Solar Photosphere: Structure, Convection, and Magnetic Fields

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.

Foreword.- I. Global Properties of the Photosphere.- Models of the Solar Photosphere.- Thermal Bifurcation of the Outer Photosphere.- Temperature Diagnostics of the Upper Photosphere.- Solar Oscillator Strengths as a Diagnostic Tool.- Average Variations of Photospheric Fe I And Fe II Line Parameters as Function of the Magnetic Filling Factor.- II. Photospheric Fine Structure.- High Resolution Observations of the Photosphere.- Properties of the Solar Granulation.- Analysis of the Solar Granulation in the Opacity Minimum Region.- Fine Structure of Photospheric Faculae.- Bright Features in the Intergranular Region.- III. Small-scale Magnetic Fields.- Empirical Models of Photospheric Flux Tubes.- Empirical Photospheric Fluxtube Models From Inversion of Stokes V Data.- Properties of Photospheric Fluxtubes Derived from Magnetograph Observations.- Small-Scale Magnetic Features Observed in the Photosphere.- High-Resolution Observations of Emerging Magnetic Fields and Flux Tubes in Active Region Photosphere.- Small Scale Motions Over Concentrated Magnetic Field Regions of the Quiet Sun.- Small Scale Magnetic Structures in Active Centers.- Theoretical Aspects of Small-Scale Photospheric Magnetic Fields.- Model Calculations of the Photospheric Layers of Solar Magnetic Fluxtubes.- Wave Heating in Magnetic Flux Tubes.- IV. Magnetohydrodynamics of the Photosphere.- Solar Magnetoconvection.- Results from 2-D Numerical Simulations of Solar Granules.- Waves and Oscillations in the Non-Magnetic Photosphere.- Waves and Oscillations in Magnetic Fluxtubes.- On the 5-Minute Photospheric Oscillation and its Modeling.- Classification of Magnetoatmospheric Modes in Sunspot Umbrae.- The Observational Signature of Flux Tube Waves and an Upper Limit on the Energy Flux Transported by them.- Magnetic Flux Concentration by Siphon Flows in Isolated Magnetic Flux Tubes.- Electric Currents in the Atmosphere of the Sun.- Generation of Magnetic Fields and Electric Currents in the Solar Photosphere.- V. Large—scale Structure and Dynamics.- Global Evolution of Photospheric Magnetic Fields.- Origin of the Sun’s Differential Rotation.- The Sun’s Rotation Rate as Inferred from Magnetic Field Data.- Generators of Solar Differential Rotation and Implications of Helioseismology.- Asymmetry of Emerging Flux Loops Caused by Radial Differential Rotation.- Convection and its Stability in the Equatorial Regions of the Convection Zone.- Inverse Cascade in Hydrodynamic Turbulence and its Role in Solar Granulation.- Generation of Torsional Oscillations in the Sun.- VI. Generation of Solar Magnetic Fields.- Order and Chaos in the Solar Cycle.- Symmetry Breaking in the Solar Dynamo: Nonlinear Solutions.- Excitation of Dynamo Modes.- Study of Sun’s “Hydromagnetic” Oscillations Using Sunspot Data.- Variation of Even and Odd Parity in the Solar Dynamo.- Self-Ordering of Photospheric Magnetic Fields.- Torsional Oscillations and the Solar Dynamo Regime.- Large-Scale Internal Magnetic Field of the Sun.- VII. Convection and Magnetic Fields in Solar—type Stars.- Observing, Modeling, and Understanding Stellar Granulation.- A Model for Stellar Convection and Spectral Line Asymmetries.- Comparative Analysis of Physical Conditions in the Solar and Procyon Atmospheres.- Magnetic Fields on Solar-Like Stars: The First Decade.- Results of Coordinated Multiwavelength Observations of Solar-Type Stars.- Nonlinear Dynamo Modes and Timescales of Stellar Activity.- Does a Common Dynamo Mechanism Exist for Lower Main Sequence Stars?.- VIII. Future Directions.- Ground Based and Space Future Prospects in Solar Interferometry Applied to the Photosphere.- New Observational Aspects.- Outstanding Theoretical Problems.- Summary Lecture.- List of Poster Papers.- Letter to G. Bush and M. Gorbachev.- List of Participants.- Author Index.