The transfer of resonance line polarization with partial frequency redistribution in the general Hanle-Zeeman regime

TL;DR

This paper develops a comprehensive numerical method to model the polarization of spectral lines in the solar atmosphere, incorporating partial frequency redistribution and magnetic effects, providing insights into the interpretation of solar magnetic diagnostics.

Contribution

It introduces a rigorous PRD theoretical approach and a numerical code for solving the polarized radiative transfer problem in the Hanle-Zeeman regime with detailed physical interpretation.

Findings

Magneto-optical effects produce noticeable U/I wing signals in strong resonance lines.

PRD effects significantly influence the linear polarization profiles.

Weak-field approximation may be inadequate when PRD effects are significant.

Abstract

The spectral line polarization encodes a wealth of information about the thermal and magnetic properties of the solar atmosphere. Modeling the Stokes profiles of strong resonance lines is, however, a complex problem both from the theoretical and computational point of view, especially when partial frequency redistribution (PRD) effects need to be taken into account. In this work, we consider a two-level atom in the presence of magnetic fields of arbitrary intensity (Hanle-Zeeman regime) and orientation, both deterministic and micro-structured. Working within the framework of a rigorous PRD theoretical approach, we have developed a numerical code which solves the full non-LTE radiative transfer problem for polarized radiation, in one-dimensional models of the solar atmosphere, accounting for the combined action of the Hanle and Zeeman effects, as well as for PRD phenomena. After briefly discussing the relevant equations, we describe the iterative method of solution of the problem and the numerical tools that we have developed and implemented. We finally present some illustrative applications to two resonance lines that form at different heights in the solar atmosphere, and provide a detailed physical interpretation of the calculated Stokes profiles. We find that in strong resonance lines sensitive to PRD effects the magneto-optical terms of the Stokes-vector transfer equation produce conspicuous U/I wing signals along with a very interesting magnetic sensitivity in the wings of the linear polarization profiles. We also show that the weak-field approximation has to be used with caution when PRD effects are considered.