Spectral Lines for Polarization Measurements of the Coronal Magnetic Field. II. Consistent Treatment of the Stokes Vector for Magnetic‐Dipole Transitions

Abstract

We present a compact, self-consistent formulation for the description of polarized radiation from magnetic-dipole transitions occurring in the magnetized solar corona. This work di †ers from earlier treatments by and House in the 1970s, in that the radiative emission coefficients for the Sahal-Bre chot four Stokes parameters, I, Q, U, and V , are treated to Ðrst order in a Taylor expansion of the line proÐle in terms of the Larmor frequency of the coronal magnetic Ðeld. In so doing, the inÑuence on the scattered radiation of both atomic polarization, induced through anisotropic irradiation, and the Zeeman e †ect is accounted for in a consistent way. It is found that the well-known magnetograph formula, relating the V proÐle to the frequency derivative of the I proÐle, must be corrected in the presence of atomic alignment produced by anisotropic irradiation. This correction is smallest for lines where collisions and cascades dominate over excitation by anisotropic radiation, but it systematically increases with height above the solar limb (up to a theoretical maximum of 100%, in the collisionless regime and in the limit of vanishing longitudinal magnetic Ðeld). Although the correction to the magnetograph formula must be calculated separately for each line as a function of heliocentric distance, it is likely to be small for some lines of practical interest, along lines of sight close to the solar limb.

Observational Test of Coronal Magnetic Field Models. I. Comparison with Potential Field Model

“…The formulae for the emergent Stokes parameter of the CEL are identical to those derived from a full-quantum mechanical formulation (Casini & Judge 1999), up to a proportional constant. However, as this classical formulation does not consider the effect of collisional depolarization, it may overestimate the degree of linear polarization at a lower height.…”

Inference of the magnetic field in spicules from spectropolarimetry of He I D$_\mathsf{3}$

“…Other ambiguities are less known, simply because less common. One of these is a 90 • ambiguity also affecting the plane-of-the-sky direction of the magnetic field and arising in very specific cases (see, e.g., Casini & Judge 1999). For the sake of simplicity, we show how this ambiguity comes about in the case of a two-level atom in the saturated regime of the Hanle effect using analytic expressions derived by Casini (2002).…”

Solar magnetism eXplorer (SolmeX)

“…At the high magnetic field strengths of active regions, the IR lines show a large splitting of the Zeeman components. This allows us to deduce the direction and the strength of the magnetic field, if the linear polarization signal produced by scattering processes is also taken into account [16]. Because the corona above an active region (when located near the limb) is bright above the limb, temporal changes on the time scale of minutes can be detected making it possible to study Alfvén waves, i.e., distortions of the magnetic field.…”