Radiative transfer modeling of the enigmatic scattering polarization in the solar NaI D1 line

TL;DR

This paper develops a detailed theoretical and numerical radiative transfer model to explain the observed scattering polarization in the solar NaI D1 line, emphasizing the importance of spectral structure and quantum interference effects.

Contribution

It introduces a novel approach combining density-matrix metalevel theory with advanced formal solvers to model complex polarization phenomena in solar spectral lines.

Findings

The model explains the polarization in the NaI D1 line without requiring ground-level polarization.

Spectral structure of anisotropic radiation significantly influences scattering polarization.

Quantum interference effects between hyperfine levels are crucial for accurate modeling.

Abstract

The modeling of the peculiar scattering polarization signals observed in some diagnostically important solar resonance lines requires the consideration of the detailed spectral structure of the incident radiation field as well as the possibility of ground level polarization, along with the atom's hyperfine structure and quantum interference between hyperfine F-levels pertaining either to the same fine structure J-level, or to different J-levels of the same term. Here we present a theoretical and numerical approach suitable for solving this complex non-LTE radiative transfer problem. This approach is based on the density-matrix metalevel theory (where each level is viewed as a continuous distribution of sublevels) and on accurate formal solvers of the transfer equations and efficient iterative methods. We show an application to the D-lines of NaI, with emphasis on the enigmatic D1 line, pointing out the observable signatures of the various physical mechanisms considered. We demonstrate that the linear polarization observed in the core of the D1 line may be explained by the effect that one gets when the detailed spectral structure of the anisotropic radiation responsible for the optical pumping is taken into account. This physical ingredient is capable of introducing significant scattering polarization in the core of the NaI D1 line without the need for ground-level polarization.