Solar Active Regions and 3D Magnetic Structure
IAU GA 2006 Joint Discussion (3)
August 16-17, 2006, during the XXVIth
IAU
General Assembly,
Prague, Czech Republic
Scientific Rationale
The magnetic field of the sun is responsible for most of its visible dynamic features, including the most energetic events that can effect the near earth space environment, producing space weather. The solar magnetic field is generated below the visible layer (the photosphere) and erupts into the solar atmosphere. The cross-section of the erupting field structure at the photosphere is observed as an active region, above which there exists a complex three-dimensional magnetic "dome". Many fundamental physical processes take place in and above active regions that govern the dynamics of the hot, magnetized plasma manifesting in the observed features. In order to understand these processes, a detailed understanding of the origin and dynamics of magnetic field is essential. In the recent past there have been spectacular advances in various topics related to this field such as: magnetic helicity, temporal evolution of magnetic field creating large-scale structure; thermal and magnetic instabilities leading to fine-scale structure; wave dissipation and reconnection providing coronal heating; instability and nonequilibrium states leading to eruptions. The future observations in infrared wavelength and from space platform combined with the sophisticated computer modeling are expected to make equally impressive advances in this field in near future. This being an important topic in solar physics having wide implication for many areas of astronomy, we propose to hold an IAU symposium around the IAU General Assembly in 2006.
The photospheric vector magnetic field of solar active regions has been measured on a synoptic basis for the last 30 years. From these, the 3D magnetic field is derived using numerical models. These measurements provide the input to 3D numerical magnetic field models. The spatial resolution of the measurements has improved steadily, and the models have been able to incorporate some departures from the force-free field approximation. Several of there models are now capable of reproducing the observed sheared coronal magnetic features.
Until now the field in the chromosphere and corona has been largely derived by extrapolating the photospheric measurements. Recently, the actual measurements of these fields have been carried out using both the Zeeman effect and Hanle effects. In the last few years, several exploratory measurements of magnetic fields in spectral lines originating at chromospheric and coronal heights have shown promising results. The technological advances in the field of detectors and polarizing optics makes it possible to design and fabricate vector magnetographs that can be used to measure vector magnetic fields simultaneously in several layers above the photosphere. Chromospheric field measurements using the Zeeman effect have been carried out on a regular basis at NSO/KP, Hawaii and Huairou in China and are being planned at NASA/MSFC and San Fernando Observatory. At the same time the Hanle effect has emerged as a new and powerful diagnostic tool, both for measuring weak horizontal magnetic fields in the solar chromosphere, and for exploring the subresolution tangled or turbulent magnetic fields that cannot be seen by the Zeeman effect, but which have been found to carry a vast amount of "hidden" magnetic flux. The recently uncovered wealth of polarization phenomena throughout the "Second Solar Spectrum", formed by coherent scattering processes, has exciting and entirely novel diagnostic potential.
The other aspect of solar magnetic field is inverting the observed polarization measurements in deriving the magnetic field. There is a considerable progress both in terms of atomic and molecular physics and radiative transfer analysis in this crucial topic.
On the other hand, helioseismic analysis is beginning to provide interesting results in the sub-photospheric structure and flows in active regions. These results can be used to derive sub-photospheric magnetic fields in active regions and may in the near future also help us in refining models about how active regions emerge.
There are many new instruments that are now under development and poised to produce impressive result in near future. The solar optical space telescope with 50 cm aperture and its back-end instrumentation on Solar-B that is scheduled to be launched in 2006 is poised to revolutionize our understanding the fine scale magnetic structure on the sun. Other space initiatives such as SUMI (MSFC) are expected to provide the results for the feasibility of measuring the transition region magnetic field. Many ground based instruments such as GREGOR, new 1-m SST at La Palma and NSO/SP adaptive optics are expected to yield new results in this field. There are other equally impressive instruments such as the chromospheric magnetograph at BBSO, Hawaii, IR spectropolarimeter at San Fernando Observatory of Cal State Northridge and Norikura Solar Observatory, Japan that are making important contribution in terms of magnetic field measurements. On the other hand radio emission is sensitive to magnetic fields throughout the solar atmosphere, and there is a long history of such measurements. A new radio facility, the Frequency Agile Solar Radiotelescope (FASR) is now being designed with a main goal of routinely measuring coronal magnetic fields at high spatial resolution. In this symposium we shall focus the measurement of solar magnetic field in different wavelengths and different techniques.
At the same time, we had IAU symposium on topics related to the present title in almost every 10 years (solar cycle!!), the last one being in early 1990s (Solar Magnetic Fields, IAU symposium 43, 1970, Solar and stellar magnetic field: origin and coronal effects, IAU symposium 102, 1982, Solar Photosphere: Structure Convection and Magnetic Fields, IAU symposium 138, 1989). The year 2006, therefore, will be ideal time to summarize the results that are being produced in this new field of research and discuss the future directions. In this symposium, we would like to discuss both observational and theoretical aspects of the three-dimensional solar magnetic field of solar active regions.
Last update 10th December 2005.
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