Inferring Solar Magnetic fields From the Polarization of the Mg II h and k Lines

TL;DR

This paper reviews how polarization measurements of Mg II h and k lines can be used to infer magnetic fields in the solar chromosphere, highlighting recent observational and interpretative advances.

Contribution

It provides a comprehensive overview of the physical mechanisms and recent progress in interpreting Mg II h and k line polarization for magnetic field diagnostics.

Findings

Successful spectropolarimetric observations confirm the diagnostic potential.

Understanding of polarization mechanisms has advanced.

Interpretation of Stokes profiles has improved significantly.

Abstract

The polarization of the Mg II h and k lines holds significant diagnostic potential for measuring chromospheric magnetic fields, which are crucial for understanding the physical processes governing the energy transport and dissipation in the solar upper atmosphere, as well as the subsequent heating of the chromosphere and corona. The Chromospheric Layer Spectropolarimeter was launched twice in 2019 and 2021, successfully acquiring spectropolarimetric observations across the Mg II h and k lines. The analysis of these observations confirms the capability of these lines for inferring magnetic fields in the upper chromosphere. In this review, we briefly introduce the physical mechanisms behind the polarization of the Mg II h and k lines, including the joint action of the Zeeman and Hanle effects, the magneto-optical effect, partial frequency redistribution, and atomic level polarization. We also provide an overview of recent progress in the interpretation of the Stokes profiles of the Mg II h and k lines.