The Hanle Effect as a Magnetic Diagnostic

The physics of the Hanle effect is briefly reviewed, and its application as a diagnostic of hot star magnetic fields is described. Emphasis is given to the practicalities of using spectropolarimetry of resolved wind emission lines to infer information about the circumstellar magnetic field strength and its geometry. A model for a weakly magnetized wind from "WCFields" theory is used as the backdrop for investigating polarized line profile effects for P Cygni resonance lines using a kind of "last scattering approximation". Model results are presented for a typical P Cygni line that forms in a spherical wind. Significant line polarizations of a few tenths of a percent can result for circumstellar fields of order 100G. Information about the field topology and surface field strength is gleaned from the Stokes-Q and U-profiles. Simplistically, the Q-profile polarization is governed by the field strength, and the U-profile symmetry (whether symmetric or anti-symmetric or even zero) is governed by the field geometry.