The 19th NSO/SP summer workshop was held in Sunspot, NM from 28 September to 2 October 1998. The topic was High Resolution Solar Physics: Theory, Observations, and Techniques. High-resolution angular, spectral, and temporal observations of the Sun are required in order to answer many important questions in solar and stellar physics. Furthermore, the Sun is the only star that can be spatially resolved well enough to allow detailed study of fundamental astrophysical processes such as magneto-convection.
Previous NSO/SP summer workshops on high-resolution solar observations were held in 1980, 1988. and 1992. It was evident during this year's workshop that substantial progress continues to be made in the solar astronomer's quest for observations of ever higher angular resolution, but much remains to be accomplished. The proceedings of the 1998 workshop will review the current state of high-resolution solar physics from the theoretical, observational, and modeling perspectives, and the state-of-the-art of instrumentation used to achieve high-resolution solar observations.
About 75 scientists from around the world participated in this workshop, with nearly half from countries outside the US, including Austria, China, Germany, India, Ireland, Italy, Japan, The Netherlands, Norway, Russia, Spain, and Sweden.
The workshop was dedicated to Richard B. Dunn, who recently retired from the NSO. Dunn's pioneering contributions in engineering, design, and observation have done much to lay the foundations of present-day high-resolution solar physics. A highlight of the workshop was the ceremony during which the Vacuum Tower Telescope at Sacramento Peak, one of the finest high-resolution solar telescopes in the world, was renamed the Richard. B. Dunn Solar Telescope, after its creator.
The workshop's keynote address was presented by J. Thomas (University of Rochester) who reviewed the scientific case for high-resolution solar physics, giving examples of important solar phenomena occurring at length scales at or below the current limit of angular resolution of about 0.2". His conclusion was that both theoretical considerations and recent observational successes bolster the case for further effort and investment in high-resolution solar telescopes and instrumentation.
The scientific case for a new, large-aperture solar telescope was further developed and restated in several workshop contributions (e.g., C. Keller, NSO/Tucson). A large aperture is required not only for improved angular resolution, but also just to collect enough photons. The high spectral and temporal resolution and polarimetric accuracy needed to study small-scale solar phenomena are the main drivers for apertures of 2-4 m for new generation solar telescopes. This theme also featured in the final summary talk by A. Title (Lockheed-Martin Solar and Astrophysics Lab), who argued that the fundamental astrophysical processes observed on the Sun at small scales are of importance to non-solar astronomers and plasma physicists, a fact that the solar community needs to convey more effectively to the broad astronomical community.
O. Steiner (Kiepenheuer Institut für Sonnenphysik, Germany), and M. Rast (High Altitude Observatory) described advances in numerical modeling techniques that are now driving the field to achieve observations of extremely high resolution. Steiner described the rather few key high-resolution experiments that have dramatically changed our understanding of the nature, formation, and evolution of flux tubes, and how the lack of high-resolution observations has prevented us from exploring this new frontier. Rast described simulations that explain granular convection as a phenomenon driven by radiative cooling effects at the very surface of the Sun. In these simulations, solar oscillations are excited by acoustic noise generated in down-drafting plumes just beneath the solar surface. P. Goode (New Jersey Institute of Technology/Big Bear Solar Observatory) presented observations which show that acoustic noise is indeed generated in the intergranular lanes where the strongest downdrafts occur.
An interesting but controversial idea, which would completely alter our current picture of sunspot penumbrae, was presented by J. Sanchez-Almeida (Instituto de Astrofisica de Canarias, Spain), who proposed that the actual size of penumbral filaments is as small as 1-10 km.
A new small-scale, weak-field component covering a large fraction of the solar surface was revealed by high-resolution polarimetric observations in the near infrared (Lin, NSO/Sac Peak). The inferred field strengths are of order 500 G or smaller; such field strengths can only be measured accurately using infrared lines at the spatial resolution currently available. The developing field of far infrared solar physics was featured in presentations by T. Ayres (University of Colorado) and D. Gezari (NASA/GSFC).
A major highlight of the meeting was the presentation of new results from NASA's Transition Region and Coronal Explorer (TRACE) satellite by A. Title and T. Berger (Lockheed-Martin Solar and Astrophysics Lab). TRACE was launched on 2 April 1998 and has since produced over 700,000 images of superb quality. The TRACE observations indicate an enormous amount of fine structure with actual spatial scales that are most likely below the 750-km resolution of TRACE in the transition region and corona. Movies produced from TRACE observations show the dynamic nature of the solar corona with its emergent and interacting loops, including a spectacular example of an oscillating post-flare loop system.
A powerful tool to study the highly dynamic upper solar atmosphere is provided by combining observations from different instruments such as TRACE, SoHO and ground-based observatories. Examples of such observations were presented by P. Brekke (Institute of Theoretical Astrophysics, Norway), who showed movies of jets and eruptions observed in the transition region. Progress in the field of flare physics and prediction of solar flares was reviewed by S. Keil (Air Force Research Laboratory) who demonstrated that dynamical motions measured in the chromosphere using Ha images may constitute precursors to flare activity and may aid in the attempt to predict flares.
An entire day of the workshop was dedicated to discussions of new instrumentation, tools and techniques. The current status of the Dutch Open-Air Telescope (DOT) on La Palma was summarized by R. Rutten (Sterrekundig Instituut, The Netherlands). The DOT saw first light about one year ago and has already produced some exciting high-resolution imagery. The experience with the DOT's open-air telescope design is particularly relevant for future solar telescopes which will have apertures of 2-4 m. For such large telescopes, the entrance windows used in existing telescopes may no longer be feasible.
Technical aspects of a ground-based Advanced Solar Telescope of about 3-m aperture were discussed by J. Beckers (NSO/Sac Peak). Although high-resolution observations are best performed from space, the cost of a 3-4 m space solar telescope makes it unlikely that such a telescope will be launched in the near future. A crucial technology for high-resolution solar observations from the ground with large aperture telescopes is adaptive optics. Substantial recent progress in the design and development of solar adaptive optics was described during the workshop by T. Rimmele (NSO/Sac Peak) and M. Shand (Compaq-Digital, France). Results of the first successful bench tests with a solar adaptive optics system based on a correlating Shack-Hartmann wavefront sensor were presented by Rimmele.
Major progress has been made recently with image reconstructive techniques such as phase diversity, described by J. Sheldon (ERIM), A. Tritschler (Kiepenheuer Institut für Sonnenphysik, Germany), M. Löfdal (Royal Swedish Academy of Sciences), and M. Koschinsky (Universitäts Sternwarte Göttingen, Germany), and speckle reconstructive techniques, presented by C. Keller (NSO/Tucson) and C. Denker (Big Bear Solar Observatory).
G. Scharmer (Royal Swedish Academy of Sciences) presented plans to upgrade the 50-cm Swedish Solar Vacuum Telescope on La Palma to approximately one-meter aperture. Efforts to modernize existing telescopes or to build new facilities and instruments to achieve high-resolution observations were described by workshop participants from Russia (A. Boulatov, Institute of Solar-Terrestrial Physics), China (B. Ye, Beijing Astronomical Observatory), India (V. Verma, Uttar Pradesh State Observatory), Germany (O. v.d. Lühe, Kiepenheuer Institut für Sonnenphysik), and Spain (V. Martinez-Pillet, Instituto de Astrofisica de Canarias). P. Bernasconi (John Hopkins University/APL) summarized the Flare Genesis experiment, which will attempt high-resolution polarimetry from an 80 cm balloon-borne telescope circling Antarctica at stratospheric altitudes, thus avoiding the adverse effects of atmospheric seeing.
A major effort of the international solar physics community for the next decade will be the Solar-B space mission. After its launch, approximately in the year 2004, the 50 cm optical telescope on board Solar-B will provide continuous imaging data and polarimetric observations of the Sun with a diffraction limited resolution of 0.2". Y. Suematsu (National Astronomical Observatory, Japan) described the scientific goals of the mission, post-focus instrumentation plans, and the status of the project.
The Workshop was sponsored by the National Solar Observatory, the National Optical Astronomy Observatories, U.S. Air Force Office of Scientific Research through its European and Asian Offices, the National Science Foundation, and the National Aeronautics and Space Administration. The proceedings will be edited by T. Rimmele, K. S. Balasubramaniam, and R. Radick and published in the ASP Conference Series.
Thomas Rimmele