On the (mis)interpretation of the scattering polarization signatures in the Ca II 8542 A line through spectral line inversions

TL;DR

This study assesses the reliability of non-LTE spectral line inversion codes in inferring magnetic fields from Ca II 8542 A line polarization, highlighting limitations when scattering polarization dominates.

Contribution

It demonstrates the conditions under which existing inversion codes accurately recover magnetic field vectors, emphasizing the impact of scattering polarization on inversion results.

Findings

Inversion codes accurately estimate transverse fields >80 G.

Below 80 G, scattering polarization causes significant errors.

Line-of-sight magnetic field estimates remain reliable.

Abstract

Scattering polarization tends to dominate the linear polarization signals of the Ca II 8542 A line in weakly magnetized areas ($B \lesssim 100$ G), especially when the observing geometry is close to the limb. In this paper we evaluate the degree of applicability of existing non-LTE spectral line inversion codes (which assume that the spectral line polarization is due to the Zeeman effect only) at inferring the magnetic field vector and, particularly, its transverse component. To this end, we use the inversion code STiC to extract the strength and orientation of the magnetic field from synthetic spectropolarimetric data generated with the Hanle-RT code. The latter accounts for the generation of polarization through scattering processes as well as the joint actions of the Hanle and the Zeeman effects. We find that, when the transverse component of the field is stronger than $\sim$80 G, the inversion code is able to retrieve accurate estimates of the transverse field strength as well as its azimuth in the plane of the sky. Below this threshold, the scattering polarization signatures become the major contributors to the linear polarization signals and often mislead the inversion code into severely over- or under-estimating the field strength. Since the line-of-sight component of the field is derived from the circular polarization signal, which is not affected by atomic alignment, the corresponding inferences are always good.