Magnetic Field Diagnostics with Strong Chromospheric Lines

TL;DR

This paper explores how strong chromospheric lines' spectropolarimetric patterns can be used to diagnose magnetic fields in the chromosphere, revealing measurable polarization effects even at small field strengths.

Contribution

It introduces advanced numerical radiative transfer methods to analyze magnetic field effects on chromospheric line polarization, highlighting new diagnostic possibilities.

Findings

Partial redistribution effects are crucial for Mg II h-k line formation in magnetic atmospheres.

Magnetic fields cause measurable broadband linear polarization modifications at small field strengths.

Circular polarization remains well described by the magnetograph formula.

Abstract

The complex spectropolarimetric patterns around strong chromospheric lines, the result of subtle spectroscopic and transport mechanisms, are sensitive, sometimes in unexpected ways, to the presence of magnetic fields in the chromosphere, which may be exploited for diagnostics. We apply numerical polarization radiative transfer implementing partially coherent scattering by polarized multi-term atoms, in the presence of arbitrary magnetic fields, in planeparallel stellar atmospheres to study a few important spectroscopic features: Mg II h-k doublet; Ca II H-K doublet and IR triplet. We confirm the importance of partial redistribution effects in the formation of the Mg II h-k doublet in magnetized atmospheres, as previously pointed out for the non-magnetic case. Morevover, we show, numerically and analytically, that a magnetic field produces measurable modications of the broadband linear polarization even for relatively small field strengths, while circular polarization remains well represented by the magnetograph formula. We note that this phenomenon has already (unknowingly) been observed by UVSP/SMM, and the interest and possibility of its observation in stars other than the Sun. The interplay between partial redistribution in the H-K doublet of Ca II and metastable level polarization in its IR triplet allow diagnosing the chromospheric magnetic field at different layers and strengths. Our results suggest several new avenues to investigate empirically the magnetism of the solar and stellar chromospheres.