explanation blue bibcodes open ADS page with paths to full text
Author name code: hasan
ADS astronomy entries on 2022-09-14
=author:"Hasan, S.S." OR =author:"Hasan, S. Sirajul"
---------------------------------------------------------
Title: National Large Solar Telescope
Authors: Hasan, S. S.; Banerjee, D.; Ravindra, B.; Sankarasubramanian,
K.; Rangarajan, K. E.
2017CSci..113..696H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Division II: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
2015IAUTB..28..106V Altcode:
The Business Meeting of Commission 10 was held as part of the Business
Meeting of Division II (Sun and Heliosphere), chaired by Valentin
Martínez-Pillet, the President of the Division. The President of
Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took
the chair for the business meeting of C10. She summarised the activities
of C10 over the triennium and the election of the incoming OC.
---------------------------------------------------------
Title: Solar Astronomy at High Altitude
Authors: Hasan, S. S.; Bagare, S. P.; Rangarajan, K. E.
2014PINSA..80..815H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Scientific Programmes with India's National Large Solar
Telescope and their contribution to Prominence Research
Authors: Hasan, S. S.
2014IAUS..300..355H Altcode:
The primary objective of the 2-m National Large Solar Telescope (NLST)
is to study the solar atmosphere with high spatial and spectral
resolution. With an innovative optical design, NLST is an on-axis
Gregorian telescope with a low number of optical elements and a high
throughput. In addition, it is equipped with a high order adaptive
optics system to produce close to diffraction limited performance. <P
/>NLST will address a large number of scientific questions with a focus
on high resolution observations. With NLST, high spatial resolution
observations of prominences will be possible in multiple spectral
lines. Studies of magnetic fields, filament eruptions as a whole,
and the dynamics of filaments on fine scales using high resolution
observations will be some of the major areas of focus.
---------------------------------------------------------
Title: Acoustic emission from magnetic flux tubes in the solar network
Authors: Vigeesh, G.; Hasan, S. S.
2013JPhCS.440a2045V Altcode: 2013arXiv1304.5193V
We present the results of three-dimensional numerical simulations to
investigate the excitation of waves in the magnetic network of the Sun
due to footpoint motions of a magnetic flux tube. We consider motions
that typically mimic granular buffeting and vortex flows and implement
them as driving motions at the base of the flux tube. The driving
motions generates various MHD modes within the flux tube and acoustic
waves in the ambient medium. The response of the upper atmosphere to
the underlying photospheric motion and the role of the flux tube in
channeling the waves is investigated. We compute the acoustic energy
flux in the various wave modes across different boundary layers
defined by the plasma and magnetic field parameters and examine the
observational implications for chromospheric and coronal heating.
---------------------------------------------------------
Title: Observations and Modeling of the Emerging Extreme-ultraviolet
Loops in the Quiet Sun as Seen with the Solar Dynamics Observatory
Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.;
DeLuca, E. E.; Hasan, S. S.; Hanslmeier, A.
2013ApJ...768...32C Altcode: 2013arXiv1303.3426C
We used data from the Helioseismic and Magnetic Imager (HMI) and the
Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory
(SDO) to study coronal loops at small scales, emerging in the quiet
Sun. With HMI line-of-sight magnetograms, we derive the integrated
and unsigned photospheric magnetic flux at the loop footpoints in the
photosphere. These loops are bright in the EUV channels of AIA. Using
the six AIA EUV filters, we construct the differential emission measure
(DEM) in the temperature range 5.7-6.5 in log T (K) for several hours
of observations. The observed DEMs have a peak distribution around
log T ≈ 6.3, falling rapidly at higher temperatures. For log T <
6.3, DEMs are comparable to their peak values within an order of
magnitude. The emission-weighted temperature is calculated, and its
time variations are compared with those of magnetic flux. We present
two possibilities for explaining the observed DEMs and temperatures
variations. (1) Assuming that the observed loops are composed of
a hundred thin strands with certain radius and length, we tested
three time-dependent heating models and compared the resulting DEMs
and temperatures with the observed quantities. This modeling used
enthalpy-based thermal evolution of loops (EBTEL), a zero-dimensional
(0D) hydrodynamic code. The comparisons suggest that a medium-frequency
heating model with a population of different heating amplitudes can
roughly reproduce the observations. (2) We also consider a loop model
with steady heating and non-uniform cross-section of the loop along
its length, and find that this model can also reproduce the observed
DEMs, provided the loop expansion factor γ ~ 5-10. More observational
constraints are required to better understand the nature of coronal
heating in the short emerging loops on the quiet Sun.
---------------------------------------------------------
Title: India's National Large Solar Telescope
Authors: Hasan, S. S.
2012ASPC..463..395H Altcode:
India's 2-m National Large Solar Telescope (NLST) is aimed primarily
at carrying out observations of the solar atmosphere with high spatial
and spectral resolution. A comprehensive site characterization program,
that commenced in 2007, has identified two superb sites in the Himalayan
region at altitudes greater than 4000-m that have extremely low water
vapor content and are unaffected by monsoons. With an innovative optical
design, the NLST is an on-axis Gregorian telescope with a low number of
optical elements to reduce the number of reflections and yield a high
throughput with low polarization. In addition, it is equipped with
a high-order adaptive optics to produce close to diffraction limited
performance. To control atmospheric and thermal perturbations of the
observations, the telescope will function with a fully open dome, to
achieve its full potential atop a 25 m tower. Given its design, NLST can
also operate at night, without compromising its solar performance. The
post-focus instruments include broad-band and tunable Fabry-Pérot
narrow-band imaging instruments; a high resolution spectropolarimeter
and an Echelle spectrograph for night time astronomy. This project
is led by the Indian Institute of Astrophysics and has national
and international partners. Its geographical location will fill the
longitudinal gap between Japan and Europe and is expected to be the
largest solar telescope with an aperture larger than 1.5 m till the
ATST and EST come into operation. An international consortium has
been identified to build the NLST. The facility is expected to be
commissioned by 2016.
---------------------------------------------------------
Title: The National Large Solar Telescope (NLST) of India
Authors: Hasan, S. S.
2012IAUSS...6E.211H Altcode:
The Indian National Large Solar Telescope (NLST) will be a
state-of-the-art 2-m class telescope for carrying out high-resolution
studies in the solar atmosphere. Recent numerical simulations suggest
that crucial physical processes like vortex flow, dissipation of
magnetic fields and the generation of MHD waves can occur efficiently
over length scales of tens of kilometers. Current telescopes are unable
to resolve solar feature to this level at visible wavelengths. NLST
will not only achieve good spatial resolution, but will also have
a high photon throughput in order to carry out spectropolarimetric
observations to accurately measure vector magnetic fields in the
solar atmosphere with a good signal to noise ratio. The main science
goals of NLST include: a) Magnetic field generation and the solar
cycle; b) Dynamics of magnetized regions; c) Helioseismology; d)
Long term variability; e) Energetic phenomena and Activity; and f)
Night time astronomy. The optical design of the telescope is optimized
for high optical throughput and uses a minimum number of optical
elements. A high order adaptive optics system is integrated as part
of the design that works with a modest Fried's parameter of 7-cm to
give diffraction limited performance. The telescope will be equipped
with a suite of post-focus instruments including a high resolution
spectrograph and a polarimeter. NLST will also be used for carrying
out stellar observations during the night. The mechanical design of
the telescope, building, and the innovative dome takes advantage of
the natural air flush which will help to keep the open telescope in
temperature equilibrium. Critical to the successful implementation of
NLST is the selection of a site with optimum atmospheric properties,
such as the number of sunshine hours and good "seeing" over long
periods. A site characterization programme carried over several
years has established the existence of suitable sites in the Ladakh
region. After its completion, currently planned for 2016, NLST will
fill a gap in longitude between the major solar facilities in the world
and will be for some years the largest solar telescope in the world.
---------------------------------------------------------
Title: Three-dimensional Simulations of Magnetohydrodynamic Waves
in Magnetized Solar Atmosphere
Authors: Vigeesh, G.; Fedun, V.; Hasan, S. S.; Erdélyi, R.
2012ApJ...755...18V Altcode: 2011arXiv1109.6471V
We present results of three-dimensional numerical simulations of
magnetohydrodynamic (MHD) wave propagation in a solar magnetic flux
tube. Our study aims at understanding the properties of a range of MHD
wave modes generated by different photospheric motions. We consider two
scenarios observed in the lower solar photosphere, namely, granular
buffeting and vortex-like motion, among the simplest mechanism for
the generation of waves within a strong, localized magnetic flux
concentration. We show that granular buffeting is likely to generate
stronger slow and fast magnetoacoustic waves as compared to swirly
motions. Correspondingly, the energy flux transported differs as a
result of the driving motions. We also demonstrate that the waves
generated by granular buffeting are likely to manifest in stronger
emission in the chromospheric network. We argue that different
mechanisms of wave generation are active during the evolution of a
magnetic element in the intergranular lane, resulting in temporally
varying emission at chromospheric heights.
---------------------------------------------------------
Title: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
2012IAUTA..28...69V Altcode:
Commission 10 of the International Astronomical Union has more than
650 members who study a wide range of activity phenomena produced by
our nearest star, the Sun. Solar activity is intrinsically related
to solar magnetic fields and encompasses events from the smallest
energy releases (nano- or even picoflares) to the largest eruptions
in the Solar System, coronal mass ejections (CMEs), which propagate
into the Heliosphere reaching the Earth and beyond. Solar activity is
manifested in the appearance of sunspot groups or active regions, which
are the principal sources of activity phenomena from the emergence of
their magnetic flux through their dispersion and decay. The period
2008-2009 saw an unanticipated extended solar cycle minimum and
unprecedentedly weak polar-cap and heliospheric field. Associated with
that was the 2009 historical maximum in galactic cosmic rays flux since
measurements begun in the middle of the 20th Century. Since then Cycle
24 has re-started solar activity producing some spectacular eruptions
observed with a fleet of spacecraft and ground-based facilities. In
the last triennium major advances in our knowledge and understanding
of solar activity were due to continuing success of space missions as
SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched
by the breathtaking images of the solar atmosphere produced by the
Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the
framework of NASA's Living with a Star program. In August 2012, at the
time of the IAU General Assembly in Beijing when the mandate of this
Commission ends, we will be in the unique position to have for the
first time a full 3-D view of the Sun and solar activity phenomena
provided by the twin STEREO missions about 120 degrees behind and
ahead of Earth and other spacecraft around the Earth and ground-based
observatories. These new observational insights are continuously
posing new questions, inspiring and advancing theoretical analysis
and modelling, improving our understanding of the physics underlying
magnetic activity phenomena. Commission 10 reports on a vigorously
evolving field of research produced by a large community. The number
of refereed publications containing `Sun', `heliosphere', or a synonym
in their abstracts continued the steady growth seen over the preceding
decades, reaching about 2000 in the years 2008-2010, with a total of
close to 4000 unique authors. This report, however, has its limitations
and it is inherently incomplete, as it was prepared jointly by the
members of the Organising Committee of Commission 10 (see the names
of the primary contributors to the sections indicated in parentheses)
reflecting their fields of expertise and interest. Nevertheless, we
believe that it is a representative sample of significant new results
obtained during the last triennium in the field of solar activity.
---------------------------------------------------------
Title: Stokes Diagnostics of Magneto-Acoustic Wave Propagation in
the Magnetic Network on the Sun
Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S.
2011SoPh..273...15V Altcode: 2011SoPh..tmp..349V; 2011arXiv1104.4069V
The solar atmosphere is magnetically structured and highly
dynamic. Owing to the dynamic nature of the regions in which the
magnetic structures exist, waves can be excited in them. Numerical
investigations of wave propagation in small-scale magnetic flux
concentrations in the magnetic network on the Sun have shown that
the nature of the excited modes depends on the value of plasma β
(the ratio of gas to magnetic pressure) where the driving motion
occurs. Considering that these waves should give rise to observable
characteristic signatures, we have attempted a study of synthesised
emergent spectra from numerical simulations of magneto-acoustic
wave propagation. We find that the signatures of wave propagation
in a magnetic element can be detected when the spatial resolution
is sufficiently high to clearly resolve it, enabling observations in
different regions within the flux concentration. The possibility to
probe various lines of sight around the flux concentration bears the
potential to reveal different modes of the magnetohydrodynamic waves
and mode conversion. We highlight the feasibility of using the Stokes-V
asymmetries as a diagnostic tool to study the wave propagation within
magnetic flux concentrations. These quantities can possibly be compared
with existing and new observations in order to place constraints on
different wave excitation mechanisms.
---------------------------------------------------------
Title: Magnetohydrostatic equilibrium in starspots: dependences on
color (T<SUB>eff</SUB>) and surface gravity (g)
Authors: Rajaguru, S. P.; Hasan, S. S.
2011IAUS..273..276R Altcode: 2010arXiv1010.0126R
Temperature contrasts and magnetic field strengths of sunspot
umbrae broadly follow the thermal-magnetic relationship obtained
from magnetohydrostatic equilibrium. Using a compilation of recent
observations, especially in molecular bands, of temperature contrasts
of starspots in cool stars, and a grid of Kurucz stellar model
atmospheres constructed to cover layers of sub-surface convection zone,
we examine how the above relationship scales with effective temperature
(T<SUB>eff</SUB>), surface gravity g and the associated changes in
opacity of stellar photospheric gas. We calculate expected field
strengths in starpots and find that a given relative reduction in
temperatures (or the same darkness contrasts) yield increasing field
strengths against decreasing T<SUB>eff</SUB> due to a combination of
pressure and opacity variations against T<SUB>eff</SUB>.
---------------------------------------------------------
Title: Spectroscopic Observation of Oscillations in the Corona During
the Total Solar Eclipse of 22 July 2009
Authors: Singh, Jagdev; Hasan, S. S.; Gupta, G. R.; Nagaraju, K.;
Banerjee, D.
2011SoPh..270..213S Altcode: 2011SoPh..tmp...36S
We performed high resolution spectroscopy of the solar corona during
the total solar eclipse of 22 July 2009 in two emission lines: the green
line at 5303 Å due to Fe XIV and the red line at 6374 Å due to Fe X,
simultaneously from Anji (latitude 30°28.1' N; longitude 119°35.4'
E; elevation 890 m), China. A two-mirror coelostat with 100 cm focal
length lens produced a 9.2 mm image of the Sun. The spectrograph using
140 cm focal length lens in Littrow mode and a grating with 600 lines
per millimeter blazed at 2 μm provided a dispersion of 30 mÅ and
43 mÅ per pixel in the fourth order around the green line and third
order around the red line, respectively. Two Peltier cooled 1k × 1k
CCD cameras, with a pixel size of 13 μm square and 14-bit readout at
10 MHz operated in frame transfer mode, were used to obtain the time
sequence spectra in two emission lines simultaneously. The duration of
totality was 341 s, but we could get spectra for 270 s after a trial
exposure at an interval of 5 s. We report here on the detection of
intensity, velocity, and line width oscillations with periodicity in
the range of 25 - 50 s. These oscillations can be interpreted in terms
of the presence of fast magnetoacoustic waves or torsional Alfvén
waves. The intensity ratios of green to red emission lines indicate
the temperature of the corona to be 1.65 MK in the equatorial region
and 1.40 MK in the polar region, relatively higher than the expected
temperature during the low activity period. The width variation of
the emission lines in different coronal structures suggests different
physical conditions in different structures.
---------------------------------------------------------
Title: National Large Solar Telescope of India
Authors: Hasan, S. S.
2011ASInC...2...37H Altcode:
The National Large Solar Telescope NLST will be a state-of-the-art
2-m class telescope for carrying out high-resolution studies of
the solar atmosphere. Sites in the Himalayan region at altitudes
greater than 4000-m that have extremely low water vapor content and
are unaffected by monsoons are under evaluation. This project is
led by the Indian Institute of Astrophysics and has national and
international partners. Its geographical location will fill the
longitudinal gap between Japan and Europe and is expected to be the
largest solar telescope with an aperture larger than 1.5 m till ATST and
EST come into operation. <P />NLST is an on-axis alt-azimuth Gregorian
multi-purpose open telescope with the provision of carrying out night
time stellar observations using a spectrograph at the final focus. The
telescope utilizes an innovative design with low number of reflections
to achieve a high throughput and low polarization. High order adaptive
optics is integrated into the design that works with a modest Fried
parameter of 7-cm to give diffraction limited performance. The telescope
will be equipped with a suite of post-focus instruments including a
high-resolution spectrograph and a polarimeter.
---------------------------------------------------------
Title: Solar physics in India: developments from the nineteenth
century to the present era
Authors: Hasan, S. S.; Choudhuri, Arnab Rai; Rajaguru, S. P.
2011ASInC...2..367H Altcode:
Modern solar astronomy took root in India with the discovery of helium
during the total solar eclipse of 1868 and followed by the setting up
of the Kodaikanal Observatory in 1899. We provide a brief overview of
the growth of this field, describe the various Indian solar observing
facilities and summarize the highlights of solar research in India
during the last few decades.
---------------------------------------------------------
Title: NLST: the Indian National Large Solar Telescope
Authors: Hasan, S. S.; Soltau, D.; Kärcher, H.; Süss, M.;
Berkefeld, T.
2010SPIE.7733E..0IH Altcode: 2010SPIE.7733E..16H
India is planning a new solar telescope with an aperture of 2-m for
carrying out high resolution studies of the Sun. Site characterization
is underway at high altitude locations in the Himalayan mountains. A
detailed concept design for NLST (National Large Solar Telescope)
has been completed. The optical design of the telescope is optimized
for high optical throughput and uses a minimum number of optical
elements. A high order AO system is integrated part of the design that
works with a modest Fried's parameter of 7-cm to give diffraction
limited performance. The telescope will be equipped with a suite
of post-focus instruments including a high resolution spectrograph
and a polarimeter. NLST will also be used for carrying out stellar
observations during the night. The mechanical design of the telescope,
building, and the innovative dome is optimized to take advantage of
the natural air flush which will help to keep the open telescope in
temperature equilibrium. After its completion (planned for 2014),
NLST will fill a gap in longitude between the major solar facilities
in USA and Europe, and it will be for years the largest solar telescope
in the world
---------------------------------------------------------
Title: NLST: India's National Large Solar Telescope
Authors: Hasan, S. S.; Soltau, D.; Kärcher, H.; Süß, M.;
Berkefeld, T.
2010AN....331..628H Altcode:
This article introduces the new Indian 2 m telescope which has been
designed by MT Mechatronics in a detailed conceptual design study
for the Indian Institute of Astrophysics, Bangalore. We describe
the background of the project and the science goals which shall be
addressed with this telescope. NLST is a solar telescope with high
optical throughput and will be equipped with an integrated Adaptive
Optics system. It is optimized for a site with the kind of seeing and
wind conditions as they are expected at a lake site in the Himalayan
mountains. The telescope can also be used for certain night time
applications. We also give the scientific rationale for this class
of telescope.
---------------------------------------------------------
Title: The Indian National Large Solar Telescope (NLST)
Authors: Hasan, S. S.
2010IAUS..264..499H Altcode:
The Indian National Large Solar Telescope (NLST) will be a
state-of-the-art 2-m class telescope for carrying out high resolution
studies of the solar atmosphere. Sites in the Himalayan region at
altitudes greater than 4000-m that have extremely low water vapor
content and are unaffected by monsoons are under evaluation. This
project is led by the Indian Institute of Astrophysics and has national
and international partners. <P />NLST is an on-axis alt-azimuth
Gregorian multi-purpose open telescope with the provision of carrying
out night time stellar observations using a spectrograph. The telescope
utilizes an innovative design with low number of reflections to achieve
a high throughput and low instrumental polarization. High order adaptive
optics is integrated into the design that works with a modest Fried's
parameter of 7-cm to give diffraction limited performance. The telescope
will be equipped with a suite of post-focus instruments including a
high resolution spectrograph and a polarimeter. A detailed concept
design of the telescope is presently being finalized and fabrication
is expected to begin in 2010 with first light in 2014.
---------------------------------------------------------
Title: Republication of: Radial Movement in Sunspots
Authors: Hasan, S. S.
2010ASSP...19....2H Altcode: 2010mcia.conf....2H
Displacements of the lines of hydrogen and calcium in the neighbourhood
of sunspots, indicating violent motions in the line of sight, is
a common characteristic of spot disturbances. Such phenomena are
frequently observed during periods of active change in spot development,
or during the genesis of a spot. These line-shifts rarely affect the
spectra of other elements than those of the higher chromospheres. In
very violent outbursts, in addition to the hydrogen and calcium lines,
those of He, Mg, Na, and some of the enhanced displacements may be
either an increase or a decrease of wave-length, and may amount to
several Ångström units, indicating movements of approach or recession
of several hundred kilometers per second. These movements are seldom
maintained for more than a few minutes at a time, and are usually to
be found in the immediate neighbourhood of sports, rarely within the
umbral area.
---------------------------------------------------------
Title: Magnetic Coupling between the Interior and Atmosphere of
the Sun
Authors: Hasan, S. S.; Rutten, R. J.
2010ASSP...19.....H Altcode: 2010mcia.conf.....H
No abstract at ADS
---------------------------------------------------------
Title: Solar Physics at the Kodaikanal Observatory: A Historical
Perspective
Authors: Hasan, S. S.; Mallik, D. C. V.; Bagare, S. P.; Rajaguru, S. P.
2010ASSP...19...12H Altcode: 2009arXiv0906.0144H; 2010mcia.conf...12H
The Kodaikanal Observatory traces its origins to the East India Company,
which started an observatory in Madras "for promoting the knowledge of
astronomy, geography, and navigation in India." Observations began in
1787 at the initiative of William Petrie, an officer of the Company,
with the use of two 3-in achromatic telescopes, two astronomical
clocks with compound pendulums, and a transit instrument. By the early
nineteenth century, the Madras Observatory had already established a
reputation as a leading astronomical center devoted to work on the
fundamental positions of stars, and a principal source of stellar
positions for most of the southern hemisphere stars. John Goldingham
(1796-1805, 1812-1830), T.G. Taylor (1830-1848),W.S. Jacob (1849-1858),
and Norman R. Pogson (1861-1891) were successive Government Astronomers
who led the activities in Madras. Scientific highlights of the work
included a catalogue of 11,000 southern stars produced by theMadras
Observatory in 1844 under Taylor's direction using the new 5-ft
transit instrument.
---------------------------------------------------------
Title: Spectropolarimetry with the NLST
Authors: Sankarasubramanian, K.; Hasan, S. S.; Rangarajan, K. E.
2010ASSP...19..156S Altcode: 2010mcia.conf..156S
India's National Large Solar Telescope (NLST) will provide
opportunities to observe the Sun with high spatial, spectral, and
polarimetric resolution. The large aperture also enables high-cadence
spectropolarimetry with moderate spatial resolution. A multi-slit
spectropolarimeter is planned as one of the back-end instruments for
this powerful telescope, primarily to measure vector magnetic fields
in both active and quiet regions. An integral-field unit added with the
multi-slit spectropolarimeter will enable fast-cadence observation. Here
we discuss the scientific requirements for such an instrument, along
with advantages and limitations of the concept and preliminary design
details.
---------------------------------------------------------
Title: Wave propagation and energy transport in the magnetic network
of the Sun
Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O.
2009A&A...508..951V Altcode: 2009arXiv0909.2325V
Aims. We investigate wave propagation and energy transport in
magnetic elements, which are representatives of small scale magnetic
flux concentrations in the magnetic network on the Sun. This is
a continuation of earlier work by Hasan et al. (2005, ApJ, 631,
1270). The new features in the present investigation include
a quantitative evaluation of the energy transport in the various
modes and for different field strengths, as well as the effect of the
boundary-layer thickness on wave propagation.<BR /> Methods: We carry
out 2D MHD numerical simulations of magnetic flux concentrations for
strong and moderate magnetic fields for which β (the ratio of gas to
magnetic pressure) on the tube axis at the photospheric base is 0.4 and
1.7, respectively. Waves are excited in the tube and ambient medium by
a transverse impulsive motion of the lower boundary.<BR /> Results: The
nature of the modes excited depends on the value of β. Mode conversion
occurs in the moderate field case when the fast mode crosses the β =
1 contour. In the strong field case the fast mode undergoes conversion
from predominantly magnetic to predominantly acoustic when waves are
leaking from the interior of the flux concentration to the ambient
medium. We also estimate the energy fluxes in the acoustic and magnetic
modes and find that in the strong field case, the vertically directed
acoustic wave fluxes reach spatially averaged, temporal maximum values
of a few times 10<SUP>6</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP> at
chromospheric height levels.<BR /> Conclusions: The main conclusions
of our work are twofold: firstly, for transverse, impulsive excitation,
flux tubes/sheets with strong fields are more efficient than those with
weak fields in providing acoustic flux to the chromosphere. However,
there is insufficient energy in the acoustic flux to balance the
chromospheric radiative losses in the network, even for the strong
field case. Secondly, the acoustic emission from the interface between
the flux concentration and the ambient medium decreases with the width
of the boundary layer.
---------------------------------------------------------
Title: Intensity Oscillation in the Corona as Observed during the
Total Solar Eclipse of 29 March 2006
Authors: Singh, Jagdev; Hasan, S. S.; Gupta, G. R.; Banerjee, D.;
Muneer, S.; Raju, K. P.; Bagare, S. P.; Srinivasan, R.
2009SoPh..260..125S Altcode:
We obtained the images of the eastern part of the solar corona in the
Fe XIV 530.3 nm (green) and Fe X 637.4 nm (red) coronal emission lines
during the total solar eclipse of 29 March 2006 at Manavgat, Antalya,
Turkey. The images were obtained using a 35 cm Meade telescope equipped
with a Peltier-cooled 2k × 2k CCD and 0.3 nm pass-band interference
filters at the rates of 2.95 s (exposure times of 100 ms) and 2.0 s
(exposure times of 300 ms) in the Fe XIV and Fe X emission lines,
respectively. The analysis of the data indicates intensity variations at
some locations with period of strongest power around 27 s for the green
line and 20 s for the red line. These results confirm earlier findings
of variations in the continuum intensity with periods in the range
of 5 to 56 s by Singh et al. (Solar Phys.170, 235, 1997). The wavelet
analysis has been used to identify significant intensity oscillations
at all pixels within our field of view. Significant oscillations with
high probability estimates were detected for some locations only. These
locations seem to follow the boundary of an active region and in the
neighborhood, rather than within the loops themselves. These intensity
oscillations may be caused by fast magneto-sonic waves in the solar
corona and partly account for heating of the plasma in the corona.
---------------------------------------------------------
Title: Processes in the magnetized chromosphere of the Sun
Authors: Hasan, S. S.
2009IAUS..257..121H Altcode:
We review physical processes in magnetized chromospheres on the Sun. In
the quiet chromosphere, it is useful to distinguish between the magnetic
network on the boundaries of supergranules, where strong magnetic
fields are organized in mainly vertical flux tubes and internetwork
regions in the cell interiors, which have traditionally been associated
with weak magnetic fields. Recent observations from Hinode, however,
suggest that there is a significant amount of horizontal magnetic flux
in the cell interior with large field strength. Furthermore, processes
that heat the magnetic network have not been fully identified. Is the
network heated by wave dissipation and if so, what is the nature of
these waves? These and other aspects related to the role of spicules
will also be highlighted. A critical assessment will be made on the
challenges facing theory and observations, particularly in light of
the new space experiments and the planned ground facilities.
---------------------------------------------------------
Title: Numerical simulation of wave propagation in magnetic network
Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O.
2009IAUS..257..185V Altcode:
We present 2-D numerical simulations of wave propagation in the magnetic
network. The network is modelled as consisting of individual magnetic
flux sheets located in intergranular lanes. They have a typical
horizontal size of about 150 km at the base of the photosphere and
expand upward and become uniform. We consider flux sheets of different
field strengths. Waves are excited by means of transverse motions at
the lower boundary, to simulate the effect of granular buffeting. We
look at the magneto-acoustic waves generated within the flux sheet
and the acoustic waves generated in the ambient medium due to the
excitation. We calculate the wave energy fluxes separating them into
contributions from the acoustic and the Poynting part and study the
effect of the different field strengths.
---------------------------------------------------------
Title: Chapter 2: Overview of the Sun
Authors: Hasan, S. S.
2008psa..book....9H Altcode:
The Sun plays a central role in two important respects: firstly, it
provides a cosmic laboratory for investigating processes that cannot
be simulated in the terrestrial environment and secondly, because
of its relative closeness it serves as a proxy for understanding
conditions in other stars. Formed about 4.6 billion years from a cloud
of gas dust and frozen ice, the Sun at the current epoch in its life
is a normal main-sequence star of spectral classification G2 with an
average surface temperature of around 5700 K. It displays an astounding
range of phenomena on myriad spatial and temporal scales that have
traditionally defied comprehension. The Sun's magnetic field, that
varies on a 22-year cycle, triggers activity and powerful eruptions
that affect regions extending from the Earth's atmosphere to the
distant edges of the solar system. Despite the inherent complexities
of these processes, some progress has been achieved in understanding
them through recent spectacular advances in observational techniques
coupled with theoretical modelling...
---------------------------------------------------------
Title: Numerical simulation of wave propagation in the presence of
a magnetic flux sheet
Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S.
2008ESPM...12.3.24V Altcode:
We model network magnetic fields as consisting of individual magnetic
flux sheets located in intergranular lanes. With a typical horizontal
size of about 150 km at the base of the photosphere, they expand upward
and merge with their neighbors at a height of about 600 km. Above
a height of approximately 1000 km the magnetic field starts to
become uniform. Granular buffeting is thought to excite waves in this
medium, which is modeled by means of transversal motions at the lower
boundary. The transverse driving, generates both fast and slow waves
within the flux sheet and acoustic waves in the ambient medium. We
consider flux sheets of different field strengths and different
boundary-layer widths. Separating the energy flux of the waves into
contributions due to the acoustic flux and the Poynting flux, we show
the longitudinal and transversal components of both and study their
temporal evolution.
---------------------------------------------------------
Title: Chromospheric dynamics
Authors: Hasan, S. S.
2008AdSpR..42...86H Altcode:
This review focuses on dynamics of the solar chromosphere, which serves
as a good proxy for understanding processes in stellar chromospheres
as well. In the quiet chromosphere it is useful to distinguish between
the magnetic network on the boundaries of supergranulation cells,
where strong magnetic fields are organized in mainly vertical magnetic
flux tubes, and internetwork regions in the cell interior, where
magnetic fields are weaker and dynamically less important. Recently,
some progress has been made in understanding the physics of the
non-magnetic chromosphere. On the other hand, the physical processes
that heat the magnetic network have not been fully identified. Is the
network heated by wave dissipation and if so, what are their physical
characteristics? These and other aspects relating to the dynamics and
energy transport mechanisms will be discussed in detail. In addition,
some of the outstanding problems in the field such the driving mechanism
for spicules and the nature of internetwork magnetic fields will also
be highlighted. Furthermore, a critical assessment will be made on the
challenges facing theory and the direction for future investigations,
particularly in the light of the new space experiments, will be
highlighted.
---------------------------------------------------------
Title: Dynamics of the Solar Magnetic Network. II. Heating the
Magnetized Chromosphere
Authors: Hasan, S. S.; van Ballegooijen, A. A.
2008ApJ...680.1542H Altcode: 2008arXiv0802.3509H
We consider recent observations of the chromospheric network and
argue that the bright network grains observed in the Ca II H and K
lines are heated by an as-yet-unidentified quasi-steady process. We
propose that the heating is caused by dissipation of short-period
magnetoacoustic waves in magnetic flux tubes (periods less than 100
s). Magnetohydrodynamic (MHD) models of such waves are presented. We
consider wave generation in the network due to two separate processes:
(1) transverse motions at the base of the flux tube and (2) the
absorption of acoustic waves generated in the ambient medium. We
find that the former mechanism leads to efficient heating of the
chromosphere by slow magnetoacoustic waves propagating along magnetic
field lines. This heating is produced by shock waves with a horizontal
size of a few hundred kilometers. In contrast, acoustic waves excited
in the ambient medium are converted into transverse fast modes that
travel rapidly through the flux tube and do not form shocks, unless
the acoustic sources are located within 100 km from the tube axis. We
conclude that the magnetic network may be heated by magnetoacoustic
waves that are generated in or near the flux tubes.
---------------------------------------------------------
Title: Wave propagation in multiple flux tubes and chromospheric
heating
Authors: Hasan, S. S.; van Ballegooijen, A.; Steiner, O.
2008IAUS..247...82H Altcode: 2007IAUS..247...82H
This investigation is a continuation of earlier work on the dynamics of
the magnetic network. In a previous calculation (Hasan et al. 2005),
we examined the response of a single flux tube to transverse motions
of its footpoints. We now extend this analysis to a more realistic
model of the network consisting of multiple flux tubes. We apply a
transverse velocity perturbation uniformly along the lower boundary
located at the base of the photosphere. Our 2-D MHD simulations enable
us to study the complex wave pattern due to waves generated in the
individual tubes as well as their interaction with those emanating
from adjacent tubes. Our results show that the dominant heating of the
chromosphere occurs due to slow magnetoacoustic waves in a region that
is close to the central region of the flux tube.
---------------------------------------------------------
Title: Inferring the chromospheric magnetic topology through waves
Authors: Hasan, S. S.; Steiner, O.; van Ballegooijen, A.
2008IAUS..247...78H Altcode: 2007IAUS..247...78H
The aim of this work is to examine the hypothesis that the wave
propagation time in the solar atmosphere can be used to infer the
magnetic topography in the chromosphere as suggested by Finsterle et
al. (2004). We do this by using an extension of our earlier 2-D MHD
work on the interaction of acoustic waves with a flux sheet. It is well
known that these waves undergo mode transformation due to the presence
of a magnetic field which is particularly effective at the surface
of equipartition between the magnetic and thermal energy density, the
β = 1 surface. This transformation depends sensitively on the angle
between the wave vector and the local field direction. At the β =
1 interface, the wave that enters the flux sheet, (essentially the
fast mode) has a higher phase speed than the incident acoustic wave. A
time correlation between wave motions in the non-magnetic and magnetic
regions could therefore provide a powerful diagnostic for mapping the
magnetic field in the chromospheric network.
---------------------------------------------------------
Title: Spatial damping of compressional MHD waves in prominences
Authors: Singh, K. A. P.; Dwivedi, B. N.; Hasan, S. S.
2007A&A...473..931S Altcode:
Aims:We study the spatial damping of linear compressional MHD waves in
a homogeneous, isothermal, and unbounded prominence. <BR />Methods: We
derive a general dispersion relation invoking the Newtonian radiation
and turbulent viscosity. The turbulent viscosity is derived from SUMER
and CDS observations for Kraichnan and Kolmogorov turbulences. Since
we are interested in the spatial damping, the dispersion relation is
solved numerically considering ω as real and k as complex corresponding
to slow, fast, and thermal modes. <BR />Results: Both the slow and
fast modes show strong damping, but the thermal mode is absent. The
turbulent viscosity derived from observations can be a viable mechanism
for the spatial damping of slow and fast modes. For a wave period of
1 s, the damping length for slow and fast modes is found to be 1.1
× 10<SUP>2</SUP> km for the Kolmogorov turbulence. Correspondingly,
the damping length of slow modes is 1.3 × 10<SUP>1</SUP> km and for
fast modes 1.9× 10<SUP>2</SUP> km for the Kraichnan turbulence. From
the damping length study of slow modes, it is found that Kraichnan
turbulence dominates for short wave periods between 10<SUP>-7</SUP> to
10<SUP>2</SUP> s, and the Kolmogorov turbulence dominates for longer
wave periods between 10<SUP>3</SUP> to 10<SUP>5</SUP> s. From the
damping length of fast modes, it is found that the Kraichnan turbulence
dominates from very short to long wave periods. <BR />Conclusions: The
Kraichnan and Kolmogorov turbulence can be a viable damping mechanism
for the spatial damping of short-period oscillations. In particular,
the short-period oscillations (5-15 min) observed in quiescent limb
prominences, which seem to be due to internal fundamental slow modes,
have damping lengths in the range 1.9-3.7× 10<SUP>3</SUP> km for
Kolmogorov turbulence and 3.5 × 10<SUP>3</SUP>-3.1 × 10<SUP>4</SUP> km
for Kraichnan turbulence. Correspondingly, for fast modes, the damping
length is in the range 2.6× 10<SUP>5</SUP>-2.3× 10<SUP>6</SUP> km for
Kolmogorov turbulence and 1.7 × 10<SUP>7</SUP>-1.5× 10<SUP>8</SUP>
km for Kraichnan turbulence. This study underlines the importance of
turbulent viscosity for explaining the damping of both slow and fast
modes, which, hitherto, has not been explored.
---------------------------------------------------------
Title: Kodai School on Solar Physics
Authors: Hasan, S. S.; Banerjee, D.
2007AIPC..919.....H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Kodai School on Solar Physics
Authors: Hasan, S. S.; Banerjee, D.
2007kssp.conf.....H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Damping of MHD Waves in Quiescent Prominences (P50)
Authors: Singh, K. A. P.; Hasan, S. S.; Dwivedi, B. N.
2006ihy..workE.142S Altcode:
The effects of radiative losses due to Newtonian cooling and MHD
turbulence have been considered to examine the damping of linear MHD
waves in unbounded quiescent prominences. Taking account of isotropic
viscosity in the momentum equation and viscous as well as radiation
terms in energy equation, we derive a general fifth-order dispersion
relation. The analytical solutions of the general dispersion relation
have been obtained. It is shown that the damping of magnetoacoustic
waves depends on the equilibrium density, magnetic field, temperature,
frequency and wave number. The fifth-order general dispersion relation
has been solved numerically. We have compared our results with the
observations taken from the VTT telescope at Sac Peak. We find that the
slow mode waves are mainly affected by radiation but fast mode waves
remain unaffected, while noting that both of them are damped due to
MHD turbulence. We also find that classical viscosity hardly plays a
role in damping the magnetoacoustic waves. The radiative losses give
acceptable damping lengths for the slow mode waves for the radiative
relaxation times in the range 10 - 103 s. It has been found that for a
given value of radiative relaxation time, the high frequency slow mode
waves are highly damped. We have also investigated the possible role
of MHD turbulence in damping of MHD waves and found that a turbulent
viscosity can re-produce the observed damping time and damping length
in prominences, especially in PCTR. We find that MHD turbulence alone
can explain the damping of magnetoacoustic waves in prominences. From
prominence seismology, the values of opacity and turbulent kinematic
viscosity have been inferred.
---------------------------------------------------------
Title: Dynamics of the Magnetized Solar Chromosphere
Authors: Hasan, S. S.
2006ihy..workE..29H Altcode:
This review focuses on dynamics of the magnetized solar chromosphere. In
the quiet chromosphere we distinguish between the magnetic network on
the boundary of super-granulation cells, where strong magnetic fields
are organized in mainly vertical magnetic flux tubes, and inter-network
regions in the cell interior, where magnetic fields are weak and
dynamically unimportant. Observations have firmly established the
presence of oscillations in the solar chromosphere. Both the network
and inter-network media show bright points (BPs), which are prominent
in the emission peaks in the cores of the Ca II H and K lines. However,
the dynamical and spectral properties of network and inter-network BPs
are quite different. In the latter the chromospheric velocity power
spectrum is dominated by oscillations having power in the 5-7 mHz
range, which can essentially be regarded as acoustic waves, whereas
the network exhibits low-frequency oscillations with periods 7-20
min. The qualitative properties of inter-network BPs are reasonably well
understood, including their formation in upward propagating acoustic
shocks that encounter downward-flowing gas. On the other hand, the
physical processes that heat the magnetic network have not been fully
identified. Are network BPs heated by wave dissipation and if so,
what is the nature of these waves? These and other aspects relating
to the dynamics and energy transport mechanisms will be discussed
in detail. Furthermore, a critical assessment will be made on the
challenges facing theory and the direction for future investigations,
particularly in the light of the new space experiments, will be
highlighted.
---------------------------------------------------------
Title: Wave Propagation in the Magnetic Network on the Sun
Authors: Hasan, S. S.; Vigeesh, G.; van Ballegooijen, A. A.
2006IAUS..233..116H Altcode:
Hasan et al. (2005) have recently presented 2-D dynamical calculations
on wave propagation in in the magnetic network of the Sun. The latter
is idealized as consisting of non-potential flux tubes in the quiet
solar chromosphere. It is of interest to understand how the nature of
wave propagation is influenced by the choice of initial equilibrium
configuration of the magnetic field. We examine this by comparing
the earlier calculations with those when the network is modelled as
a potential structure. Our calculations demonstrate that the nature
of the wave propagation is significantly different, particularly
the transport of energy which for the potential case, occurs more
isotropically than for the non-potential configuration.
---------------------------------------------------------
Title: Chromospheric Dynamics
Authors: Hasan, S. S.
2006cosp...36.3643H Altcode: 2006cosp.meet.3643H
This review focuses on dynamics of the solar chromosphere which serves
as a good proxy for understanding processes in stellar chromospheres
In the quiet chromosphere we distinguish between the magnetic network
on the boundary of supergranulation cells where strong magnetic fields
are organized in mainly vertical magnetic flux tubes and internetwork
regions in the cell interior where magnetic fields are weak and
dynamically unimportant Observations have firmly established the
presence of oscillations in the solar chromosphere Both the network
and internetwork media show bright points BPs which are prominent in
the emission peaks in the cores of the Ca II H and K lines However the
dynamical and spectral properties of network and internetwork BPs are
quite different In the latter the chromospheric velocity power spectrum
is dominated by oscillations having power in the 5-7 mHz range which can
essentially be regarded as acoustic waves whereas the network exhibits
low-frequency oscillations with periods 7-20 min The qualitative
properties of internetwork BPs are reasonably well understood including
their formation in upward propagating acoustic shocks that encounter
downward-flowing gas On the other hand the physical processes that
heat the magnetic network have not been fully identified Are network
BPs heated by wave dissipation and if so what is the nature of these
waves These and other aspects relating to the dynamics and energy
transport mechanisms will be discussed in detail Furthermore a critical
---------------------------------------------------------
Title: Influence of magnetic field on the Doppler measurements
of velocity field in the solar photosphere and implications for
helioseismology
Authors: Rajaguru, S. P.; Wachter, R.; Hasan, S. S.
2006ilws.conf...21R Altcode:
Shapes of spectral lines and their sensitivity to fluid motions are
strongly altered in magnetised regions of the solar atmosphere. Sunspots
and plages (bright network regions) are prime examples. Here we study
the temporal behaviour of Ni I (6768 A, used by MDI onboard SOHO) in
sunspots and plages using realistic models of sunspots and network flux
tubes as input to a radiative transfer code SPINOR (Frutiger et al.,
2000) that models spectral line formation in magnetic fields. We examine
the sunspot case based on a simple model of oscillations superposed on
the Maltby model (empirical) of sunspot atmosphere. The plage regions
are studied using a 2-d MHD simulation of oscillatory motions (Hasan
et al., 2005) in network flux tubes. We discuss the changes that the
altered line profiles would cause in the Doppler measurements of the
velocity field, especially those by helioseismic imaging instrument
MDI onboard SOHO.
---------------------------------------------------------
Title: Probing the internal magnetic field of slowly pulsating
B-stars through g modes
Authors: Hasan, S. S.; Zahn, J. -P.; Christensen-Dalsgaard, J.
2005A&A...444L..29H Altcode: 2005astro.ph.11472H
Context: .<BR /> Aims: .We suggest that high-order g modes can be used
as a probe of the internal magnetic field of SPB (slowly pulsating
B) stars. The idea is based on earlier work by the authors hich
analytically investigated the effect of a vertical magnetic field on p
and g modes in a plane-parallel isothermal stratified atmosphere. It
was found that even a weak field can significantly shift the g-mode
frequencies - the effect increases with mode order.<BR /> Methods:
.In the present study we adopt the classical perturbative approach
to estimate the internal field of a 4 solar mass SPB star by looking
at its effect on a low-degree (l=1) and high-order (n=20) g mode with
a period of about 1.5 d. <BR /> Results: .We find that a polar field
strength of about 110 kG on the edge of the convective core is required
to produce a frequency shift of 1%. Frequency splittings of that order
have been observed in several SPB variables, in some cases clearly too
small to be ascribed to rotation. We suggest that they may be due to
a poloidal field with a strength of order 100 kG, buried in the deep
interior of the star.<BR /> Conclusions: .
---------------------------------------------------------
Title: Dynamics of the Solar Magnetic Network: Two-dimensional
MHD Simulations
Authors: Hasan, S. S.; van Ballegooijen, A. A.; Kalkofen, W.;
Steiner, O.
2005ApJ...631.1270H Altcode: 2005astro.ph..3525H
The aim of this work is to identify the physical processes that occur
in the network and contribute to its dynamics and heating. We model the
network as consisting of individual flux tubes, each with a nonpotential
field structure, that are located in intergranular lanes. With a typical
horizontal size of about 150 km at the base of the photosphere, they
expand upward and merge with their neighbors at a height of about 600
km. Above a height of approximately 1000 km the magnetic field starts
to become uniform. Waves are excited in this medium by means of motions
at the lower boundary. We focus on transverse driving, which generates
both fast and slow waves within a flux tube and acoustic waves at
the interface of the tube and the ambient medium. The acoustic waves
at the interface are due to compression of the gas on one side of
the flux tube and expansion on the other. These longitudinal waves
are guided upward along field lines at the two sides of the flux
tube, and their amplitude increases with height due to the density
stratification. Being acoustic in nature, they produce a compression
and significant shock heating of the plasma in the chromospheric part of
the flux tube. For impulsive excitation with a time constant of 120 s,
we find that a dominant feature of our simulations is the creation of
vortical motions that propagate upward. We have identified an efficient
mechanism for the generation of acoustic waves at the tube edge, which
is a consequence of the sharp interface of the flux concentration. We
examine some broad implications of our results.
---------------------------------------------------------
Title: How Similar are Starspots to Sunspots?
Authors: Rajaguru, S. P.; Kurucz, R. L.; Hasan, S. S.
2005BASI...33..362R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of the Magnetized Solar Atmosphere
Authors: Hasan, S. S.
2005BASI...33..338H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Erratum: Dynamics and heating of the magnetic network on the
Sun. Efficiency of mode transformation
Authors: Hasan, S. S.; Ulmschneider, P.
2004A&A...428.1017H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Modulation in the solar irradiance due to surface magnetism
during cycles 21, 22 and 23
Authors: Jain, K.; Hasan, S. S.
2004A&A...425..301J Altcode:
Magnetic field indices derived from synoptic magnetograms of the
Mt. Wilson Observatory, i.e. Magnetic Plage Strength Index (MPSI)
and Mt. Wilson Sunspot Index (MWSI), are used to study the effects of
surface magnetism on total solar irradiance variability during solar
cycles 21, 22 and 23. We find that most of the solar cycle variation in
the total solar irradiance can be accounted for by the absolute magnetic
field strength on the solar disk, if fields associated with dark and
bright regions are considered separately. However, there is a large
scatter in the calculated and observed values of TSI during solar cycle
21. On the other hand, the multiple correlation coefficients obtained
for solar cycles 22 and 23 are 0.88 and 0.91 respectively. Furthermore,
separate regression analyses for solar cycles 22 and 23 do not show
any significant differences in the total solar irradiance during these
cycles. Our study further strengthens the view that surface magnetism
indeed plays a dominant role in modulating solar irradiance.
---------------------------------------------------------
Title: Dynamics and heating of the magnetic network on the
Sun. Efficiency of mode transformation
Authors: Hasan, S. S.; Ulmschneider, P.
2004A&A...422.1085H Altcode: 2004astro.ph..6626H
We aim to identify the physical processes which occur in the magnetic
network of the chromosphere and which contribute to its dynamics
and heating. Specifically, we study the propagation of transverse
(kink) MHD waves which are impulsively excited in flux tubes through
footpoint motions. When these waves travel upwards, they get partially
converted to longitudinal waves through nonlinear effects (mode
coupling). By solving the nonlinear, time-dependent MHD equations
we find that significant longitudinal wave generation occurs in the
photosphere typically for Mach numbers as low as 0.2 and that the
onset of shock formation occurs at heights of about 600 km above the
photospheric base. We also investigate the compressional heating due
to longitudinal waves and the efficiency of mode coupling for various
values of the plasma β, that parameterises the magnetic field strength
in the network. We find that this efficiency is maximum for field
strengths corresponding to β≈ 0.2, when the kink and tube wave
speeds are almost identical. This can have interesting observational
implications. Furthermore, we find that even when the two speeds are
different, once shock formation occurs, the longitudinal and transverse
shocks exhibit strong mode coupling.
---------------------------------------------------------
Title: Reconstruction of the past total solar irradiance on short
timescales
Authors: Jain, Kiran; Hasan, S. S.
2004JGRA..109.3105J Altcode:
The aim of this investigation is to present a new analysis of short-term
variations in total solar irradiance by developing regression models
and to extend these to epochs when irradiance measurements were not
available. In our models the sunspot area is used to quantify sunspot
darkening while facular brightening is calculated using facular area,
10.7 cm radio flux and Mg II core-to-wing ratio. Models developed
with various proxies are compared with a view to identify the role of
key parameters in solar variability. We also study the relationship
between different facular proxies and show that the facular area and
10.7 cm radio flux do not vary linearly with the Mg II core-to-wing
ratio. We emphasize that the facular term in current empirical models
(using facular area or radio flux proxies) on short time scale needs to
have a nonlinear component in order to obtain a better correlation with
observed irradiance. Our analysis demonstrates that the correlation for
daily variations in solar irradiance improves by 10% using a quadratic
term in the model based on radio flux as a facular proxy, which is
a significant improvement on earlier models. On the other hand, the
correlation remains unchanged in the model using Mg II core-to-wing
ratio. Thus we point out that various proxies for facular brightenings
contribute differently to solar irradiance. We estimate the solar
irradiance variations at epochs before irradiance observation began,
in particular to the start of the radio flux measurements, and find that
there is no drastic increase in radiative output during the most active
solar cycle 19 while for cycle 20 we observe a much lower irradiance
during maximum.
---------------------------------------------------------
Title: Reconstruction of Total Solar Irradiance on Multiple Time
Scales
Authors: Jain, Kiran; Hasan, S. S.
2003BASI...31..315J Altcode:
We have developed regression models of total solar irradiance on
different time scales by parameterizing the combined influence of
sunspots and faculae. These models are useful in identifying the key
parameters responsible for temporal variations.
---------------------------------------------------------
Title: Kink and Longitudinal Oscillations in the Magnetic Network
on the Sun: Nonlinear Effects and Mode Transformation
Authors: Hasan, S. S.; Kalkofen, W.; van Ballegooijen, A. A.;
Ulmschneider, P.
2003ApJ...585.1138H Altcode:
We examine the propagation of kink and longitudinal waves in the solar
magnetic network. Previously, we investigated the excitation of network
oscillations in vertical magnetic flux tubes through buffeting by
granules and found that footpoint motions of the tubes can generate
sufficient wave energy for chromospheric heating. We assumed that
the kink and longitudinal waves are decoupled and linear. We overcome
these limitations by treating the nonlinear MHD equations for coupled
kink and longitudinal waves in a thin flux tube. For the parameters
we have chosen, the thin tube approximation is valid up to the layers
of formation of the emission features in the H and K lines of Ca II,
at a height of about 1 Mm. By solving the nonlinear, time-dependent MHD
equations we are able to study the onset of wave coupling, which occurs
when the Mach number of the kink waves is of the order of 0.3. We also
investigate the transfer of energy from the kink to the longitudinal
waves, which is important for the dissipation of the wave energy in
shocks. We find that kink waves excited by footpoint motions of a
flux tube generate longitudinal modes by mode coupling. For subsonic
velocities, the amplitude of a longitudinal wave increases as the square
of the amplitude of the transverse wave, and for amplitudes near Mach
number unity, the coupling saturates and becomes linear when the energy
is nearly evenly divided between the two modes.
---------------------------------------------------------
Title: Magnetic Flux Tubes and Activity on the Sun
Authors: Hasan, S. S.
2003LNP...619..173H Altcode: 2003lsp..conf..173H
Activity on the Sun is associated with magnetic fields, involving a
complex interaction between the field and plasma. In this review I focus
on three fundamental aspects of magnetic activity: (a) the generation,
storage and emergence of magnetic fields from the solar interior;
(b) the nature of the surface magnetic fields, especially in the form
of small-scale flux tubes; and (c) dynamical processes in flux tubes
and heating of the magnetic chromosphere.
---------------------------------------------------------
Title: Physics of Photospheric Magnetic Field (Invited review)
Authors: van Ballegooijen, A. A.; Hasan, S. S.
2003ASPC..286..155V Altcode: 2003ctmf.conf..155V
No abstract at ADS
---------------------------------------------------------
Title: The dynamics of the quiet solar chromosphere
Authors: Kalkofen, W.; Hasan, S. S.; Ulmschneider, P.
2003dysu.book..165K Altcode:
Wave propagation in the nonmagnetic chromosphere is described for
plane and spherical waves, and excitation by means of impulses in small
source regions in the photosphere; excitation for flux tube waves in
the magnetic network is described for large, single impulses and for a
fluctuating velocity field. Observational signatures of the various wave
types and their effect on chromospheric heating are considered. It is
concluded that calcium bright points in the nonmagnetic chromosphere
are due to spherical acoustic waves, and that for the oscillations
in the magnetic network, transverse waves are more important than
longitudinal waves; they may penetrate into the corona, giving rise
to some coronal heating.
---------------------------------------------------------
Title: Dynamics of Chromospheres
Authors: Hasan, S. S.
2002AAS...200.5303H Altcode: 2002BAAS...34..729H
This review focuses on dynamics of the solar chromosphere, which serves
as a good proxy for understanding processes in stellar chromospheres. In
the quiet chromosphere we distinguish between the magnetic network on
the boundary of supergranulation cells, where strong magnetic fields
are organized in mainly vertical magnetic flux tubes, and internetwork
regions in the cell interior, where magnetic fields are weak and
dynamically unimportant. Observations have firmly established the
presence of oscillations in the solar chromosphere. The internetwork
medium is dominated with oscillations having power in the 5-7 mHz
range, which can essentially be regarded as acoustic waves. Significant
progress has been made recently in modeling wave propagation in the
non-magnetic medium and applying these calculations to interpreting
the properties of K<SUB>2V</SUB> grains. Nevertheless, there are still
several open questions which need to be addressed, specifically the
departure from 1-D geometry and the inclusion of oblique propagation:
these can have important consequences. The dynamics of the magnetic
network, on the other hand, is dominated by low frequency waves with
periods in the 4-15 min. range, which can be interpreted as transverse
MHD waves, generated in thin flux tubes by granular buffeting. Through
nonlinear effects, these modes generate longitudinal MHD waves, that
form shocks and dissipate in the low to middle chromosphere. Alternative
theoretical scenarios for interpreting network oscillations will also
be discussed as well as their observational consequences. Finally, we
consider some implications of the above models to stellar chromospheres.
---------------------------------------------------------
Title: Convective Intensification of Magnetic Flux Tubes in Stellar
Photospheres
Authors: Rajaguru, S. P.; Kurucz, R. L.; Hasan, S. S.
2002ApJ...565L.101R Altcode: 2002astro.ph..1026R
The convective collapse of thin magnetic flux tubes in the
photospheres of Sun-like stars is investigated using realistic
models of the superadiabatic upper convection zone layers of these
stars. The strengths of convectively stable flux tubes are computed as
a function of surface gravity and effective temperature. We find that
while stars with T<SUB>eff</SUB>>=5500 K and logg>=4.0 show flux
tubes highly evacuated of gas, and hence strong field strengths due to
convective collapse, cooler stars exhibit flux tubes with lower field
strengths. Observations reveal the existence of field strengths close
to thermal equipartition limits even in cooler stars, implying highly
evacuated tubes, for which we suggest possible reasons.
---------------------------------------------------------
Title: Coronal Heating by Kink Waves
Authors: Hasan, S. S.; Kalkofen, W.; Ulmschneider, P.
2001AGUSM..SH41B01H Altcode:
We examine the hypothesis that kink waves contribute to coronal
heating. In earlier work we demonstrated that the excitation of
kink oscillations flux tubes in the magnetic network of the Sun
through their footpoint motions can provide sufficient energy for
chromospheric heating. This calculation assumed that: (a) the waves
could be treated using the linear approximation, and (b) the kink
and longitudinal waves were decoupled. These approximations, although
valid in the lower atmosphere, break down in the upper chromosphere,
where the wave amplitude becomes comparable with the tube speed. We
overcome the earlier limitations by numerically solving the nonlinear
MHD equations for coupled kink and longitudinal waves. Using a specified
form of the footpoint motions, which is compatible with observations,
we solve the nonlinear time-dependent MHD equations for a thin flux
tube extending vertically from the sub-photosphere to the base of the
corona. Our code is able to resolve shocks and also self-consistently
treats mode transformation. We calculate the energy fluxes in vertically
propagating kink waves and show that there is in principle adequate
energy in the waves to heat the corona.
---------------------------------------------------------
Title: Photospheric Flow Fields and Properties of Embedded Small-scale
Magnetic Flux Concentrations
Authors: Rajaguru, S. P.; Srikanth, R.; Hasan, S. S.
2001IAUS..203..205R Altcode:
The association between the different scales of convection on the solar
photosphere and the field strengths/flux contents of discrete magnetic
flux elements are analysed using simultaneously recorded SOHO MDI high
resolution intensity, velocity (Doppler) images and magnetograms. The
convective flow patterns are mapped using the Local Correlation Tracking
(LCT) algorithm. The locations and strengths of the flux elements
with respect to the flow cells are shown to reflect the depths of the
associated downflows. This property is in turn, in combination with
results of calculations on the convective collapse process that forms
strong field elements, used to derive some properties of the different
scales of convection.
---------------------------------------------------------
Title: Radiative Transfer Effects and the Dynamics of Small-Scale
Magnetic Structures on the Sun
Authors: Rajaguru, S. P.; Hasan, S. S.
2000ApJ...544..522R Altcode:
The dynamical consequences of radiative energy transport on the
evolution of gas confined to small-scale magnetic structures on the Sun
are studied. Convective collapse, which transforms weak-field structures
into intense structures of field strengths in the 1-2 kG range on
the photosphere, is strongly influenced by radiative heating from the
surroundings and cooling due to losses in the vertical direction. We
first present analytic results in the quasi-adiabatic approximation
to attempt a qualitative understanding of the influence of radiative
effects on the convective stability of flux tubes. We demonstrate the
destabilizing action of vertical radiative losses, that tend to enhance
convective collapse and produce strong tubes at a relatively smaller
horizontal scale than those expected from calculations based solely on
horizontal radiative energy transport. Our calculations clearly point to
an asymmetry between upflow and downflow perturbations-only the latter
are amplified in the presence of vertical radiative transport. Using
a realistic model of the solar atmospheric structure and treating
radiative transfer in the diffusion and Eddington approximations,
we next perform numerical stability analyses and produce size
(flux)-strength relations for solar flux tubes. Our results provide
a physical explanation for the observed flux-dependent (equivalently
size-dependent) field strengths of the solar small-scale magnetic
structures in the form of weak intranetwork and strong network
components.
---------------------------------------------------------
Title: Results from a revisit to the K<SUB>2V</SUB> bright points
Authors: Sivaraman, K. R.; Gupta, S. S.; Livingston, W. C.; Damé,
L.; Kalkofen, W.; Keller, C. U.; Smartt, R.; Hasan, S. S.
2000A&A...363..279S Altcode:
We have used pairs of temporally simultaneous CaII K-line
spectroheliograms and magnetic area scans to search for spatial
correlation between the CaII K<SUB>2V</SUB> bright points in the
interior of the network and corresponding magnetic elements. We find
that about 60% of the K<SUB>2V</SUB> bright points spatially coincide
with magnetic elements of flux density > 4 Mx cm<SUP>-2</SUP>. About
25% of the K<SUB>2V</SUB> bright points with equally enhanced emission
lie over bipole elements where the fields are > 4 Mx cm<SUP>-2</SUP>
for both polarity elements which merge and presumably cancel and
result in low fields. The rest, 15%, of the bright points coincide
with areas of fields < 4 Mx cm<SUP>-2</SUP> which is the noise
level set by us for the magnetic scans. When magnetic elements of
opposite polarity merge and form bipoles, the associated K<SUB>2V</SUB>
bright points show excess emission. Although such excess emission is a
magnetic-field-driven phenomenon, the measured value of the field at the
site of the bipole is typically low, and these cases would therefore be
excluded in the count of coincidences of excess emission with excess
magnetic fields. In our opinion, these cases of excess emission at
the sites of the bipoles, as well as at the sites of fields >
4 Mx cm<SUP>-2</SUP>, are both instances of magnetic-field-related
emissions. If the former are not taken into account as coincidences,
the correlation will drop down and this might be interpreted as not an
obvious correlation. Our present results, taking into account the low
fields of merging bipoles, establish the association of K<SUB>2V</SUB>
bright points with magnetic elements.
---------------------------------------------------------
Title: Dynamical Processes in Flux Tubes and their Role in
Chromospheric Heating
Authors: Hasan, S. S.
2000JApA...21..283H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Excitation of Oscillations in the Magnetic Network on the Sun
Authors: Hasan, S. S.; Kalkofen, W.; van Ballegooijen, A. A.
2000ApJ...535L..67H Altcode: 2000astro.ph..4246H
We examine the excitation of oscillations in the magnetic network of
the Sun through the footpoint motion of photospheric magnetic flux
tubes located in intergranular lanes. The motion is derived from a
time series of high-resolution G-band and continuum filtergrams using
an object-tracking technique. We model the response of the flux tube
to the footpoint motion in terms of the Klein-Gordon equation, which
is solved analytically as an initial value problem for transverse
(kink) waves. We compute the wave energy flux in upward-propagating
transverse waves. In general we find that the injection of energy into
the chromosphere occurs in short-duration pulses, which would lead
to a time variability in chromospheric emission that is incompatible
with observations. Therefore, we consider the effects of turbulent
convective flows on flux tubes in intergranular lanes. The turbulent
flows are simulated by adding high-frequency motions (periods 5-50 s)
with an amplitude of 1 km s<SUP>-1</SUP>. The latter are simulated by
adding random velocity fluctuations to the observationally determined
velocities. In this case, we find that the energy flux is much
less intermittent and can in principle carry adequate energy for
chromospheric heating.
---------------------------------------------------------
Title: The Nature of Wave Excitation in the Magnetic Network
Authors: Hasan, S. S.; Kalkofen, W.
2000SPD....31.0129H Altcode: 2000BAAS...32..806H
We examine the nature of wave excitation in the magnetic network of
the Sun through the footpoint motion of photospheric magnetic flux
tubes located in intergranular lanes. This contribution builds on
earlier work (Hasan & Kalkofen 1999, ApJ 519, 899) where it was
suggested that impulsively generated MHD kink, or transverse, waves can
contribute efficiently to chromospheric heating. On the other hand, it
was argued that this scenario for heating the magnetic network would
produce "strongly intermittent chromospheric emission consisting of
brief, intense flashes superimposed on a very low background" (Hasan,
Kalkofen & van Ballegooijen 2000, ApJL, in press). In this paper,
we analyse in greater detail the consequences based on the above picture
by adopting different forms for the footpoint point velocity. We compute
the transverse wave energy flux injected into the chromosphere through
(a) impulsive and (b) turbulent footpoint motions. Finally, we point
out observational implication of our calculations.
---------------------------------------------------------
Title: Excitation of Oscillations in Photospheric Flux Tubes through
Buffeting by External Granules
Authors: Hasan, S. S.; Kalkofen, W.
1999ApJ...519..899H Altcode:
We examine the excitation of transverse (kink) and longitudinal
(sausage) waves in magnetic flux tubes by granules in the solar
photosphere. The investigation is motivated by the interpretation
of network oscillations in terms of flux tube waves. We model the
interaction between a granule, with a specified transverse velocity,
and a vertical flux tube in terms of the Klein-Gordon equation, which
we solve analytically as an initial value problem for both wave modes,
assuming the same external impulse. The calculations show that for
magnetic field strengths typical of the network, the energy flux in
transverse waves is higher than in longitudinal waves by an order
of magnitude, in agreement with the chromospheric power spectrum of
network oscillations observed by Lites, Rutten, & Kalkofen. But
for weaker fields, such as those that might be found in internetwork
regions, the energy fluxes in the two modes are comparable. This result
implies that if there are internetwork oscillations in magnetic flux
tubes, they must show the cutoff periods of both longitudinal and
transverse modes at 3 minutes and at 7 minutes or longer. We also find
that granules with speeds of about 2 km s<SUP>-1</SUP> can efficiently
excite transverse oscillations in frequent short-duration (typically
1 minute) bursts that can heat the corona.
---------------------------------------------------------
Title: Excitation of oscillations in the magnetic network on the Sun
Authors: Hasan, S. S.; Kalkofen, W.
1999AAS...194.9309H Altcode: 1999BAAS...31..990H
We examine the excitation of oscillations in the magnetic network of
the Sun through the buffeting action of external granules on vertical
magnetic flux tubes extending through the photosphere. We assume that
the granules motions are turbulent with an extended Kolmogorov energy
spectrum. We model the interaction of the external flow field with
the flux tube in terms of the Klein-Gordon equation, that is solved
analytically as an initial value problem for transverse (kink) and
longitudinal (waves). We compute the wave energy flux in both the modes
for various magnetic field strengths and compare these results with our
previous calculations in which the network oscillations are excited in
short duration pulses by the occasional rapid motions of granules. We
examine the consequences of our results for coronal heating.
---------------------------------------------------------
Title: 2D radiative equilibrium models of magnetic flux tubes
Authors: Hasan, S. S.; Kalkofen, W.; Steiner, O.
1999ASSL..243..409H Altcode: 1999sopo.conf..409H
No abstract at ADS
---------------------------------------------------------
Title: The excitation of oscillations in network bright points.
Authors: Kalkofen, W.; Hasan, S. S.
1999joso.proc..137K Altcode:
The authors study the excitation of flux tube waves in the photosphere
by granular buffeting of magnetic flux tubes and show that for the
strong magnetic fields of the network the emitted energy flux appears
mainly in transverse (kink) waves, and only to a much smaller extent
in longitudinal (sausage) waves.
---------------------------------------------------------
Title: Radiative Cooling, Convective Downflows and the Formation of
Small-Scale Strong Magnetic Field Structures on the Sun
Authors: Rajaguru, S. Paul; Hasan, S. S.
1999soho....9E..16R Altcode:
Radiative energy transport has important dynamical consequences for
the evolution of gas confined in small-scale magnetic structures on
the Sun. The convective collapse process which transforms weak field
structures into intense structures of strength in the range 1-2 kG,
which eventually form sunspots in the emerging flux regions, is
affected by the radiative heating from the surroundings and cooling
due to vertical losses. Through a simple analysis of such radiative
transfer effects in the diffusion approximation for a thin magnetic flux
tube, we show the destabilizing action of vertical radiative losses in
the form of an accelerated downflow, thereby verifying theoretically
the thermal origin of the convective collapse process, as suggested
by Parker (1978). We also perform numerical stability calculations
involving the generalised Eddington approximation for radiative transfer
and produce size(flux)-strength relations for the solar small-scale
magnetic structures. Our results provide a physical explanation for
the flux dependent (equivalently size dependent) field strength of
the solar small-scale magnetic structures and the two-component (weak
intra-network and the strong network) distribution is explained in terms
of physical parameters that control the convective collapse process.
---------------------------------------------------------
Title: Spectral line radiation from solar small-scale flux tubes. II
Authors: Hasan, S. S.; Kneer, F.; Kalkofen, W.
1998A&A...332.1064H Altcode:
We examine spectral line radiation from small-scale magnetic flux tubes
in the solar atmosphere. This is a continuation of work by Kneer et
al. (1996). The main difference with the previous investigation is in
the choice of the external atmosphere. Earlier we adopted an atmosphere
resembling the empirical quiet Sun model for the ambient medium. In the
present study, we iteratively adjust the temperature structure of the
external atmosphere to fit the Stokes I and V profiles and the average
continuum intensities with those obtained from observations. Our
models are hotter in the uppermost photospheric layers and cooler
in the deeper layers than the quiet Sun model and agree well with
semi-empirical flux tube models.
---------------------------------------------------------
Title: Excitation of Longitudinal Modes in Solar Magnetic Flux Tubes
by p-Modes
Authors: Hasan, S. S.; Kalkofen, W.
1998ASPC..154..767H Altcode: 1998csss...10..767H
This is a continuation of earlier work by Hasan (1997) on the
interaction of longitudinal (sausage) waves in a slender flux tube
with p-modes in the ambient medium. We use a realistic stratification
for the flux tube and external atmospheres based upon the models of
Hasan & Kalkofen (1994). The MHD equations for a thin flux tube
are solved as an initial value problem incorporating radiative and
convective energy transport. Our calculations confirm the linear
prediction that the interaction is non-resonant. We find that the
response (for a fixed order) increases with mode degree l up to a
maximum and then falls off sharply as l increases. For the f-mode,
l_max ~650. The amplitude of the oscillations tend to become stationary
implying a balance between energy input from p-modes and losses through
radiative damping and leakage from boundaries. Low order p-modes with
degrees of several hundred appear to be most efficient for exciting
longitudinal oscillations in flux tubes. The energy flux in these
oscillations appears to be insufficient for chromospheric heating,
but may contribute partially to the required flux.
---------------------------------------------------------
Title: Why Does the Sun Have Kilogauss Magnetic Fields?
Authors: Hasan, S. S.; van Ballegooijen, A. A.
1998ASPC..154..630H Altcode: 1998csss...10..630H
Magnetic fields in the solar photosphere are concentrated in flux
tubes with kilogauss field strength surrounded by nearly field-free
plasma. Observations show that the flux tubes are located in convective
downdrafts where the temperature is lower than average. We assume that
the convective downdrafts extend to large depths in the convection
zone, and that flux tubes follow the downdrafts to these depths. We
develop a model for the magnetic field strength B(z) in the flux
tubes as a function of depth z below the surface. Our calculations
reveal that epsilon, the ratio of magnetic pressure to gas pressure,
has a large depth variation: at the base of the convection zone where
epsilon ~10^{-5} (B ~10^5 G), while at the top epsilon ~1, in broad
agreement with solar observations. Thus the model can explain why the
field strength at the photosphere is around 1 kG.
---------------------------------------------------------
Title: Radial Modes of Rotating Neutron Stars in the
Chandrasekhar-Friedman Formalism
Authors: Datta, B.; Hasan, S. S.; Sahu, P. K.; Prasanna, A. R.
1998IJMPD...7...49D Altcode:
Eigenfrequencies of radial pulsations of "slowly" rotating neutron
stars are calculated in a general relativistic formalism given by
Chandrasekhar & Friedman. It is found that the square of the
frequencies are always a decreasing function of the central density of
the neutron star. The decrease of the squared frequency is sensitive
to the equation of state of neutron star matter, and is illustrated
using realistic models.
---------------------------------------------------------
Title: The influence of magnetic fields on radiative damping of
magnetoatmospheric oscillations
Authors: Banerjee, Dipankar; Hasan, S. S.; Christensen-Dalsgaard, J.
1998IAUS..185..423B Altcode:
We examine the non adiabatic effects on the modes of an isothermal
stratified magnetic atmosphere. The present investigation
is a continuation of earlier work by Banerjee, Hasan &
Christensen-Dalsgaard (1995, 1996, 1997), where the interaction of
various elementary modes in a stratified magnetised atmosphere
was studied in the purely adiabatic limit. The inclusion of
radiative dissipation in magnetoatmospheric wave problem - even in
the over simplified Newtonian Cooling approximation - is of great
importance. Including Newtonian Cooling, in the weak field limit, an
analytic expression for the dispersion relation is derived which allows
the effect of weak magnetic field on the modes to be studied. We examine
the nature of the eigen frequency curves in the diagnostic diagram and
find that, similar to the previous analysis, the modes undergo avoided
crossings. We study the full frequency spectrum and the interaction
amongst various modes. Strong mode coupling in the avoided crossing
regions permits energy leakage. Our results, find application in the
analysis of waves in flux-tubes on the Sun. We study the influence of
magnetoatmospheric waves on spectral line profiles.
---------------------------------------------------------
Title: The Excitation of Oscillations in Network Bright Points
Authors: Kalkofen, W.; Hasan, S. S.
1998joso.proc..137K Altcode:
We study the excitation of flux tube waves in the photosphere by
granular buffeting of magnetic flux tubes and show that for the strong
magnetic fields of the network the emitted energy flux appears mainly
in transverse (kink) waves, and only to a much smaller extent in
longitudinal (sausage) waves.
---------------------------------------------------------
Title: Structure of small-scale magnetic flux tubes and excitation
of sausage mode oscillations
Authors: Hasan, S. S.
1998BASI...26..181H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Influence of Magnetic Flux Tubes on Their Environment
Authors: Hasan, S. S.; Kalkofen, W.
1998ASPC..154..838H Altcode: 1998csss...10..838H
We present new calculations for model atmospheres in magnetic flux
tubes extending vertically through the photosphere and convection zone
of the Sun. This study is a continuation of earlier work by Hasan &
Kalkofen (1994) on the equilibrium structure of intense magnetic flux
tubes. We construct static models of flux tubes by solving the equations
of energy transport and radiative transfer. The most significant aspect
of our study is the inclusion of multidimensional radiative transfer in
cylindrical geometry and the influence of the flux tube on the ambient
medium. Our models satisfy the condition of radiative equilibrium;
the lower boundary intensity includes the effect of convection. We
determine the structure of the thermal boundary layer at the interface
of the flux tube and the ambient medium. We find that the temperature
does not change abruptly from its value on the flux tube axis to the
ambient value far from the tube. Rather, there is a transition layer
at the interface, where there is a significant horizontal temperature
gradient. Detailed calculations have been carried out to examine the
physical conditions in this layer as well as its horizontal extent.
---------------------------------------------------------
Title: The influence of radiative damping on the modes of a magnetized
isothermal atmosphere
Authors: Banerjee, D.; Hasan, S. S.; Christensen-Dalsgaard, J.
1997ASSL..225..277B Altcode: 1997scor.proc..277B
We examine the influence of radiative damping on the modes of an
isothermal magnetized plasma. This investigation is an extension of
earlier work by Hasan \& Christensen-Dalsgaard (1992) and Banerjee,
Hasan \& Christensen-Dalsgaard (1995, 1996), which treated the
effects of a uniform vertical magnetic field on the adiabatic modes
of a stratified atmosphere. We focus on the properties of the Lamb
mode and examine its interaction with a magnetic mode. We show that
radiative dissipation can significantly alter the Lamb-mode frequency
and lead to a enhanced damping of the waves at the avoided crossings
in the $k-\omega$ diagram. This mechanism could be important for wave
damping in flux tubes on the Sun.
---------------------------------------------------------
Title: Dynamical phenomena in sunspots. I. Time dependent relaxation
to equilibrium.
Authors: Gangadhara, R. T.; Hasan, S. S.
1997A&A...327..786G Altcode:
We have adapted a general purpose time-dependent 2-D code to study
dynamical phenomena in sunspots. In the first part of our investigation,
we numerically simulate the dynamical relaxation to equilibrium of
a sunspot. Treating the sunspot as a thick axisymmetric flux tube
in cylindrical geometry, we solve the time dependent MHD equations
to examine the evolution of a sunspot towards equilibrium, starting
from an arbitrary initial state. Initially, we choose a potential
magnetic field configuration and assume hydrostatic equilibrium along
field lines, which allows the pressure variation along the field to
be determined, for a known temperature distribution. We also assume
that all quantities in the tube have a smooth and continuous radial
variation. In particular the pressure increases radially from the
tube axis to the photospheric value. The absence of Lorentz forces
to balance the radial pressure gradient, leads to an inflow of gas
towards the axis accompanied by an increase in the magnetic field
strength. A complex flow pattern develops in the tube, which eventually
dies out due to escape of matter upwards along the field lines. In the
quasi-equilibrium state it is found that the field lines near the center
of a large spot assume a configuration which is almost potential while
those at the periphery depart significantly from the initial state,
due to being pushed inwards by the gas flow. Our method is applicable
to both thin and thick flux tubes. Further it can be readily extended
to any coordinate system with 2 or 3 coordinates, and to discontinuous
configurations such as current sheets. Forthcoming studies will focus on
an extension of the present study to an analysis of dynamical effects
in sunspots associated with nonlinear waves and examine the transport
of energy by these to the corona.
---------------------------------------------------------
Title: The Linear Response of a Magnetic Flux Tube to Buffeting by
External p-Modes. I.
Authors: Hasan, S. S.
1997ApJ...480..803H Altcode:
The linear response of a thin vertical magnetic flux tube to buffeting
by p-modes in the ambient atmosphere is examined with the aim of
understanding the interaction of acoustic modes with sausage tube
waves. The idealized case of an isothermal atmosphere is considered,
which has the mathematical advantage that the differential equation
for the vertical component of the Lagrangian displacement in the tube
can be solved analytically. A lower boundary condition is employed
that permits the tube wave to leak out through this boundary. This
has the important consequence that the p-mode interaction with flux
tubes does not exhibit a resonant behavior. The detailed behavior of
the vertical displacement in the tube and its dependence on various
parameters are examined. An equation for the wave energy in a thin flux
tube is derived along with analytic expressions for the wave energy
density and vertical energy flux. The variation of the tube response Ξ
(defined as the ratio of the total wave energy, integrated over the
length of the tube, to the p-mode energy in the external atmosphere)
is investigated for different values of the dimensionless external
horizontal wavenumber K<SUB>x</SUB>, mode order n, and β, where
β is the ratio of the gas to magnetic pressure in the tube that,
by assumption, is constant with depth. It is found that when n is
small, the response of the tube increases gradually with K<SUB>x</SUB>
until reaching a maximum, and thereafter it drops very sharply. As n
increases, the maximum shifts to lower values of K<SUB>x</SUB>. For
fixed values of K<SUB>x</SUB> and β, Ξ increases with n and then
falls off after reaching a maximum. A similar dependence of Ξ on β is
found. Line widths of p-modes are also calculated, and their dependence
on K<SUB>x</SUB> and frequency is studied. Finally, an application of
the results to the solar atmosphere is discussed, and the limitations
of the model are pointed out.
---------------------------------------------------------
Title: Effect of Newtonian Cooling on Waves in a Magnetized Isothermal
Atmosphere
Authors: Banerjee, Dipankar; Hasan, S. S.; Christensen-Dalsgaard, J.
1997SoPh..172...53B Altcode: 1997ESPM....8...53B
We examine the influence of nonadiabatic effects on the modes of an
isothermal stratified magnetic atmosphere. The present investigation is
a continuation of earlier work by Hasan and Christensen-Dalsgaard (1992)
and Banerjee, Hasan, and Christensen-Dalsgaard (1995, 1996), where
the interaction of various elementary modes in a stratified magnetized
atmosphere was studied in the purely adiabatic limit. The inclusion of
radiative dissipation based on Newton's law of cooling demonstrates
the importance of this effect in the study of magnetoatmospheric
waves. We analyze the physical nature of magnetoacoustic gravity (or
MAG) oscillations in the presence of Newtonian cooling and find that
the eigenfrequency curves in the diagnostic diagram, as in the previous
analysis, undergo avoided crossings. However, the qualitative nature of
the mode interaction is strongly influenced by radiative dissipation,
which leads to strong mode damping in the avoided-crossing regions. We
demonstrate this effect for the interaction between the Lamb mode and
a magnetic mode. Our results could be important in the analysis of
waves in flux tubes on the Sun.
---------------------------------------------------------
Title: Excitation of longitudinal modes in Magnetic Flux Tubes by
external p-modes
Authors: Hasan, S. S.; Kalkofen, W.
1997SPD....28.0237H Altcode: 1997BAAS...29R.899H
This is a continuation of earlier work by Hasan (1997, ApJ 480,
in press) on the interaction of sausage waves in a slender flux
tube with p-modes in the ambient medium. In the above paper, the
time-asymptotic response of a vertical tube in the solar photosphere
due to the buffeting action by external p-modes was examined using
linear theory. For mathematical tractability an isothermal atmosphere
was assumed, which allowed the problem to be solved exactly. This
study provided insight into the behavior of the tube response and
its dependence on various parameters. The previous analysis has
now been extended in three important respects; firstly, a realistic
stratification for the flux tube and external atmospheres has been
used based upon the models of Hasan and Kalkofen (1994, ApJ 436,
355). Secondly, the interaction has been treated as an initial value
problem, which enables the gradual buildup of energy in flux tube
oscillations to be studied. Thirdly, non-adiabatic effects involving
radiative transport have been taken into account. Our calculations
confirm the linear prediction that the interaction is non-resonant. We
find that the response does not exhibit a monotonic variation with the
p-mode degree l (for a fixed order), but increases to a maximum and
then falls off sharply as l increases. For the f- mode, l<SUB>max</SUB>
~ 700. The amplitude of the oscillations tend to become stationary
(i.e. constant in time) implying a balance between energy input
from p-modes and losses through radiative damping and leakage from
boundaries. The dominant contribution to the energy flux in the upper
photosphere comes from the enthalpy flux. This flux by itself appears to
be insufficient for chromospheric heating, but may contribute partially
to the required flux. P-modes of low order and degree appear to be
most efficient for exciting longitudinal oscillations in flux tubes.
---------------------------------------------------------
Title: The effect of p-modes on thin magnetic flux tubes
Authors: Hasan, S. S.; Bogdan, T. J.
1996BASI...24..125H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Wave leakage in a magnetized isothermal atmosphere
Authors: Banerjee, Dipankar; Hasan, S. S.; Christensen-Dalsgaard, J.
1996BASI...24..325B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Modelling sunspot equilibrium through a solution of the time
dependent MHD equations
Authors: Gangadhara, R. T.; Hasan, S. S.
1996BASI...24..341G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamical Effects in Solar Photospheric Flux Tubes
Authors: Hasan, S. S.
1996Ap&SS.243..155H Altcode: 1996IAUCo.154..155H
The interaction of an intense flux tube, extending vertically through
the photosphere, with p-modes in the ambient medium is modelled
by solving the time dependent MHD equations in the thin flux tube
approximation. It is found that a resonant interaction can occur,
which leads to the excitation of flux tube oscillations with large
amplitudes. The resonance is not as sharp as in the case of an
unstratified atmosphere, but is broadened by a factor proportional
toH <SUP>-2</SUP>, whereH is the local pressure scale height. In
addition, the inclusion of radiative transport leads to a decrease in
the amplitude of the oscillations, but does not qualitatively change
the nature of the interaction.
---------------------------------------------------------
Title: Spectral line radiation from solar small-scale magnetic
flux tubes.
Authors: Kneer, F.; Hasan, S. S.; Kalkofen, W.
1996A&A...305..660K Altcode:
We consider spectral line radiation from small-scale magnetic model flux
tubes in the solar atmosphere. The structure of the tube is determined
from the magnetostatic equations in the thin flux tube approximation. We
assume that the tube is in energy equilibrium and pressure balance
with the ambient medium. For the latter, we construct a quiet sun model
with an artificial heating term in order to reproduce the VAL C model,
treating the medium as a plane-parallel atmosphere. The flux tube models
are parameterized by the plasma β_0_ (the ratio of gas the pressure
to the magnetic pressure), the convective efficiency parameter α,
and the radius R_0_ at height z=0 (τ_5000_=1) in the quiet sun. The
Stokes I and V profiles emerging from the models and averaged over
areas that include the neighbourhood of the flux tube are calculated
for various spectral lines with different sensitivity for magnetic
field strength and temperature. The profiles are compared with high
spatial resolution observations of plages near disc centre that have
been obtained with the Gregory Coude Telescope at the Observatorio del
Teide/Tenerife. The information contained in both I and V profiles is
found to be very useful in constraining the theoretical models. The
best match of models with observations is achieved for values of β_0_
between 0.3 and 0.5. For a sufficiently wide separation of the V extrema
of the strongly split lines, a broadening mechanism is required. Pure
velocity (microturbulent) broadening compatible with observations
of strongly split lines gives too much broadening for weakly split
lines. A broadening that is proportional to the Lande factor, i.e.,
magnetic broadening, appears to be more appropriate. This suggests
dynamic models with temporary enhancement of the magnetic field
strength. The continuum intensity of our models is higher and the
absorption and V amplitude in the FeII 6149A line are stronger than
observed. An improvement in the match between model predictions and
observations is likely to come from models in which the ambient gas
has a lower temperature as well as a lower temperature gradient than
are found in the quiet, field-free sun. Such models are currently
under development for cylindrical flux tubes.
---------------------------------------------------------
Title: The Influence of a Vertical Magnetic Field on Oscillations
in an Isothermal Stratified Atmosphere. II.
Authors: Banerjee, Dipankar; Hasan, S. S.; Christensen-Dalsgaard, J.
1995ApJ...451..825B Altcode:
We examine the effect of a uniform vertical magnetic field on the modes
of an isothermal stratified atmosphere. The present investigation is
a continuation of earlier work by Hasan & Christensen-Dalsgaard in
which this problem was studied for rigid boundary conditions. In this
paper, the earlier results are extended to different sets of boundary
conditions. We demonstrate explicitly how these boundary conditions
affect the various elementary wave modes present in the atmosphere. In
the weak-field limit, an analytic expression for the dispersion relation
is derived, which allows the effect of a weak magnetic field on the
modes to be studied. We show that, to lowest order in our perturbation
expansion, the oscillation spectrum can be analyzed in terms of (a) p-
and g-like modes; (b) a magnetic Lamb mode; (c) magnetic or slow modes;
and (d) a gravity- Lamb mode. The first three of these were present
in the previous analysis for rigid boundaries, whereas the last is
a consequence of the vertical gradients of the displacements at the
boundaries. We focus our attention on the properties of this mode and
show that it is present even in the moderate to strong field case as a
magnetogravity-Lamb mode. The recognition and physical interpretation
of this mode is a new feature of the present analysis. We also examine
the nature of the eigenfrequency curves in the diagnostic (or K-Ω)
diagram and find that, similar to the previous analysis, the modes
undergo avoided crossings. However, the nature of the solutions in
the present case is more complicated, especially when triple-mode
interactions occur. Furthermore, the connectivity of the curves in
the K-Ω diagram can be strongly influenced by the choice of boundary
conditions. Our results, though somewhat idealized, find application
in the analysis of waves in sunspots. It is conjectured that conditions
for the existence of the magnetogravity-Lamb mode may also be satisfied
in the subphotospheric layers of the Sun.
---------------------------------------------------------
Title: Helioseismology - Impact and Implications
Authors: Hasan, S. S.
1995BASI...23..337H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Equilibrium Structure of Solar Magnetic Flux Tubes: Energy
Transport with Multistream Radiative Transfer
Authors: Hasan, S. S.; Kalkofen, W.
1994ApJ...436..355H Altcode:
We examine the equilibrium structure of vertical intense magnetic
flux tubes on the Sun. Assuming cylindrical geometry, we solve the
magnetohydrostatic equations in the thin flux-tube approximation,
allowing for energy transport by radiation and convection. The radiative
transfer equation is solved in the six-stream approximation, assuming
gray opacity and local thermodynamic equilibrium. This constitutes
a significant improvement over a previous study, in which the
transfer was solved using the multidimensional generalization of the
Eddington approximation. Convection in the flux tube is treated using
mixing-length theory, with an additional parameter alpha, characterizing
the suppression of convective energy transport in the tube by the
strong magnetic field. The equations are solved using the method of
partial linearization. We present results for tubes with different
values of the magnetic field strength and radius at a fixed depth in
the atmosphere. In general, we find that, at equal geometric heights,
the temperature on the tube axis, compared to the ambient medium,
is higher in the photosphere and lower in the convection zone, with
the difference becoming larger for thicker tubes. At equal optical
depths the tubes are generally hotter than their surroundings. The
results are comparatively insensitive to alpha but depend upon whether
radiative and convective energy transport operate simultaneously or
in separate layers. A comparison of our results with semiempirical
models shows that the temperature and intensity contrast are in broad
agreement. However, the field strengths of the flux-tube models are
somewhat lower than the values inferred from observations.
---------------------------------------------------------
Title: Thin flux tube models with multistream radiative transfer
Authors: Hasan, S. S.; Kalkofen, W.
1994smf..conf..334H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Influence of a Vertical Magnetic Field on Oscillations
in an Isothermal Stratified Atmosphere
Authors: Hasan, S. S.; Christensen-Dalsgaard, J.
1992ApJ...396..311H Altcode:
The effect of a uniform vertical magnetic field on the modes
of an isothermal stratified atmosphere is examined. The general
solutions of the wave equation for an isothermal magnetized medium
are given. Asymptotic expansions of these solutions are presented in
the strong- and weak-field limits. For a weak field, it is found that,
to lowest order of the present perturbation expansion, the oscillation
spectrum can be analyzed in terms of p- and g-like modes, magnetic
Lamb modes, and magnetic or slow modes. The frequency corrections
for each of the modes due to coupling with the remaining modes are
calculated. It is demonstrated that when the frequencies of different
modes are almost identical, strong mode coupling occurs and the waves
acquire a mixed character. An analysis of the solutions in the vicinity
of the degenerate frequencies is carried out. The solutions reveal the
presence of avoided crossings, which occur at the otherwise degenerate
frequencies. Examples of different types of avoided crossings are given.
---------------------------------------------------------
Title: Indian Institute of Astrophysics. Annual report 1990 - 91.
Authors: Parthasarathy, M.; Hasan, S. S.; Srinivasan, R.
1992iiaa.book.....P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Heating in Intense Flux Tubes (With 4 Figures)
Authors: Hasan, S. S.
1991mcch.conf..408H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetoatmospheric Oscillations in Sunspot Umbrae
Authors: Hasan, S. S.
1991ApJ...366..328H Altcode:
The nature of umbral oscillations is investigated using an empirical
model for the sunspot umbra, based on the model of Maltby et
al. (1986). Approximating the sunspot as a thick flux tube of circular
cross section, the axisymmetric normal modes of magnetoatmospheric
waves were determined, and a diagnostic diagram was generated
for different field strengths. The diagram shows the existence of
'avoided crossings' in the solution of magnetoatmospheric modes in a
sunspot atmosphere. It was found that, for low values of the radial
wavenumber k, corresponding to observed oscillations, a simple global
classification is not possible. The oscillations in the low photosphere
and below are of either fast or mixed type, but tend to acquire a slow
or acoustic character above the temperature minimum.
---------------------------------------------------------
Title: Indian Institute of Astrophysics. Annual report 1989 - 90.
Authors: Parthasarathy, M.; Hasan, S. S.
1991iiaa.book.....P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamical effects and energy transport in intense flux tubes
Authors: Hasan, S. S.
1990GMS....58..157H Altcode:
The present effort to obtain a realistic model of conditions inside
intense flux tubes employs calculations which treat radiative transport
in the Eddington approximation, while allowing for convective energy
transport within the flux tube. After constructing an equilibrium
atmosphere within the tube, this equilibrium is perturbed through
the introduction of a small downflow. Complex oscillatory behavior
is observed in which upflow and downflow phases do not appear to be
symmetric. Vertical energy transport through radiation emerges as
very important, especially in the proximity of continuum optical-depth
unity. Observational implications are noted.
---------------------------------------------------------
Title: Wave Propagation in Sunspots
Authors: Hasan, S. S.
1990IAUS..142..189H Altcode:
Wave propagation in sunspot umbrae is analyzed; the stratification
in a typical umbra is approximated by a model atmosphere, extending
vertically from a depth of a few thousand kilometers below the
photosphere, to the transition region. A Rayleigh-Ritz variational
technique is used to calculate the normal mode frequencies of the
umbra for different values of the horizontal wave number. The nature
of the wave modes is examined by decomposing the eigenvectors into
longitudinal and transverse components.
---------------------------------------------------------
Title: Classification of Magnetoatmospheric Modes in Sunspot Umbrae
Authors: Hasan, S. S.; Sobouti, Y.
1990IAUS..138..255H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Wave modes in thick photospheric flux tubes - Classification
and diagnostic diagram
Authors: Hasan, S. S.; Abdelatif, T.
1990GMS....58...93H Altcode:
The nature of wave motions in thick photospheric flux tubes is
analyzed. The aim of this investigation is to determine the normal modes
of a stratified atmosphere with a vertical magnetic field and to discuss
their properties. The results are displayed in the form of a diagnostic
diagram. An interesting feature of the solutions is the existence of
'avoided crossings', which occur when adjacent order modes approach
each other in the diagnostic diagram. In general, the character of
a mode changes with height in the atmosphere. Results are applied to
umbral oscillations, and it is found that the observed oscillations
with periods in the range 2-3 min, correspond to low-order modes in
these calculations. For low horizontal wave number K, the modes, in
the photosphere, have almost equal contributions from longitudinal
and transverse components. As K increases, the transverse component
begins to dominate. In the chromosphere, the modes are essentially
transverse and can be identified with slow modes.
---------------------------------------------------------
Title: Indian Institute of Astrophysics. Annual report 1987 - 88.
Authors: Hasan, S. S.
1989iiaa.book.....H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Energy Transport in Intense Flux Tubes on the
Sun. I. Equilibrium Atmospheres
Authors: Hasan, S. Sirajul
1988ApJ...332..499H Altcode:
Model calculations are performed to determine self-consistently the
equilibrium atmosphere in an intense flux tube. The generalization
of the Eddington approximation to three dimensions is used to develop
a zeroth-order transfer equation valid for an axisymmetric thin flux
tube. Convection is included by using a mixing length formalism and
a parameter to incorporate its inhibition by the magnetic field. The
results suggest that the temperature on the axis of a flux tube is
lower than the ambient medium at the same geometric level. At equal
optical depths, however, the temperature in the tube is higher. At an
optical depth of unity, the temperature difference is typically about
500 K. In the optically thin layers, horizontal exchange of heat is
efficient and the temperature inside the tube is insensitive to the
magnetic field strength. The equilibrium stratification is almost
independent of the degree of convective inhibition.
---------------------------------------------------------
Title: Mode classification and wave propagation in a magnetically
structured medium
Authors: Hasan, S. S.; Sobouti, Y.
1987MNRAS.228..427H Altcode:
The authors examine the structure of motions that can occur in a
vertical magnetic flux tube with a rectangular cross-section. A
polytropic stratification is assumed in the vertical direction. The
authors use a gauged version of Helmholtz's theorem, the decompose
the perturbations into an irrotational component and a solenoidal
component, which they further split into the sum of poloidal and
toroidal components. These components are identified with p, g and
toroidal modes of a fluid. The normal modes of the tube are determined
using a Rayleigh-Ritz variational technique. The authors' technique
efficiently isolates all the modes to high orders. They first consider
some special cases, in order to highlight some interesting properties of
the modes. Then, they choose a parameter range to study the properties
of oscillations in intense flux tubes on the Sun.
---------------------------------------------------------
Title: Stability of cool flux tubes in the solar chromosphere. Linear
analysis.
Authors: Hasan, S. S.; Kneer, F.
1986A&A...158..288H Altcode:
The linear stability of cool flux tubes in the solar chromosphere
which are initially in radiative equilibrium is examined. Owing to
the presence of carbon monoxide, there exists a narrow region near the
temperature minimum where the temperature gradient becomes steep enough
to drive a convective instability. The thin flux tube equations are
used and in a simple manner radiative heat exchange with the ambient
medium are included. Initial states of constant beta (where beta is
the ratio of gas to magnetic pressure) are considered. Results for
various values of beta are presented. It is found that for beta less
than 5.7 the tube is overstable with periods in the range 300-600 s. At
beta = 5.7 a bifurcation occurs into two purely growing modes. Growth
rates, eigenvectors of the fundamental modes are calculated and phase
relationships are examined. It is suggested that overstable oscillations
should invariably be associated with cool flux tubes. These oscillations
transport energy and can thus change the thermodynamic structure of flux
tubes. It is conjectured that the CO overstability may be responsible
for spicules.
---------------------------------------------------------
Title: Oscillatory motions in intense flux tubes
Authors: Hasan, S. S.
1986MNRAS.219..357H Altcode:
The intricate nature of oscillatory motions in intene flux tubes is
examined and states in which the ratio of gas to magnetic pressure
(Beta) is constant are considered. Results are presented for both
polytropic and real atmospheres by means of linear analysis; in the
latter case, a height-dependent radiation exchange time constant
is used. For purposes of comparison with earlier studies, results
for the adiabatic case are also given. These results indicate that
states, characteristicaly stable in the adiabatic limit, can be
driven overstable when heat exchange is included in the analysis. It
was found that for solar stratification, oscillatory behavior occurs
for Beta less than Beta<SUB>c,</SUB> where Beta<SUB>c</SUB> denotes
a (tube) radius-dependent critical value. A bifurcation at Beta =
Beta<SUB>c</SUB> was also found owing to overstability into two
purely unstable modes. The height dependence of the eigenvectors
is discussed along with the sensitivity of the results on boundary
conditions. Observational consequences of the study are also noted.
---------------------------------------------------------
Title: One-Dimensional Model Calculations of Flux Tubes
Authors: Hasan, S. S.
1986ssmf.conf..121H Altcode:
The author presents 1-D model calculations of intense flux tubes on the
Sun. Assuming an initial state in hydrostatic and thermal equilibrium
(with the ambient medium), he models the collapse of such a tube due
to a convective instability by numerically solving the time dependent
MHD equations in the thin flux tube approximation. The results indicate
that the collapse produces a final state with field strengths in the
kG range. This state is not steady, but one exhibiting overstable
oscillations (in time). Furthermore, flows are generated in the
tube during the collapse with a peak amplitude 1 - 2 km/s, but with a
time-averaged value around zero. Some of the observational implications
are discussed.
---------------------------------------------------------
Title: Heating of solar magnetic elements by downflows
Authors: Hasan, S. S.; Schuessler, M.
1985A&A...151...69H Altcode:
The idea that magnetic elements in the photosphere and lower
chromosphere of the sun are heated by downflowing gas is quantitatively
examined. The time-dependent hydromagnetic equations are solved
numerically in the slender flux tube approximation. Viscous terms are
retained, and the radiative exchange of heat between the flux tube
and the ambient medium are included. Hydrogen ionization and its
thermodynamic consequences are treated self-consistently. Starting
from a state of hydrostatic and thermal equilibrium, the temporal
response due to the onset of a downflow in the tube is studied. After a
transient phase lasting a few minutes, a stationary state results that
is substantially hotter than the ambient medium over a fairly large
height range. Chapman's facular model can be reproduced remarkably well
by adjusting the mass flux entering the tube at the upper boundary. The
results are comparatively insensitive to viscosity (nu less than or
equal to 10 to the 12th sq cm/s), while radiative heat exchange is
significant. Some observational implications are discussed, and it is
suggested that the necessary mass flux could be provided by overstable
oscillations during their downflow phase.
---------------------------------------------------------
Title: Convective instability in a solar flux tube. II - Nonlinear
calculations with horizontal radiative heat transport and finite
viscosity
Authors: Hasan, S. S.
1985A&A...143...39H Altcode:
Convective instability in a thin flux tube is examined in the presence
of horizontal radiative heat transport and finite viscosity. The
temporal behaviour of flux tubes initially in hydrostatic and thermal
equilibrium is studied by solving the nonlinear time dependent equations
for a thin flux tube. An important result of the investigation is the
demonstration of overstable oscillations with periods typically about
1000s in intense flux tubes on the Sun. Overstability is a direct
consequence of radiative exchange between the flux tube and the
surrounding medium. Detailed calculations are presented for a broad
range of parameters which characterize the strength of the magnetic
field and the tube radius in the intial state. An initial stratification
based on a model atmosphere is used. The results indicate that that
overstable oscillations are produced as an end state of the convective
instability. It is found that the surface magnetic field that results
is in the observed kG range. Furthermore, there is also an oscillating
flow (unidirectional at any given instant) with an amplitude between
1 and 2 km s<SUP>-1</SUP>, with an average value that is approximately
zero. The observational implications of these results are discussed.
---------------------------------------------------------
Title: Convective collapse and overstable oscillations in solar
flux tubes.
Authors: Hasan, S. S.
1984ESASP.220..227H Altcode: 1984ESPM....4..227H
The collapse of solar flux tubes by a convective instability is
investigated. The final state consists of overstable oscillations with
periods typically around 1000 s.
---------------------------------------------------------
Title: Time-dependent convective collapse of flux tubes
Authors: Hasan, S. S.
1983IAUS..102...73H Altcode:
The time-dependent collapse of a slender flux tube extending vertically
in the convection zone of the sun is modelled. Starting from an
initial state in which the flux tube is in hydrostatic equilibrium, the
non-linear MHD equations are used to examine its temporal evolution. A
detailed study of the flow variables and magnetic field within the tube
is presented. It is seen that asymptotically in time a unique state of
dynamic equilibrium is established, irrespective of the value of beta-O
(the ratio of the thermal to magnetic pressure at the initial epoch).
---------------------------------------------------------
Title: Transient Response of the Solar Wind to Changes in Flow
Geometry - Flows in Coronal Holes
Authors: Hasan, S. S.; Venkatakrishnan, P.
1982SoPh...80..385H Altcode:
The transient response of the solar wind to changes in geometry is
examined. An initial stationary flow in a configuration that diverges
as r<SUP>2</SUP> is assumed. This state corresponds to the usual solar
wind solution. The effect on the flow through a tube whose area A(r,
t) diverges faster than r<SUP>2</SUP>, with the degree of divergence
increasing in time, is considered. The asymptotic form of A(r, t)
is chosen to mimic the form inferred in coronal holes. A detailed
parameter study relating the form of A(r, t) to the pattern of flow in
the tube is presented. It is observed that in the limit of large time
(large compared to τ, the time constant for change in geometry of a
flow tube) the solutions obtained from a time-dependent analysis can
depend upon τ. For sufficiently large τ, the asymptotic solution is
the same as the steady state solution obeying the correct boundary
conditions and possessing a smooth sonic transition. However, if
the geometry changes rapidly enough, solutions exhibiting shock-like
discontinuities can also exist. This is essentially a new feature that
emerges from the present investigation. Finally, it is suggested that
this study may be useful in describing flows in evolving coronal holes.
---------------------------------------------------------
Title: Comment on the paper `a new resonance in the solar atmosphere'
by Joseph V. Hollweg
Authors: Venkatakrishnan, P.; Hasan, S. S.
1982SoPh...75...79V Altcode:
In the absence of genuine forcing terms, there is no resonance between
linear fast mhd and gravito-acoustic waves.
---------------------------------------------------------
Title: A Time Dependent Model for Spicule Flow
Authors: Hasan, S. S.; Venkatakrishnan, P.
1981SoPh...73...45H Altcode:
A time dependent model for the flow of gas in a spicule is studied. In
this model, the flow occurs in a magnetic flux sheath. Starting from
hydrostatic equilibrium, the flux sheath is allowed to collapse normal
to itself. The collapse induces a flow of gas along the magnetic field
and this flow is identified as a spicule. A variety of sheath geometries
and velocity patterns for the normal flow have been studied. It is
observed that a large curvature in the field geometry and a large
initial value for the normal flow are necessary to achieve spicule-like
velocities. The duration for which a large velocity of normal flow is
required is much shorter than the average lifetime of a spicule. It is
proposed that the initial rapid collapse occurs during an `impulsive
spicule' phase and it is the subsequent gradual relaxation of the flow
which is observed as a spicule.
---------------------------------------------------------
Title: Time-dependent interaction of granules with magnetic flux tubes
Authors: Venkatakrishnan, P.; Hasan, S. S.
1981JApA....2..133V Altcode:
The time-dependent interaction of the granulation velocity field with
a magnetic flux tube is investigated here. It is seen that when a
magnetic field line is displaced normal to itself so as to simulate
the buffeting action of granules, a flow of gas is initiated along the
field. By choosing a lateral velocity field which is consistent with
observations of granules, it is found that the resulting gas motion is
a downward flow with a velocity compatible with the observed downflow
in isolated photospheric flux tubes. It is therefore proposed that the
observed photospheric downflow is a manifestation of the interaction
of granules with flux tubes.
---------------------------------------------------------
Title: A time dependent model for spicule flow.
Authors: Hasan, S. S.; Venkatakrishnan, P.
1981BASI....9...74H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetohydrodynamic Equilibrium and Stability of Pre-Flare
Loops - Constant Pitch Field
Authors: Hasan, S. S.
1980SoPh...67..267H Altcode:
The equilibrium and stability of a loop in which energy storage occurs
prior to a solar flare is discussed. Working on the hypothesis, that
the onset of the flare begins only after sufficient magnetic energy has
been stored in the loop typical values of parameters which describe the
equilibrium are found for a magnetic field with a constant twist. The
stability of this configuration is examined next and it is shown that
for the force-free case, the structure is always unstable to kinking for
any degree of twist. However, a slight deviation from the force-free
configuration, through the presence of a small positive transverse
pressure gradient, can stabilize the loops for moderate degrees of
twist. The range of wave-numbers for which instability occurs and the
maximum growth rates are also presented. Lastly, it is shown that the
pressure gradients required to stabilize a pre-flare loop do not lead
to conflict with observations.
---------------------------------------------------------
Title: Flow of Gas Along a Magnetic Field with Time Dependent Geometry
Authors: Hasan, S. S.; Venkatakrishnan, P.
1980KodOB...3....6H Altcode:
The flow of gas along a magnetic field with time dependent geometry
has been studied. It is seen that the velocity of the flow in the
direction of the magnetic field depends both on the magnitude of the
velocity of flow perpendicular to the field as well as on its spatial
variation. Further, the nature of the flow is not very sensitive to
the choice of base temperature and polytropic index. The application
of this study to magnetofluid dynamic flow on the Sun is discussed.
---------------------------------------------------------
Title: A Magneto-Hydrodynamic Study of Pre-Flare Loops.
Authors: Hasan, S. S.
1979phsp.coll..233H Altcode: 1979IAUCo..44..233H; 1979phsp.conf..233H
The magnetohydrodynamic equilibrium and stability of a configuration
which may apply to solar pre-flare loops is analyzed based on the
following scenario: (1) the loops exist much prior to the flare and
in equilibrium with their surroundings; (2) a few hours before the
flare the configuration gradually acquires energy in the form of
currents; (3) during the period of energy build-up, the loops are
magnetohydrodynamically stable; and (4) the flare occurs only after
there is adequate energy in the currents. Assuming a cylindrical
geometry, equations are then presented and solved for the pressure
and magnetic field equilibrium distribution within the loop using
a typical energy value associated with a subflare and assuming the
case of an approximately force-free field. An equation for the MHD
stability of the system is then solved as an eigenvalue problem for
the frequency of the normal modes. It is shown that the force-free
configuration is unstable for all cases considered, with the wavenumber
region for instability increasing inversely with pitch. It is concluded
that stable configurations for loops possessing adequate energy for a
flare are possible only if positive pressure gradients of sufficient
magnitude exist.
---------------------------------------------------------
Title: The Alfvén-Carlquist Double-Layer Theory of Solar Flares
Authors: Hasan, S. S.; Ter Haar, D.
1978Ap&SS..56...89H Altcode:
We use the Vlasov equations for ions and electrons to develop a theory
of a double layer in which there are both free and trapped electrons
and ions. We find the equations which replace the Langmuir condition
and the Bohm conditions and by numerically solving the resultant
differential equation we find for particular choices of distribution
functions the potential distribution in the layer. We discuss the
applicability of this theory to solar flares, and show that conditions
in solar flares may be such that double layers can exist for which the
free particles have a power-law energy distribution. These particles
will be accelerated in a double layer and may in this way account for
the production of high-energy particles during the impulsive phase of
solar flares.