Modeling the quantum interference signatures of the Ba II D2 4554 A line in the second solar spectrum

TL;DR

This study models the polarization signatures of the Ba II D2 line in the solar spectrum, emphasizing the importance of partial frequency redistribution and isotope effects, and highlights the line's sensitivity to atmospheric temperature for solar diagnostics.

Contribution

It introduces a comprehensive radiative transfer model incorporating PRD and isotope effects to accurately reproduce observed polarization profiles of the Ba II D2 line.

Findings

PRD is essential for modeling the line's polarization

Line center polarization is sensitive to atmospheric temperature

Complete frequency redistribution fails to match observations

Abstract

Quantum interference effects play a vital role in shaping the linear polarization profiles of solar spectral lines. The Ba II D2 line at 4554 A is a prominent example, where the F-state interference effects due to the odd isotopes produce polarization profiles, which are very different from those of the even isotopes that have no F-state interference. It is therefore necessary to account for the contributions from the different isotopes to understand the observed linear polarization profiles of this line. Here we do radiative transfer modeling with partial frequency redistribution (PRD) of such observations while accounting for the interference effects and isotope composition. The Ba II D2 polarization profile is found to be strongly governed by the PRD mechanism. We show how a full PRD treatment succeeds in reproducing the observations, while complete frequency redistribution (CRD) alone fails to produce polarization profiles that have any resemblance with the observed ones. However, we also find that the line center polarization is sensitive to the temperature structure of the model atmosphere. To obtain a good fit to the line center peak of the observed Stokes Q/I profile, a small modification of the FALX model atmosphere is needed, by lowering the temperature in the line-forming layers. Because of the pronounced temperature sensitivity of the Ba II D2 line it may not be a suitable tool for Hanle magnetic-field diagnostics of the solar chromosphere, because there is currently no straightforward way to separate the temperature and magnetic-field effects from each other.