Solar Magnetism and the Second Solar Spectrum

Abstract

The key to a unified understanding of the fundamental astrophysical building blocks, like magnetoturbulence, flux tubes, dynamo processes, and heating mechanisms, is to be found at the smallest scales. All instruments have a resolution cut-off, beyond which much of the physics is hidden from view. Larger telescope apertures are needed, not only to extend the spatial resolution cut-off to allow us to see structures at the scales of the pressure scale height and photon mean free path, but also to enhance the polarimetric sensitivity, since high-resolution solar observations are always photon starved. An increasingly sophisticated array of indirect diagnostic techniques are becoming available, allowing us to explore the physics beyond any spatial-resolution cut-off in ever greater detail. In particular the applications of the Hanle effect have matured to the extent that we may now go from the simplistic one-parameter models to more realistic interpretative schemes based on continuous distributions for a magnetoturbulent spectrum and use both Zeeman and Hanle observations as joint constraints for a Stokes inversion that aims to determine the free parameters of the magnetoturbulent distributions. The “second solar spectrum” that is due to coherent scattering processes offers a wealth of new diagnostic possibilities, but it has also revealed new physics that is not yet fully understood, since it seems to imply paradoxical properties of the Sun’s magnetic field that appear incompatible with our basic understanding of solar magnetism.