Jan Olof Stenflo - Böcker

Solar and stellar photospheres constitute the layers most accessible to observations, forming the interface between the interior and the outside of the stars. The solar atmosphere is a rich physics laboratory, in which the whole spectrum of radiative, dynamical, and magnetic processes that tranfer energy into space can be observed. As the fundamental processes take place on very small spatial scales, we need high· resolution observations to explore them. On the other hand the small-scale processes act together to form global properties of the sun, which have their origins in the solar interior. The rapid advances in observational techniques and theoreticallllodelling over the past decade made it very timely to bring together scientists from east and west to the first lAU Symposium on this topic. The physics of the photosphere involves complicated interactions between magnetic fields, convection, waves, and radiation. During the past decade our understanding of these gener­ ally small-scale structures and processes has been dramatically advanced. New instrumen­ tations, on ground and in space, have given us new means to study the granular convection. Diagnostic methods in Stokes polarimetry have allowed us to go beyond the limitations of spatial resolution to explore the structure and dynamics of the subarcsec magnetic struc­ tures. Extensive numerical simulations of the interaction between convection and magnetic fields using powerful supercomputers are providing deepened physical insight. Granulation, magnetic fields, and dynamo processes are being explored in the photospheres of other stars, guided by our improved understanding of the solar photosphere.

Magnetic fields are responsible for much of the variability and structuring in the universe, but only on the Sun can the basic magnetic field related processes be explored in detail. While several excellent textbooks have established a diagnostic foundation for exploring the physics of unmagnetized stellar atmospheres through spectral analysis, no corresponding treatise for magnetized stellar atmospheres has been available. The present monograph fills this gap. The theoretical foundation for the diagnostics of stellar magnetism is developed from first principles in a comprehensive way, both within the frameworks of classical physics and quantum field theory, together with a presentation of the various solar applications. This textbook can serve as an introduction to solar and stellar magnetism for astronomers and physicists at the graduate or advanced undergraduate level and will also become a resource book for more senior scientists with a general interest in cosmic magnetic fields.

Instruments for high-precision imaging polarimetry have opened new possibilities, not only for diagnostics of magnetic fields, but also for exploring effects in radiative scattering, atomic physics, spectral line formation and radiative transfer. The observational advances have stimulated various theoretical developments, for instance in vector radiative transfer and techniques for inverting polarized line profiles.

Traditionally, solar and stellar physics have been two separate branches of astronomy, which independently of each other have developed their own scientific goals and methods. During the last decade, however, we have witnessed a gradual convergence of these two areas: The solar physicists realize more and more that the sun has to be seen as a special case in a large family of stars of various properties. A more complete understanding of the sun can only be achieved by considering it in this broader context. The stellar physicists on the other hand have become aware that the detailed knowledge of the physical processes that the solar physicists have reached has a more general significance and can be applied to a variety of other astrophysical objects. Observational techniques developed in solar work can frequently be adapted for other stars as well. This unified approach to solar and stellar physics is often called the "solar-stellar connection". One main goal of this approach has been to understand the general nature and causes of stellar activity. The pioneering and visionary program to search for activity cycles on other stars started by Olin Wilson at the Mount Wilson Observatory 16 years ago has born fruit: in his sample of 91 stars, cyclic behaviour similar to that of the sun is found to be quite common, but many stars also show irregular activity fluctua.tions of large amplitude.

Traditionally, solar and stellar physics have been two separate branches of astronomy, which independently of each other have developed their own scientific goals and methods. During the last decade, however, we have witnessed a gradual convergence of these two areas: The solar physicists realize more and more that the sun has to be seen as a special case in a large family of stars of various properties. A more complete understanding of the sun can only be achieved by considering it in this broader context. The stellar physicists on the other hand have become aware that the detailed knowledge of the physical processes that the solar physicists have reached has a more general significance and can be applied to a variety of other astrophysical objects. Observational techniques developed in solar work can frequently be adapted for other stars as well. This unified approach to solar and stellar physics is often called the "solar-stellar connection". One main goal of this approach has been to understand the general nature and causes of stellar activity. The pioneering and visionary program to search for activity cycles on other stars started by Olin Wilson at the Mount Wilson Observatory 16 years ago has born fruit: in his sample of 91 stars, cyclic behaviour similar to that of the sun is found to be quite common, but many stars also show irregular activity fluctua.tions of large amplitude.

Magnetic fields are responsible for much of the variability and structuring in the universe, but only on the Sun can the basic magnetic field related processes be explored in detail. While several excellent textbooks have established a diagnostic foundation for exploring the physics of unmagnetized stellar atmospheres through spectral analysis, no corresponding treatise for magnetized stellar atmospheres has been available. The present monograph fills this gap. The theoretical foundation for the diagnostics of stellar magnetism is developed from first principles in a comprehensive way, both within the frameworks of classical physics and quantum field theory, together with a presentation of the various solar applications. This textbook can serve as an introduction to solar and stellar magnetism for astronomers and physicists at the graduate or advanced undergraduate level and will also become a resource book for more senior scientists with a general interest in cosmic magnetic fields.

Novel instruments for high-precision imaging polarimetry have opened new possibilities, not only for diagnostics of magnetic fields, but also for exploring effects in radiative scattering, atomic physics, spectral line formation and radiative transfer. The observational advances have stimulated various theoretical developments, for instance in vector radiative transfer and techniques for inverting polarized line profiles. The present volume gives a comprehensive and up-to-date account of this rapidly evolving and interdisciplinary field of science. It is based on the oral presentations given at the 2nd International Workshop on Solar Polarization held in Bangalore, India, in October 1998.