The magnetic sensitivity of the Mg ii k line to the joint action of Hanle, Zeeman and magneto-optical effects

TL;DR

This study investigates how the Mg ii k line's polarization signals are affected by magnetic fields through Hanle, Zeeman, and magneto-optical effects, revealing new diagnostic capabilities for solar chromosphere magnetism.

Contribution

It provides the first detailed analysis of magneto-optical effects on the Mg ii k line, demonstrating their impact on polarization signals in the line wings and enhancing magnetic field diagnostics.

Findings

Linear polarization at line center is sensitive to 5-50 G magnetic fields.

Circular polarization signals are prominent for fields >50 G.

Magneto-optical effects cause polarization rotation in the line wings.

Abstract

We highlight the main results of a radiative transfer investigation on the magnetic sensitivity of the solar Mg ii k resonance line at 2795.5 angstrom, accounting for the joint action of the Hanle and Zeeman effects as well as partial frequency redistribution (PRD) phenomena. We confirm that at the line center, the linear polarization signals produced by scattering processes are measurable, and that they are sensitive, via the Hanle effect, to magnetic fields with strengths between 5 and 50 G, approximately. We also show that the Zeeman effect produces conspicuous circular polarization signals, especially for longitudinal fields stronger than 50 G, which can be used to estimate the magnetization of the solar chromosphere via the familiar magnetograph formula. The most novel result is that magneto-optical effects produce, in the wings of the line, a decrease of the Q/I scattering polarization pattern and the appearance of U/I signals (i.e., a rotation of the plane of linear polarization). This sensitivity of the Q/I and U/I wing signals to both weak (around 5 G) and stronger magnetic fields expands the scientific interest of the Mg ii k line for probing the chromosphere in quiet and active regions of the Sun