The Effect of Sunspots and Faculae upon the Solar Constant and Atmospheric Transmission Authors: Foukal, P.; Vernazza, J. Bibliographic Code: 1978BAAS...10R.620F Abstract @-------------------------------------------------------------------- Title: Magnetic loops, downflows, and convection in the solar corona Authors: Foukal, P. Bibliographic Code: 1978ApJ...223.1046F Abstract Optical and extreme-ultraviolet observations of solar loop structures show that flows of cool plasma from condensations near the loop apex are a common property of loops associated with radiations whose maximum temperature is greater than approximately 7000 K and less than approximately 3,000,000 K. It is suggested that the mass balance of these structures indicates reconnection by means of plasma motion across field lines under rather general circumstances (not only after flares). It is shown that the cool material has lower gas pressure than the surrounding coronal medium. The density structure of the bright extreme ultraviolet loops suggests that downflows of cool gas result from isobaric condensation of plasma that is either out of thermal equilibrium with the local energy deposition rate into the corona, or is thermally unstable. The evidence is thought to indicate that magnetic fields act to induce a pattern of forced convection. @-------------------------------------------------------------------- Title: Supergranulation and the dynamics of gas and magnetic field below the solar photosphere Authors: Foukal, P. Bibliographic Code: 1977ApJ...218..539F Abstract Results from recent calculations of convection are used to discuss the interpretation of three observations which seem to bear upon the dynamics of gas and magnetic fields below the photosphere but which are not dependent on the difficult direct study of the wave number, phase, and symmetry of large-scale low-amplitude motions on the sun. These observations include the scale of supergranulation, the rotation rate of sunspots, and the tilt of magnetic flux tubes at the photosphere. It is suggested that the discrete scale of supergranular flow, the high rotation rate of sunspots, and the azimuthal tilt of magnetic fields from a radial orientation are most simply explained if one supposes that a transition occurs in the structure of the magnetic field at a depth of about 15,000 km below the photosphere. The gas and magnetic field would be strongly coupled dynamically and comoving below this depth; above the transition, the field would be confined to a small fraction of the total volume, and the convecting gas would be largely nonmagnetic. In this case, the supergranulation could be the convection mode that reflect simply the full depth of the transition layer. Implications of this layer are considered for the dynamics and energy balance of magnetic features at the photosphere. @-------------------------------------------------------------------- Title: The effect of sunspots and faculae on the solar constant Authors: Foukal, P. V.; Mack, P. E.; Vernazza, J. E. Bibliographic Code: 1977ApJ...215..952F Abstract We study the available measurements of the solar constant made at ground sites and from recent space observations to determine whether sunspots or faculae produce a detectable modulation of either the solar flux or the earth's atmospheric transmission. The data from radiometers on Mariners 6 and 7 rule out any relative change of the solar constant in space due directly to faculae or spots exceeding 0.03%. This limit is two orders of magnitude smaller than previous values obtained from ground measurements. The measurements made at mountain stations of the Smithsonian Astrophysical Observatory between 1923 and 1952 show a marginally significant increase of solar constant at the level of 0.1%, related specifically to high projected facular area. Since this increase is not seen in the space measurements, we suggest that it may reflect a change in the earth's atmospheric transmission, possibly due to a change in ozone concentration induced by variation of facular ultraviolet radiation. @-------------------------------------------------------------------- Title: Magnetic fields and solar convection and rotation. Authors: Foukal, P. V. Bibliographic Code: 1977BAAS....9..375F Abstract @-------------------------------------------------------------------- Title: INVITED PAPER - Magnetic Fields and Solar Convection and Rotation. Authors: Foukal, P. Bibliographic Code: 1977BAAS....9..374F Abstract @-------------------------------------------------------------------- Title: Dynamics of Gas and Magnetic Field in the Supergranular Layer Authors: Foukal, Peter V. Bibliographic Code: 1977lsms.proc..113F Abstract @-------------------------------------------------------------------- Title: The pressure and energy balance of the cool corona over sunspots Authors: Foukal, P. V. Bibliographic Code: 1976ApJ...210..575F Abstract The 22 largest sunspots observed with the Skylab SO55 spectrometer are studied for a relation between their EUV radiation and their umbral size or magnetic classification. The ultimate goal is to determine why the coronal plasma is so cool over a sunspot and how this cool plasma manages to support itself against gravity. Based on the time behavior of the EUV emission, a steady-state model is developed for the pressure and energy balance of the cool coronal-plasma loops over the spots. Analysis of the temperature structure in a typical loop indicates that the loop is exceedingly well insulated from the outside corona, that its energy balance is determined purely by internal heating and cooling processes, and that a heat input of about 0.0001 erg/cu cm per sec is required along the full length of the loop. It is proposed that: (1) coronal material flows steadily across the field lines at the tops of the loops and falls downward along both sides under gravity; (2) the corona is heated by mechanical-energy transport across the very thin transition region immediately over network-cell interiors; and (3) strong magnetic fields tend to inhibit mechanical-energy dissipation in the corona. @-------------------------------------------------------------------- Title: Plasma diagnostic techniques in the ultraviolet - The C III density-sensitive lines in the sun Authors: Dupree, A. K.; Foukal, P. V.; Jordan, C. Bibliographic Code: 1976ApJ...209..621D Abstract Spectra and spectroheliograms of the C III transitions at 977 and 1176 A are obtained with the Harvard extreme-ultraviolet spectrometer on Skylab. Analysis of the intensities of these lines, and of their density-sensitive ratio, indicates a wide range of temperature gradients and electron densities in the transition region of various solar features. From values of the observed ratio, we suggest necessary revisions to the excitation rates, and propose a relationship between the ratio and density. The significantly higher ratio found in active regions indicates a density increase of about a factor 2 relative to the network. In the quiet sun, there is no significant difference in density between network and cell interiors, but the uncertainty is as large as a factor 3. The very central 10% of the areas of cell interiors shows a significantly higher density than the mean value for cell interiors. @-------------------------------------------------------------------- Title: The Relative Rotation of Gas and Magnetic Field at the Photosphere Authors: Foukal, P. V. Bibliographic Code: 1976BAAS....8..344F Abstract @-------------------------------------------------------------------- Title: Spectroscopic evidence for a higher rotation rate of magnetized plasma at the solar photosphere Authors: Foukal, P. Bibliographic Code: 1976ApJ...203L.145F Abstract @-------------------------------------------------------------------- Title: Ultraviolet observations of C III transitions in the sun. Authors: Dupree, A. K.; Foukal, P.; Jordan, C. Bibliographic Code: 1976BAAS....8..292D Abstract @-------------------------------------------------------------------- Title: Extreme-ultraviolet transients observed at the solar pole Authors: Withbroe, G. L.; Jaffe, D. T.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E. Bibliographic Code: 1976ApJ...203..528W Abstract Extreme-ultraviolet (EUV) observations of two polar transient features ('macrospicules') are described. These features appear to be caused by jets of chromospheric material that shoot upward to a height of 35,000 km above the limb and then fall back into the chromosphere, reaching terminal velocities of about 140 km/s. On the basis of a model developed from the EUV measurements, it is found that the energy required to produce each event is about 3 by 10 to the 26th power ergs, about two orders of magnitude more than that required to produce an ordinary spicule. This indicates that macrospicules may be an important factor in the energy balance of the chromosphere and corona. @-------------------------------------------------------------------- Title: Initial results from the EUV spectroheliometer on ATM Authors: Reeves, E. M.; Timothy, J. G.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Vernazza, J. E.; Withbroe, G. L.; Schmahl, E. J. Bibliographic Code: 1976skls.conf...73R Abstract The Harvard College Observatory photoelectric spectroheliometer on the Apollo Telescope Mount operated correctly in orbit from May 29, 1973 to February 7, 1974. During this period, many thousands of spatial and spectral scans at extreme ultraviolet wavelengths were recorded during observations of a variety of solar features. The construction and modes of operation of the instrument are outlined, and the principal scientific results from a preliminary analysis of the data are described. @-------------------------------------------------------------------- Title: The temperature structure and pressure balance of magnetic loops in active regions Authors: Foukal, P. Bibliographic Code: 1975SoPh...43..327F Abstract EUV observations show many active region loops in lines formed at temperatures between 10,000 and 2,000,000 K. The brightest loops are associated with flux tubes leading to the umbrae of sunspots. It is shown that the high visibility of certain loops in transition region lines is due principally to a sharp radial decrease of temperature to chromospheric values toward the loop axis. The plasma density of these cool loops is not significantly greater than in the hot gas immediately surrounding it. Consequently, the internal gas pressure of the cool material is clearly lower. The hot material immediately surrounding the cool loops is generally denser than the external corona by a factor 3-4. When the active region is examined in coronal lines, this hot high pressure plasma shows up as loops that are generally parallel to the cool loops but significantly displaced laterally. @-------------------------------------------------------------------- Title: Time variations in extreme-ultraviolet emission lines and the problem of coronal heating Authors: Vernazza, J. E.; Foukal, P. V.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Withbroe, G. L.; Huber, M. C. E. Bibliographic Code: 1975ApJ...199L.123V Abstract We have analyzed the time structure of the intensity of solar chromospheric and coronal extreme-ultraviolet lines, obtained by the Harvard College Observatory spectrometer aboard Skylab. We find changes in the intensity of up to 50 percent in times as short as 1 minute, but not periodic oscillations. Some evidence is found for the presence of shock waves in the chromosphere and the transition region. It is suggested that the solar chromosphere and corona are heated by nonperiodic waves. @-------------------------------------------------------------------- Title: On the rotation of gas and magnetic fields at the solar photosphere Authors: Foukal, P.; Jokipii, J. R. Bibliographic Code: 1975ApJ...199L..71F Abstract We point out that observations of a 5 percent velocity difference between photospheric gas and magnetic structures at a given latitude may simply result from angular momentum conservation by fluid elements in the convection zone. Estimates of the viscosity and magnetic drag are considered, and we conclude that they probably are not large enough to enforce strictly rigid rotation. @-------------------------------------------------------------------- Title: The Pressure Balance and Currents in Active Region Loop Structures Authors: Foukal, P. V. Bibliographic Code: 1975BAAS....7..346F Abstract @-------------------------------------------------------------------- Title: The Contribution of Active Regions to Solar Variation in the Visible and Near Infrared Authors: Foukal, P. Bibliographic Code: 1975scea.conf..109F Abstract @-------------------------------------------------------------------- Title: EUV observations of the active sun from the Harvard experiment on ATM Authors: Noyes, R. W.; Foukal, P. V.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.; Huber, M. C. E. Bibliographic Code: 1975IAUS...68....3N Abstract Some extreme UV observations of solar active regions made with a scanning spectroheliometer are described. Spectroheliograms constructed from digital data using a computer-driven cathode-ray tube display show clearly how the appearance of an active region changes as a function of temperature. Flare studies indicate that the impulsive rise in EUV emission occurs essentially simultaneously at all levels from the transition zone to the corona. Observations of sunspots reveal a very intense emission in transition zone lines. A matrix of Mg x rasters covering the entire sun reveals several hundred bright points having dimensions of 30 arc seconds or less. Other observations include coronal holes and prominences. @-------------------------------------------------------------------- Title: Solar prominences in the extreme ultraviolet as observed from the Apollo Telescope Mount Authors: Schmahl, E. J.; Foukal, P. V.; Noyes, R. W.; Reeves, E. M.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L.; Huber, M. C. E. Bibliographic Code: 1974SoPh...39..337S Abstract Observations of quiescent solar prominences with the Harvard College Observatory spectrometer aboard Skylab show that prominence material is optically thick in the Lyman alpha line and the Lyman continuum. The color temperature of the Lyman continuum has a mean of 6600 K and an upward gradient toward the top of the prominence. The departure coefficient of the ground state of hydrogen is found to be of the order of unity, as expected from theory. The optical depth of the C III sheath region is determined directly from the observation of the limb through the prominence and is used to infer the mean electron density and the temperature gradient of the sheath. The result implies that the sheath density is about 0.4, and the temperature gradient about 1.4 times the respective value in the C III transition zone of the quiet sun. The C III triplet-singlet ratio for the prominence is found to give a density compatible, within the uncertainty of the atomic parameters, with the density obtained from the optical depth. @-------------------------------------------------------------------- Title: Extreme-ultraviolet observations of coronal holes - Initial results from SKYLAB Authors: Huber, M. C. E.; Foukal, P. V.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1974ApJ...194L.115H Abstract We compare the appearance and physical parameters of the solar chromosphere, transition zone, and corona in areas of coronal holes with that of quiet areas outside the hole. Measurements of the height of emission of various ions in a coronal hole appearing at the polar limb give a quantitative indication of the increased thickness of the transition zone underlying coronal holes. @-------------------------------------------------------------------- Title: Extreme-ultraviolet observations of sunspots with the Harvard spectrometer on the Apollo Telescope Mount Authors: Foukal, P. V.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Wilhbroe, G. L.; Huber, M. C. E. Bibliographic Code: 1974ApJ...193L.143F Abstract Abstract image available at: http://esoads.eso.org/abs/1974ApJ...193L.143F @-------------------------------------------------------------------- Title: Initial results from the EUV spectroheliometer on ATM Authors: Reeves, E. M.; Timothy, J. G.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Schmahl, E. J.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1974aiaa.conf.....R Abstract The Harvard College Observatory photoelectric spectroheliometer on the Apollo Telescope Mount operated correctly in orbit from May 29, 1973 to Feb. 7, 1974. During this period many thousands of spatial and spectral scans at extreme ultraviolet wavelengths were recorded during observations of a variety of solar features. The construction and modes of operation of the instrument are outlined and the principal scientific results from a preliminary analysis of the data are described. @-------------------------------------------------------------------- Title: A three-component concept of the chromosphere and transition region Authors: Foukal, P. Bibliographic Code: 1974SoPh...37..317F Abstract It is proposed that present observations of the chromosphere and transition region in EUV, optical and mm wavelengths are best described by a three-component concept. The three components are taken to be the interiors of supergranular cells, the hot plagettes overlying faculae, and the cooler, transient mottles which surround them in the network boundaries. The enhanced emission of the hot plagettes in transition ions is interpreted as a direct result of the increased pressure scale height over faculae relative to the cell interiors. @-------------------------------------------------------------------- Title: Extreme Ultraviolet Solar Spectra from Skylab-Apollo Telescope Mount. Authors: Dupree, A. K.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1974BAAS....6..349D Abstract @-------------------------------------------------------------------- Title: A Measurement of Electron Temperature in the Orion Nebula from the Intensity Ratio of Forbidden Transitions in S III Authors: Foukal, Peter Bibliographic Code: 1974PASP...86..211F Abstract Abstract image available at: http://esoads.eso.org/abs/1974PASP...86..211F @-------------------------------------------------------------------- Title: Extreme Ultraviolet Solar Observations from the Harvard ATM Experiment Authors: Withbroe, G. L.; Foukal, P. K.; Huber, M. C. E.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E. Bibliographic Code: 1974BAAS....6V.297W Abstract @-------------------------------------------------------------------- Title: ATM Observations of the Time Dependent Intensity Fluctuations in the Extreme Ultraviolet Authors: Vernazza, J. E.; Foukal, P. K.; Huber, M. C. E.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Withbroe, G. L. Bibliographic Code: 1974BAAS....6R.296V Abstract @-------------------------------------------------------------------- Title: Observations of the Chromospheric Network: Initial Results from the Apollo Telescope Mount Authors: Reeves, E. M.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1974ApJ...188L..27R Abstract Abstract image available at: http://esoads.eso.org/abs/1974ApJ...188L..27R @-------------------------------------------------------------------- Title: Solar EUV Photoelectric Observations from SKYLAB Authors: Reeves, E. M.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1974IAUS...57..497R Abstract @-------------------------------------------------------------------- Title: Observations of a Coronal Hole Boundary in the Extreme Ultraviolet. Authors: Huber, M. C. E.; Foukal, P. V.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1973BAAS....5..446H Abstract @-------------------------------------------------------------------- Title: ATM Observations of Solar Flares in the Extreme Ultraviolet. Authors: Noyes, R. W.; Foukal, P. V.; Huber, M. C. E.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1973BAAS....5..433N Abstract @-------------------------------------------------------------------- Title: Solar Prominences-in the EUV as Observed from ATM. Authors: Schmahl, E. J.; Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Reeves, E. M.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1973BAAS....5..432S Abstract @-------------------------------------------------------------------- Title: A Study of the Active Region McMath 12417 with the Harvard ATM EUV Spectrometer. Authors: Foukal, P. V.; Huber, M. C. E.; Noyes, R. W.; Reeves, E. M.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1973BAAS....5..432F Abstract @-------------------------------------------------------------------- Title: Preliminary Solar Extreme Ultraviolet Observations from the ATM with the Harvard Instrument. Authors: Reeves, E. M.; Foukal, P. K.; Huber, M. C. E.; Noyes, R. W.; Schmahl, E. J.; Timothy, J. G.; Vernazza, J. E.; Withbroe, G. L. Bibliographic Code: 1973BAAS....5..419R Abstract @-------------------------------------------------------------------- Title: Reply to `The relations between chromospheric features and photospheric magnetic fields' by E. N. Frazier Authors: Foukal, Peter; Zirin, Harold Bibliographic Code: 1972SoPh...26..148F Abstract @-------------------------------------------------------------------- Title: Measurement of the Electron Temperature of Small 3-cm Radio Bursts Authors: Foukal, Peter Bibliographic Code: 1972SoPh...24..411F Abstract @-------------------------------------------------------------------- Title: Magnetic Coupling of the Active Chromosphere to the Solar Interior Authors: Foukal, Peter Bibliographic Code: 1972ApJ...173..439F Abstract Abstract image available at: http://esoads.eso.org/abs/1972ApJ...173..439F @-------------------------------------------------------------------- Title: Observations of Diffuse Nebulae in the lambda9532 Forbidden Line of SIII. Authors: Foukal, P. Bibliographic Code: 1972BAAS....4..233F Abstract @-------------------------------------------------------------------- Title: Image-Tube Photography of Diffuse Nebulae in [s III] lambda9532. Authors: Foukal, Peter Bibliographic Code: 1972ApJ...172..591F Abstract Abstract image available at: http://esoads.eso.org/abs/1972ApJ...172..591F @-------------------------------------------------------------------- Title: Halpha Fine Structure and the Chromospheric Field Authors: Foukal, Peter Bibliographic Code: 1971SoPh...20..298F Abstract Abstract image available at: http://esoads.eso.org/abs/1971SoPh...20..298F @-------------------------------------------------------------------- Title: Morphological Relationships in the Chromospheric Halpha Fine Structure Authors: Foukal, Peter Bibliographic Code: 1971SoPh...19...59F Abstract Abstract image available at: http://esoads.eso.org/abs/1971SoPh...19...59F @-------------------------------------------------------------------- Title: A morphological theory of the Halpha structure of the active chromosphere. Authors: Foukal, P. Bibliographic Code: 1971BAAS....3R.261F Abstract @-------------------------------------------------------------------- Title: The Development and Flaring of an Active Region Exhibiting Unusual Magnetic Structure. II. Active Regions Authors: Foukal, Peter Bibliographic Code: 1970SoPh...13..330F Abstract Abstract image available at: http://esoads.eso.org/abs/1970SoPh...13..330F @-------------------------------------------------------------------- Title: Erratum Authors: Foukal, P. Bibliographic Code: 1970Ap&SS...6..340F Abstract @-------------------------------------------------------------------- Title: The Electron Temperature Distributions and Internal Kinematics of Seven Diffuse Nebulae Authors: Foukal, Peter Bibliographic Code: 1969Ap&SS...5..469F Abstract A Fabry-Pérot spectrophotometer is used to derive values of the intensity ratio Halpha/[N ii] at 98 points in the seven bright diffuse nebulae M8, M20, M16, M17, NGC7000, M42, IC434. The fraction of nitrogen in the singly ionized state is estimated in the different objects, and is found to be sufficiently constant within any one nebula so that the above intensity ratio may be used to derive accurate electron temperature distributions. The position of the peak of the nebular line, its excess non-thermal width, its shape and relative intensity are used to derive kinematical models of these objects. It is found that values of Halpha/[N ii]~=1 are representative of the bright central cores of these nebulae. Temperatures between 7000K and 12000K were derived in the different objects. Although some of this apparent variation is due to the different conditions of excitation in the various nebulae, it is shown that a convincing progression of temperature in M8, M16, M17 is supported by radio recombination line results. The temperature variation within any one object was generally significantly less than 1500K. No evidence was found for velocities of mass motion at more than twice the speed of sound. Relative radial velocities of generally less than 15 km sec^-1 characterized the velocity fields of M8, M20, M16, M42. The velocities in M17 were measured as about 20 km sec^-1. Motions in NGC 7000 and IC 434 were much lower (˜5 km sec^-1) although here the number of points taken was too small to construct meaningful kinematical models. It is concluded that the internal motions of radiatively ionized Hii regions of Pop. I will not significantly affect the results of existing surveys for determining the rotation of the galaxy with radial velocities deduced from nebular emission lines. @-------------------------------------------------------------------- Title: The Temperature and Internal Kinematics of M8 Authors: Foukal, Peter Bibliographic Code: 1969Ap&SS...4..127F Abstract A photoelectric Fabry-Pérot spectrometer is used to record the line profiles of Halpha and [N ii] at 22 points in the nebula. The ratio of intensity Halpha/[N ii] is used to derive an electron temperature distribution with values between 5700° and 9100° showing a peak at the centre of density. These temperatures are compared with the Halpha Doppler temperatures to estimate excess velocities of mass motion. Together with the shifts of the Halpha line centres, these lead to an evaluation of the velocity field in the nebula. It is suggested that the nebula consists of a core expanding at about ±10 km/sec^-1 surrounded by a thick peripheral shell in which large scale mass motions are small. Non-thermal broadening suggesting turbulent velocities at about the speed of sound is observed in this shell and is attributed to small scale dynamic effects in a non-smooth density distribution. The effect of such expanding cores on heliocentric velocities of galactic Hii regions is discussed. @-------------------------------------------------------------------- Title: A Possible observational correlation of lunar luminescence with the K_p index Authors: Foukal, Peter Bibliographic Code: 1968Icar....9..162F Abstract Simultaneous three-color photometry of the lunar craters Menelaus, Copernicus, Kepler, and Gassendi was carried out on seven nights between June 21st and July 1st 1967, to monitor possible short-period color changes on a time scale of less than 10 min. Abnormally strong brightness variations in the red and infrared bands, amounting to deviations at high as 10% from the mean value were recorded on the morning of July 28th at a phase angle of about 75%. Comparison with K_p values for these dates indicated a good, although not perfect, correlation between the amount of time variation in the red color index and the K_p value. It is tentatively concluded that if such observations can be continued it could be possible to prove the irrelevance of magnetospheric effects to the question of lunar luminescence.