Other DOT science goals consist of studying the emergence and decay patterns of solar magnetic fields, the photospheric-chromospheric dynamic connection in non-magnetic regions, the fine structure of plage and sunspots, and the fine structure of prominences.
More detail is given below, especially for tomographic "proxy magnetometry" which constitutes the principal DOT science strength.
In terms of pure science, solar magnetism provides the Rosetta stone of cosmic magnetism, not only in other stars but also in accretion disks, cataclysmic variables, galactic dynamics, active galactic nuclei and other cosmic objects in which MHD and plasma processes control the structure and energy partitioning. The sun is close enough to study these processes in observable detail.
Terrestrial plasma confinement machines do not reach the scales, densities and temperatures exhibited in the solar atmosphere. Solar physics complements plasma instability studies in fusion research, the sun representing a non-terrestrial plasma physics laboratory.
Space weather (the solar influence on the near-earth environment and the Earth's climate) is set by solar magnetism. Solar activity modulation affects satellite orbits, influences jet stream patterns and contributes to the causes of minor, possibly major, ice ages.
Solar surface magnetism is the key for advance in these research areas. At the surface, the magnetic field wins from the gas pressure: the plasma beta parameter (ratio of gas to magnetic pressure) flips from large to small across unity, so that the field role switches from being dominated by gas motions to dominating gas motions. At the solar surface, the field displays patterning imposed by the subsurface dynamo and convective flows while, at the same time, it controls flows and wave motions to the outer atmosphere. This flip in domination makes it desirable to observe the magnetic field patterning simultaneously in the photosphere and the overlying chromosphere. The DOT is specifically suited to such solar surface magnetometry with high spatial (angular) resolution.
The solar atmosphere changes dramatically between different regimes and presents drastically different scenes to the terrestrial observer at different wavelengths. The solar photosphere, defined as the layer where the bulk of the electromagnetic radiation escapes as visible light (a dramatic transition from near-equilibrium photon enclosure, killing off the subsurface convection into the shallow pancake pattern called granulation) is also the layer where magnetic fields take over from gas dynamics in dictating the structuring and in supplying the key processes, and it is also the outermost layer where the sun may be regarded as spherical in zero-order approximation. The chromosphere is magnetically split into network and internetwork in quiet regions and is very finely structured above active regions. The transition to the corona consists of tiny fibrils with much variation in length, inclination, and ordering. The closed-field parts of the corona outside coronal holes are made up of bundles of very thin coronal loops. Yet unidentified processes supply energy to the gas in these loops, reaching a balance against cooling by X-ray photon losses at temperatures of 1-2 million K.
The loops are magnetically anchored (in unknown fashion) to the strong-field flux tubes that break out of the photosphere and respond dynamically on a wide variety of timescales to the footpoint forcing. These magnetic connections between very disparate regimes, from gas dynamics via magnetohydrodynamics to plasma physics and from LTE radiation enclosure to X-ray photon drain, require simultaneous study of structures, processes and radiation in the photosphere, chromosphere, transition region and corona. This calls for tomographic diagnostics sampling the different regimes at the same time.
Reference: "Solar Surface Magnetism", Edited by R.J. Rutten and C.J. Schrijver, NATO ASI Series C433, Kluwer, Dordrecht
This long-duration high-resolution multi-wavelength imaging capability presents a unique opportunity within solar physics to study the horizontal topology and vertical structure of solar magnetic fields. Such tomographic high-resolution imaging is also desirable as context information to high-resolution spectropolarimetry at other telescopes employing adaptive optics, and to coronal field mapping using ultraviolet and X-ray image sequences from space. This section discusses DOT tomographic imaging in terms of its "proxy magnetometry" information content in some depth.
Solar surface magnetism consists of a remarkable hierarchy of discrete strong-field structures. The basic entity is the flux tube, a key concept of MHD astrophysics. Solar flux tubes have tiny cross-sections (0.2 arcsec) but they have nevertheless become observable with Canary-Island seeing quality, particularly in the Fraunhofer G-band around 4305 Å. At this wavelength, the flux tubes show up as tiny bright points as in the magnified inset in this DOT image. At high angular resolution, G-band imaging provides a useful proxy diagnostic for charting photospheric flux tube patterns and their development (see Title & Berger 2001).
The brightness of the intergranular magnetic elements is explained by the "flux tube hot wall" effect (e.g. Spruit & Zwaan 1981): the tube is relatively empty because the inside magnetic pressure balances the outside gas pressure, and so it acts as a viewing tube through which radiation escapes from layers below the outside surface, with the hot tube walls producing brighter emission than the surroundings. In the G band, around 4305 Å (430.5 nm, labeled G by Joseph von Fraunhofer in his 1814 inventory of the solar spectrum) this contrast is enhanced because the CH molecules which make up most lines in this dark feature dissociate in the flux tube, so that the CH lines vanish and the tube gas gains even more transparency compared to the outside gas than at other wavelengths (see Rutten et al. 2001).
The intrinsic sharpness of the photospheric flux tubes is very high because they are very thin and the G-band photons are emitted thermally and are not much spread by scattering on their way out. At 100 km resolution most resulting G-band bright points are not resolved, but at least they are identifiable so that they can be located and traced in time. At somewhat lower resolution (say 0.5 arcsec) they vanish (because they are mostly located within dark intergranular lanes so that smearing by atmospheric seeing cancels bright against dark, see Title & Berger 1996).
Higher up in the chromosphere, the Ca II H & K lines of once-ionised calcium and the Balmer Halpha line also provide valuable proxies for mapping magnetic topology. The low chromosphere sampled by Ca II H or K shows clusters of bright grains outlining the magnetic network and greyish internetwork features marking acoustic oscillation interference patterns. The high chromosphere sampled by Halpha portrays the intricate structure of the magnetic canopies expanding from the network in the form of fine fibrils.
The Ca II H & K lines are the strongest Fraunhofer lines and sample the chromosphere at about 1000 km above the white-light surface. At this height, the magnetic network is enhanced by yet unidentified heating processes so that the Ca II line-center intensity (at about 0.1 nm bandwidth) provides an excellent magnetogram proxy (but unsigned). This fact has been exploited extensively in gauging the magnetic activity of sun-like stars.
The sharpness of Ca II H or K images is intrinsically less than for the G band, partially because the line photons are scattered considerably on their way out before their final escape towards the observer, and partially because the flux tubes expand and merge with height. The same regime and the same patterning are also sampled by imaging in the near-UV continua, but these scatter as badly.
The Balmer Halpha line comes from the most abundant element but is much less strong than Ca II H & K in the spatially averaged solar spectrum because its lower-level population has very low weight in the Boltzmann population partitioning over the hydrogen energy levels. Nevertheless, its high excitation energy causes this line to respond to gas at high temperature and also to shocked cool gas, so that it maps low-lying fibrils in the high chromosphere when these are sufficiently dense. This regime has often been modeled as a spherical shell, but a high-resolution Halpha movie immediately shows the fallacy of such modeling by displaying a mass of fibrils with no semblance of sphericity whatsoever. The "moss" phenomenon discovered with TRACE indicates that 2 million-K plasma actually descends down to between the Halpha fibrils in plage (see Berger et al. 1999).
It is becoming increasingly clear that the chromospheric fibril topology mapped by Halpha supplies a missing link between photospheric magnetograms (such as from MDI onboard SOHO and from HMI onboard SDO) and the coronal field topology. Nonlinear force-free-field extrapolation from such magnetograms is much better determined and much more realistic when Halpha fibril topology is entered as a constraint in the extrapolation procedure (see Bobra et al. 2008). It is probable that such Halpha insertion in field topology estimation will be decisive in setting up reliable techniques for forecasting solar eruptions and near-earth space weather.
Using Halpha as magnetograph proxy to derive the field topology from the observed fibrils is not straightforward because the latter harbor length-wise flows that modulate the apparent fibril contrast through substantial Dopplershifts. The resulting mix of brightness and Dopplershift variations requires full profile modeling, so that filtergrams must be taken at a number of wavelengths and interpreted through inversions based on sophisticated radiative transfer modeling.
The intrinsic Halpha resolution can be exceedingly high because the fibrils may be effectively or even optically thin, imposing their scale on the emergent radiation without radiative-transfer smearing. This is particularly the case in filaments and prominences (the latter are filaments seen off-limb where the background radiation along the line of sight vanishes). These amazing structures, keeping very cool gas up amidst the hot corona and persisting very long, are rich sources of MHD physics. They probably consist of very thin, highly dynamic magnetic fibrils in complex topologies that are best encoded in Halpha radiation.
The DOT is operated from the SST building in close cooperation with the Swedish collagues. The Swedish 1-m Solar Telescope (SST) achieves high-resolution imaging, spectrometry and polarimetry using adaptive optics. The two telescopes are complementary. Together, they represent a formidable facility for high-resolution solar physics.
Specific DOT research topics:
Here is a high-resolution Dopplergram which resulted from a test of a of a special tunable filter from Irkutsk. The test was done at the former SVST and is described here in detail. This filter selects a tunable narrow band in the Ba II 4554 line which possesses enhanced sensitivity to non-thermal motions due to the large atomic mass of barium atoms.
The test showed that a new domain of flow mapping opens up at the high resolution obtainable with this line, speckle restoration and La Palma image quality. At this unprecedented Dopplergram resolution, it becomes possible to chart flows in and around small basic structures such as the magnetic elements that make up the chromospheric network, in and between the fibrils making up penumbrae, and along the thin threads constituting filaments and prominences.
We have, thanks to support from INTAS and the Pieter Langerhuizen Lambertuszoon Fonds, installed the Irkutsk filter on the DOT. It was then found that it needed complete overhaul, for which R.H. Hammersclag and F.C.M. Bettonvil took it to Irkutsk in 2008. It operates on the DOT since June 2009, but the poor camera used (for lack of funding) limits the signal-to-noise ratio severely.
The DOT is particularly suited for such magnetometry. First, it provides the required angular resolution. Second, because it is a reflector the focus is co-spatial at all wavelengths. Third, the parallactic mount and the absence of image rotation make it suited to high-precision polarimetry. Fourth, its high pointing precision permits astrometric tracking in solar coordinates. Finally, the projected aperture increase will give it the high sensitivity needed for precision polarimetry.
We had hoped to turn the Irkutsk Ba II 4554 filter into a Stokes vector magnetograph by adding liquid-crystal retarders, but the present camera is too noisy to make this a realistic extension.
Our DOT development plan was to install remote targeting, integration into the worldwide Virtual Solar Observatory, aperture tripling (resolution down to 0.07 arcsec), field tripling (up to 300 arcsec), or yet larger upgrades. More detail is given below. Obviously, realization depends on renewed sponsoring,
The DOT team is very small but there is no shortage of ideas to exploit and enhance the unique DOT capacity for high-resolution solar physics. Unfortunately, the speed of realisaton is limited by funding and manpower - the DOT funding is much lower than for comparable solar telescope projects elsewhere, and much much lower than for comparable solar telescopes in space.
The following projects constitute our original DOT to-do list, roughly in time order. The first two were completed with respect to the DOT, but have new life with respect to future telescope projects such as the EST. Currently the DOT fulfills projects 3 through 5 but only when its usage is funded. Project 6 materialized with the installation of the Irkutsk Lyot filter for Ba II 4554 in June 2006 but its camera is too nosiy for project 7. We have no manpower for project 8. Projects 9 to 11 remain dreams awaiting sponsoring.
(1) Verification of DOT technology
The initial DOT post-focus equipment was limited to the verification imaging needed to prove that the DOT fulfills its design criteria. The completion and the installation of the DOT were funded by the Dutch Technology Foundation STW on the basis of the technological innovations that make the DOT a stable pointer even while exposed to strongly varying wind loads. This new (mostly mechanical) technology has indeed been demonstrated to work. There are no windshake problems, and the telescope, tower and clamshell canopy have survived multiple fierce La Palma winters without problem.
(2) Verification of the open principle
From the solar physicist point of view, DOT verification consists of testing its open principle. The DOT is the first solar telescope that relies on the ambient wind (often very strong at La Palma) to inhibit image-spoiling turbulence within the telescope. The DOT's demonstration that this strategy of wind flushing works has been a pivotal factor in the current wave of large open-telescope projects, in particular GREGOR, NST and ATST, and also inspires the GISOT concept.
(3) Proxy magnetometry
The initial observing program consisted of proxy magnetometry as described above, using imaging in the G-band to chart magnetic field structures and patterns and their evolution in the photosphere. We added imaging in Ca II H to do the same for the low chromosphere, and narrow-band imaging in Halpha to do the same for the upper chromosphere. The bandwidths are 1 nm for the G-band, 0.3 nm for Ca II H and 0.025 nm for Halpha. The first two permit the use of simple interference filters but the Halpha passband must not only be narrow but also rapidly tunable. The DOT team has a high-quality Halpha Lyot filter that was earlier employed at the Ottawa River Solar Observatory. It is mounted adjacent to the incoming beam (see DOT top design drawing) and secondary optics design drawing. This program aims in particular at obtaining high-resolution image sequences in conjunction with the MDI magnetograph and UV spectrometers onboard the SOHO mission, the high-resolution UV imaging by the TRACE mission, the photospheric diagnostics of the Hinode mission, and the upcoming large-volume solar monitoring of the SDO mission.
(4) Image restoration
High angular resolution is the science driver for the DOT - as for most other solar telescopes excepting helioseismology. Thanks to the combination of the wind-swept site, the wind-flushed open tower and telescope, and the consistent application of speckle restoration, the DOT furnishes diffraction-limited image quality whenever the seeing is only reasonably good, i.e. with Fried parameter (effective resolution permitted by the Earth's atmosphere expressed in corresponding telescope diameter) of order 6-10 cm. A major advantage of speckle processing over the adaptive-optics systems being developed for future solar telescopes is that speckle reconstruction restores the full field of observation, not just a single isoplanatic patch. The major disadvantage is that it takes a large amount of post-processing. This was remedied with the DOT speckle processor.
(5) Common-user operation
In 2004 we opened the DOT as common-user facility to the international solar physics community. Many solar observing programs at other telescopes (both groundbased and in space) can then benefit from high-resolution context imaging with the DOT. Details: White paper on future DOT observing modes.
(6) DOT data search engine
Completed in 2008 by T. van Werkhoven, see http://dotdb.strw.leidenuniv.nl/search.
(7) Doppler mapping
A test done at the SVST in its last season (summer 2000) demonstrated the tantalizing science capability of the special tunable Lyot filter operating in the Ba II 4554 that was developed at Irkutsk by V.I. Skomorovsky and G.N. Domishev. They brought it to the SVST where it was combined with the DOT speckle acquisition system, on funding from SOZOU, LKBF and NOVA. The superb angular resolution obtained by the speckle processing and the large sensitivity of the Ba II 4554 line to motions (due to the combination of very large atomic mass and insensivity of the line opacity to temperature variations) combine to Dopplergrams of extraordinary information content as detailed here. An accommodation study has shown that the DOT can harbour this large and heavy Lyot filter with telecentric re-imaging optics within the telescope top (design drawing). Funding to realize its installation was been obtained from INTAS, the Pieter Langerhuizen Lambertuszoon Fonds, and SOZOU, but then it was found that the filter needed extensive refurbishment, for which R.H. Hammerschlag and F.C.M. Bettonvil took it back to Irkutsk in 2008. It operates on the DOT since June 2009 but the camera is too noisy for science uitlization including extension into a magnetograph.
(8) Stokes magnetometry
The development and installation of a Stokes vector magnetograph is a natural desire. Stokes vector magnetometry requires spectral selection of one or more narrow bandpasses within one or more Zeeman-sensitive lines. Our intention was to outfit the Irkutsk Ba II 4554 birefringent filter with ferro-electric liquid crystal retarders but the present camera is too bad. Another option for a compact instrument fitting the cramped DOT top is to use a multiple Fabry-Perot interferometer. A larger-scale option is the use of optical fibers to transfer the image to a classical grating spectrometer for two-dimensional small-field spectrometry by using a fiber field-to-slit reformatter combined with phase-diverse data acquisition.
(9) Remote targeting
Many international multi-telescope campaigns address phenomena related to solar activity and therefore require selection of the common field of view on short to very short notice (the latter for example when observing flares or filament eruptions). Co-pointing diverse instruments at different telescopes is therefore greatly assisted by facilities for (nearly) instantaneous remote targeting. At the DOT, such a remote pointing interface can be a low-bandwidth emulation of the actual telescope control interface in the DOT control room (already remote itself). It will also enable staff elsewhere to look over the shoulder of observers on La Palma. Details are also given in the white paper on future DOT observing modes.
Triple aperture increase
The mechanical structure of the DOT accepts a larger mirror than the current 45 cm one, at least 140 cm without replacing the telescope mount nor the multi-wavelength secondary optics and camera system. A detailed design has been made to verify the opto-mechanical feasibility (optics diagram). A new light-weight parabolic primary mirror of 140 cm diameter will feed the existing multi-wavelength recording system via new beam folding and reformatting optics. A super-reflective, water-cooled field stop with air suction around it in prime focus reflects most of the solar image and passes the field of view to the relay mirrors, of which the first one is parabolic and cancels the coma of the parabolic primary, producing a large field of view. A telecentric region can harbour polarisation encoders for magnetometry. A choice of beam reformatting lens combinations and a fixed pupil stop permits flexible user-selectable trade-off between resolution and field size, producing a 1:45 beam into the multi-wavelength system just as in the present 45-cm DOT. The projection will be via a low-order adaptive optics system which serves to increase the signal-to-noise ratio of the speckle processing. The G-band channel will no longer be on-axis so that the central obscuration will be small. The beam reformatter permits flexibility in usage. At superb seeing, when speckle restoration will work down to the 0.07 arcsec diffraction limit of the 140-cm aperture, the aperture tripling will fully regain the DOT's role of tomographic imager even in the Hinode era -- and actually enhance Hinode through co-observing: the DOT can then image at 0.07 arcsec (50 km on the solar surface, equal to the photon mean free path in the photosphere) what Hinode diagnoses at 0.2 arcsec. When the seeing is less good one opts for lower resolution but a concomitantly larger field of view on the camera chips. A pupil shifter then permits selection of an obscuration-free off-axis aperture.
Triple field increase
The introduction of a parabolic secondary mirror will cancel coma from the parabolic primary which presently limits the useful DOT field of view to three arcmin. Since much larger CCD chips with the 10 frames/s readout speed needed for speckle burst registration should become affordable with time, we intend later in this decade to revamp the DOT cameras and speckle pipeline with state-of-the-art hardware and so increase the field of view considerably at any resolution. For example, during non-superb seeing 4K x 4K chips would register 300 x 300 arcsec at 0.2 arcsec resolution and enable studies of the topology and dynamics of whole active regions including complete coronal loop anchoring. When the seeing turns excellent - as flagged by our reliable Seykora-Beckers scintillometer - shift to 0.07 arcsec resolution then reduces the field to 100 x 100 arcsec, still large enough to contain a complete mature sunspot with its moat. Increasing the disk storage (or accelerating the speckle processing to real-time turnaround) will increase the maximum sequence duration.
Upgrade options beyond tripling
Preliminary designs for yet larger-scale upgrades of the telescope, the tower, and the platform are described in Aperture Increase Options for the Dutch Open Telescope.
Rob Rutten 2018-01-16