Astrophysical Spectropolarimetry and Magnetic Field Diagnostics

This paper highlights the increasing interest of astrophysical spectropolarimetry by showing how remote sensing techniques based on the Hanle and Zeeman effects are allowing us to investigate the magnetism of the extended solar atmosphere. Particular emphasis is given to the development of new diagnostic windows on the weakest magnetic fields of the photosphere, chromosphere and corona. Spectropolarimetry in the He i 10830 Å multiplet is allowing us to infer the three-dimensional geometry of the magnetic fields that confine the plasma of solar prominences. Multilevel modelling of the Hanle and Zeeman effects in the Ca II IR triplet and other chromospheric lines is helping us to decipher the strength and topology of chromospheric magnetic fields in regions where photospheric magnetograms show the well-known ‘salt and pepper’ patterns of mixed polarities. The Hanle effect in molecular lines, as well as in atomic lines of rare-earth elements like Ce II, offers a novel diagnostic tool for empirical investigations of ‘turbulent’ magnetic fields in relatively deep regions of the ‘quiet’ solar photosphere.