New possibilities for the diagnostics of solar magnetic fields

The polarized spectrum that is produced by coherent scattering processes has a structural richness comparable to that of the intensity spectrum but different in appearance and physical origin. The amplitudes of the polarization features are influenced by magnetic fields via the Hanle effect in a way that is very different different from the ordinary Zeeman effect. While the main contribution to Zeeman-effect observations comes from the strong fields of the photospheric magnetic flux tubes, the Hanle effect is sensitive to weak magnetic fields, turbulent fields of mixed polarities, and chromospheric fields. As different spectral lines respond very differently to the Hanle effect, the scattering polarization offers novel and rich diagnostic opportunities. In the present overview we illustrate some of these new effects and indicate what can be learnt from them.