The transfer of polarized radiation in resonance lines with partial frequency redistribution, J-state interference, and arbitrary magnetic fields. A radiative transfer code and useful approximations

TL;DR

This paper develops a comprehensive radiative transfer code for polarized spectral lines considering complex physical effects and evaluates various approximations for modeling scattering polarization in solar atmosphere lines.

Contribution

The authors introduce a new radiative transfer code that accounts for partial frequency redistribution, J-state interference, hyperfine structure, and arbitrary magnetic fields, along with assessing useful approximations.

Findings

Neglecting Doppler redistribution causes minimal error in Mg II wings.

Elastic collisions significantly affect the intensity component of emissivity.

Approximations are suitable for Mg II wings but not for Lyman alpha wings.

Abstract

We present the theoretical framework and numerical methods we have implemented to solve the problem of the generation and transfer of polarized radiation in spectral lines out of local thermodynamical equilibrium, while accounting for scattering polarization, partial frequency redistribution, J-state interference, and hyperfine structure. The resulting radiative transfer code allows modeling the impact of magnetic fields of arbitrary strength and orientation through the Hanle, incomplete Paschen-Back, and magneto-optical effects. We also evaluate the suitability of a series of approximations for modeling the scattering polarization in the wings of strong resonance lines that would be particularly useful for the numerically intensive case of three-dimensional radiative transfer. We examine the suitability of the considered approximation using our radiative transfer code to model the Stokes profiles of the Mg II h & k lines and of the H I Lyman alpha line in magnetized one-dimensional models of the solar atmosphere. Neglecting Doppler redistribution in the scattering processes that are unperturbed by elastic collisions produces a negligible error in the scattering polarization wings of the Mg II resonance lines and a minor one in the Lyman alpha wings, although it is unsuitable to model the cores of these lines. For both lines, the scattering processes that are perturbed by elastic collisions give a significant contribution only to the intensity component of the emissivity. Neglecting collisional as well as Doppler redistribution represents a rough but suitable approximation for the wings of the Mg II resonance lines, but a very poor one for the Lyman alpha wings. The magnetic sensitivity in the scattering polarization wings of the considered lines can be modeled by accounting for the magnetic field only in the eta I and rho V coefficients of the Stokes-vector transfer equation.