Homepage of the 20th NSO/SP Summer Workshop

Advanced Solar Polarimetry - Theory, Observation, and Instrumentation

11. - 15. September 2000



Workshop Proceedings:

20th International Sacramento Peak Summer Workshop
Adcanced Solar Polarimetry - Theory, Observation, and Instrumentation
ASP Conference Series, Vol. 236, 2001
M. Sigwarth, ed.
 
 
 

 
 
Press Release from August 31, 2000
 
 
 
 
Group picture of the Workshop participants in front of the Sunspot Astronomy & Visitor Center where the meeting took place from 
Sept. 11 - 15, 2000

 
NSO 20th Annual International Summer Workshop
Michael Sigwarth

The 20th NSO/SP Summer Workshop on Advanced Solar PolarimetryTheory,  Observation, and Instrumentation was held at Sunspot, NM from September 11  15, 2000, with80 participants more than half of whom were from  overseas:  China, Finland, France, Germany, India, Japan, Netherlands, Russia, Saudi Arabia, Spain, and Switzerland.

Solar activity and variability is driven and controlled by magnetic fields,  and polarimetryanalysis of the polarization state of light from the Sun induced by the presence of magnetic fields in the solar atmosphereis one of the most important tools for obtaining information on the magnetohydrodynamic (MHD) processes on the Sun.

The previous NSO summer workshop on solar polarimetry was held ten years ago.  Since then, major progress has been made and new research fields in this area have opened. Theoretical models and numerical experiments have become important tools for understanding and predicting fundamental physical processes involving solar magnetic fields. In the past, most polarimetric investigations were based on the Zeeman effect. Atomic polarization and the Hanle effect have since become important tools for investigating very weak magnetic fields on the solar surface. The goal of this year's workshop was to present an overview of new research areas, the latest results and developments, including instrumentation, and to develop a perspective on solar polarimetry for the next decade.

Michael Knölker (HAO) opened the workshop with provocative comments, encouraging theoreticians, observers, and instrumentalists to go beyond the boundaries that are often based on traditions descended from the "solar grandfathers".  High-resolution polarimetry, for example, should no longer focus on the photon mean free path but on the scale MHD models predicted to be significant for the formation of flux tubes as small as a few kilometers. The solar community acknowledges that the time is ripe for new, sophisticated instrumentation like the advanced technology solar telescope (ATST), with minimum 4-m aperture and fully corrected by a high-order AO system, being proposed by NSO and its partners. Other key issues are the need for three-dimensional numerical MHD codes, the need to measure the vector magnetic field in the corona which may well be a potential source of surprises, and the need for extremely high spatial and spectral resolution in polarimetric measurements.

The workshop was thematically organized in the following sessions: New Instrumentation; Weak Polarization and Coronal Magnetic Fields; IR Polarimetry and the Physics of Active Regions; MHD Simulations; High-Resolution Polarimetry and the Physics of Flux Tubes; and The Analysis of Stokes Profiles.

A presentation on the French-Italian THEMIS telescope, specifically designed and built for polarimetric observations and started this year with regular observing campaigns, was made by F. Paletou (THEMIS). C. Keller (NSO) gave a presentation on the SOLIS vector spectromagnetograph (VSM) that will provide the solar community with full disc magnetograms and precise vector magnetic field measurements. R. Rutten and P. Sütterlin (both from Utrecht) presented impressive high-resolution movies obtained with the Dutch Open Telescope (DOT).  Success of the open design of the DOT is of significance for the proposed construction of an advanced technology large-aperture solar telescope by NSO and its partners.  B. Lites (HAO) and T. Tarbell (Lockheed Martin) described the spectropolarimeter and the narrow-band filter imager for SOLAR-B, a 50-cm space-based optical solar telescope that will allow for diffraction-limited polarimetric measurements in the visible. Several other new and improved instrumentation for ground- and space-based experiments were presented by oral or poster contributions.

The session on weak polarization a relatively young research area started with two review talks: an overview of scattering polarization observations by Jan Stenflo (ETH-Zürich) and an introduction by Javier Trujillo Bueno (IAC) on theoretical approaches that try to explain the observed spectral features induced by atomic polarization and Hanle effect.  Several oral and poster presentations demonstrated the intensive research activity going on in the field of weak polarization. Richard Ignace (Iowa) showed possible applications of the Hanle effect to investigate magnetic fields in stellar atmospheres and circumstellar envelopes. For the first time an intensive exchange between the Zeeman and the Hanle camps started. Through the course of the meeting it became clear that scattering polarization measurements have the potential to supplement the Zeeman measurements that are insensitive to very weak magnetic field. Therefore the open theoretical questions have to be solved and further precise observations are needed.

Manuel Collados (IAC) opened one of the classical polarimetry sessions with a review on IR Polarimetry, pointing out the advantages of doing polarimetry in the infrared (e.g., large Zeeman splitting, less influence from seeing, and instrumental polarization), and presenting some observations of sunspot penumbrae and network magnetic field. The fact that IR measurements of the IAC group (presented by I. R. Hidalgo) do not show any evidence for a field amplification by convective collapse led to a lively discussion on the validity of the convective collapse model.

The session on MHD simulations was introduced by M. Schüssler (Max-Planck Institute, Lindau) with an elegant review on numerical simulations of magnetoconvection from the infancy of this reseach area in the 1960's to the present. Realistic simulations and the Stokes line diagnostics based on models that have evolved over the years underscored the impressive quality these simulations have reached.  Schüssler offered the provocative conclusion that simulations of non-magnetic convection are already consistent with existing observations so that the achieved spatial resolution in these models of 10 - 25 km is sufficient, but that MHD simulations should go well beyond this, i.e., to scales in the order of 1 km.  T. Emonet (Chicago) presented a survey of numerical simulations for different magnetic regimes, i.e., from quiet Sun fields to sunspot umbrae, and showed once more the state-of-the-art in visualization of numerical simulations. S. Ploner (Max Planck Institute, Lindau) presented 2D MHD simulations of photospheric magneto-convection that show flux recycling and reconnection. Based on these models, he found a variety of unusually shaped Stokes profiles from which he discussed the formation of Stokes V profiles with only one wing by basically horizontal, weak magnetic fields.  J. Sakai (Toyama Univ.) ended the session with simulations of the collisions between shock waves and a magnetic flux tube. He pointed out that such events are an efficient way to transport energy into the corona.

Spectropolarimetry at high spatial resolution suffers from the limited resolution and low photon flux provided by the current class of 0.5- to 0.8-m aperture telescopes. In his opening talk for the high resolution session, C. Keller (NSO) reaffirmed the urgent need for a new, large-aperture solar telescope, which, for example, would make resolution of the smallest magnetic features on the Sun (flux tubes) possible  This will be essential for developing and proving physical models of the basic processes that determine the main part of photospheric magnetic fields.  Results from the recent flight of the Flare Genesis balloon telescope were presented by P. Bernasconi (Johns Hopkins). Another effort to overcome the seeing-limited spatial resolution was shown by K. Janssen (Göttingen) by combining speckle image reconstruction with 2D spectro-polarimetry. Additional talks addressed the dynamics of magnetic elements (M. Demidov, ISTP) and the origin of helicity in the quiet Sun (A. Pevtsov, NSO).

The last session of the workshop addressed another classical core of solar polarimetry: the analysis and interpretation of Stokes profiles. H. Socas-Navarro (HAO) presented an overview of existing methods to invertStokes data, including current efforts to build and introduce a "community code" that would provide the solar community easy access to inversion codes. The important issue of computing time required by data inversion was addressed in several of the session's presentations. The use of neuronal networks (T. Carroll, Astrophysical Institute, Potsdam) and pattern recognition and group theory approach (A. López Ariste, HAO) were discussed. Instruments like the SOLIS VSM will produce huge amounts of Stokes data that need to be inverted quickly in order to obtain the magnetic field vector and other physical parameters.

P. Judge (HAO) introduced a code to simulate forbidden coronal lineslines that are potentially useful for deriving information on coronal magnetic fields. Current efforts to investigate the magnetic field structure in the corona were presented in the corona session. Possibilities for using Zeeman signals from molecular lines were discussed by S. Berdyugina (Oulu, Finland) and B. Livingstion (NSO).  The session was completed by discussing Stokes profiles of unusual shapes that are observed in sunspot penumbrae (K.D. Leka, Colorado Research) and in network fields (M. Sigwarth, NSO). O. Steiner (Kiepenheuer Institute) finally presented a possible explanation for some of these profiles that could be produced as a consequence of magnetopauses.

A. von Ballegooijen (Harvard) concluded the meeting with a summary of the scientific discussions that took place during the four-day workshop. The quality of contributions during the week was perhaps most aptly described by Rob Rutten (Utrecht): There was no boring talk so that I could take a nap.

There were several social events during the workshop, including a classic Sac Peak barbeque, an evening slide show by Bill Livingston (NSO), Cao Wenda (Beijing) and Yuanyong Deng (Beijing) of their recent "seeing" tour of the Himalayas, and an evening picnic on the gypsum dunes at White Sands National Monument.

This successfully stimulating week of scientific exchange was made possible by the tireless efforts of Rebecca Coleman and members of the local organizing committee, as well as the NSO/SP staff who kept everything running smoothly from behind the scenes. The workshop was sponsored by the National Solar Observatory, National Science Foundation, National Aeronautics and Space Administration, and the U.S. Air Force Office of Scientific Research through its European and Asian offices. The workshop proceedings will be edited by M. Sigwarth and published in the ASP Conference Series.
 

     Announcements and further information: Last updated April 18, 2001

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