The impact of angle-dependent partial frequency redistribution on the scattering polarization of the solar Na i D lines

TL;DR

This study evaluates the accuracy of angle-averaged versus angle-dependent partial frequency redistribution models in simulating the scattering polarization of the solar Na i D lines, highlighting the importance of AD effects for precise magnetic diagnostics.

Contribution

The paper demonstrates that angle-dependent PRD modeling is crucial for accurately reproducing the polarization profiles of Na i D lines, improving solar magnetic field diagnostics.

Findings

PRD-AA produces antisymmetric Q/I profiles unlike PRD-AD.

PRD-AA introduces artificial features in D2 line core.

PRD-AD is essential for accurate polarization modeling.

Abstract

The long-standing paradox of the linear polarization signal of the Na i D1 line was recently resolved by accounting for the atom's hyperfine structure and the detailed spectral structure of the incident radiation field. That modeling relied on the simplifying angle-averaged (AA) approximation for partial frequency redistribution (PRD) in scattering, which potentially neglects important angle-frequency couplings. This work aims at evaluating the suitability of a PRD-AA modeling for the D1 and D2 lines through comparisons with general angle-dependent (AD) PRD calculations, both in the absence and presence of magnetic fields. We solved the radiative transfer problem for polarized radiation in a one-dimensional semi-empirical atmospheric model with microturbulent and isotropic magnetic fields, accounting for PRD effects, comparing PRD-AA and PRD-AD modelings. The D1 and D2 lines are modeled separately as two-level atomic system with hyperfine structure. The numerical results confirm that a spectrally structured radiation field induces linear polarization in the D1 line. However, the PRD-AA approximation greatly impacts the Q/I shape, producing an antisymmetric pattern instead of the more symmetric PRD-AD one, while presenting a similar sensitivity to magnetic fields between 10 and 200 G. Under the PRD-AA approximation, the Q/I profile of the D2 line presents an artificial dip in its core, which is not found for the PRD-AD case. We conclude that accounting for PRD-AD effects is essential to suitably model the scattering polarization of the Na i D lines. These results bring us closer to exploiting the full diagnostic potential of these lines for the elusive chromospheric magnetic fields.