Polarization of Absorption Lines as a Diagnostics of Circumstellar, Interstellar, and Intergalactic Magnetic Fields: Fine‐Structure Atoms

The relative population of the fine structure sublevels of an atom's groundstate is affected by radiative transitions induced by an anisotropic radiationflux. This causes the alignment of atomic angular momentum. In terms ofobservational consequences for the interstellar and intergalactic medium, thisresults in the polarization of the absorption lines. In the paper we considerthe conditions necessary for this effect and provide calculations ofpolarization from a few astrophysically important atoms and ions with multipleupper and lower levels for an arbitrary orientation of magnetic fields to thea) source of optical pumping, b) direction of observation, c) absorbed source.We also consider an astrophysically important ``degenerate'' case when thesource of optical pumping coincides with the source of the absorbed radiation.We present analytical expressions that relate the degree of linear polarizationand the intensity of absorption to the 3D orientation of the magnetic fieldwith respect to the pumping source, the source of the absorbed radiation, andthe direction of observations. We discuss how all these parameters can bedetermined via simultaneous observations of several absorption lines andsuggest graphical means that are helpful in practical data interpretation. Weprove that studies of absorption line polarization provide a unique tool tostudy 3D magnetic field topology in various astrophysical conditions.Comment: 22 pages, 10 figures, ApJ, in pres