Why study sun and star spots?
Sunspots are responsible for temporal-varying properties of the sun, including the solar irradiance. Combined study of spots on the sun and other stars will provide a greater understanding of sunspot formation on a long-term basis.
Sunspots are the oldest features observed on the sun. Around 1610 AD, Galileo Galilei and Thomas Harriot opened a new era in solar observing by being the first known astronomers to view sunspots through a telescope. Three hundred years later, the first significant paper about the “Solar Magnetic Field” by George Hale was published in 1910. With modern instrumentation from ground and space, we are poised to make significant discoveries about the detailed structures of these objects that affect our daily lives through their influence on space weather. In the last several years, similar spots have been discovered on several other stars -- star spots. While sunspots are more thoroughly studied, star spots remain obscure phenomena. Combining the two fields of research would benefit both disciplines: solar research will gain insight for the long-term evolution of solar magnetism and stellar research will gain insight into the finer aspects of spot phenomena.
The astronomical community's recent and current interest in advanced observational and theoretical studies of sun and star spots prompted IAU Symposium 273, The Physics of Sun and Star Spots.
2010 will mark 400 years of telescopic viewing of sunspots and 100 years since the discovery, at Mt. Wilson (in the Los Angeles area), of sunspot magnetic fields. We briefly review the current state of knowledge of sun and star spots in the following paragraphs.
The magnetic field of the sun is responsible for most of the sun's visible dynamic features, including the most energetic events capable of affecting 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, governing 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: 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 non-equilibrium states leading to eruptions. Future observations in the infrared wavelength and from space-based platforms combined with sophisticated computer modeling are expected to make equally impressive advances in this field.
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 measurements has improved steadily, and models are able to incorporate some departures from the force-free field approximation. Several of these 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 from photospheric measurements. Recently, the actual measurement of these fields has been carried out using both the Zeeman 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. 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." Another aspect of solar magnetic field involves inverting observed polarization measurements in deriving the magnetic field. There is considerable progress both in terms of atomic and molecular physics and radiative transfer analysis on this crucial topic. Additionally, 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 help to refine models of active region emergence.
A number of new instruments are currently under development and poised to produce impressive results in the near future. The solar optical space telescope with a 50 cm aperture and its back-end instrumentation on Hinode is making superb observations of fine-scale magnetic structure on the sun. Many ground-based instruments such as GREGOR and the new 1-m SST at La Palma, along with NSO/SP adaptive optics are expected to yield new results in this field. Equally impressive instruments such as the chromospheric magnetograph at BBSO, Hawaii, IR spectropolarimeter at Norikura Solar Observatory, Japan are making important contributions in terms of magnetic field measurements. It is also the case that 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 the goal of routinely measuring coronal magnetic fields at high spatial resolution.
Unlike solar magnetic field research, research in stellar magnetic field is still quite young. However, significant advances have been made. In the past few years, advanced techniques such as "Doppler Imaging" have made significant contributions to the observational study of star spots. The use of space telescopes like Chandra and the Hubble Space Telescope has greatly increased our knowledge of stellar activity.
The Physics of Sun and Star Spots focuses on the measurement of solar and stellar magnetic field in different wavelengths and using different techniques with the goal of developing a unified understanding of spot phenomena in different types of stars. Historically, IAU symposia on related topics have been held approximately every ten years (every solar cycle!!): 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; and IAU Joint Discussion on the Three Dimensional Magnetic Structure of Sunspots in 2006. And what better place to celebrate the 100th anniversary of solar magnetic fields than within a stone's throw of their discovery!