Magnetic Diagnostics of the Solar Chromosphere with the Mg II h-k Lines

TL;DR

This study develops a numerical model to analyze how magnetic fields influence the polarization of Mg II h-k lines in the solar chromosphere, enhancing weak-field diagnostic capabilities.

Contribution

It introduces a new polarized radiative transfer code that accounts for partial redistribution effects and magnetic fields in the formation of Mg II lines.

Findings

Magnetic fields cause measurable linear polarization modifications at ~10 G.

Circular polarization remains consistent with classical magnetograph formulas.

Partial redistribution effects are crucial in modeling Mg II line formation.

Abstract

We investigated the formation of the Mg II h-k doublet in a weakly magnetized atmosphere (20-100 G) using a newly developed numerical code for polarized RT in a plane-parallel geometry, which implements a recent formulation of partially coherent scattering by polarized multi-term atoms in arbitrary magnetic field regimes. Our results confirm the importance of partial redistribution effects in the formation of the Mg II h and k lines, as pointed out by previous work in the non-magnetic case. We show that the presence of a magnetic field can produce measurable modifications of the broadband linear polarization even for relatively small field strengths (~10 G), while the circular polarization remains well represented by the classical magnetograph formula. Both these results open an important new window for the weak-field diagnostics of the upper solar atmosphere.