Spectropolarimetric inversions of the Ca ii 8542 \AA\ line in a M-class solar flare

TL;DR

This study uses high-resolution spectropolarimetry and non-LTE inversions to analyze the magnetic and thermal structure of an M-class solar flare, revealing significant heating and magnetic field sensitivity changes during the flare.

Contribution

It presents the first detailed semi-empirical models of the flaring chromosphere using spectropolarimetric inversions of the Ca ii 8542 Å line in an M-class flare.

Findings

Strong heating of the flare ribbon during peak.

Increased polarization signals in the flare ribbon.

Enhanced sensitivity of the Ca ii 8542 Å line to lower atmospheric magnetic fields during the flare.

Abstract

We study the M1.9 class solar flare SOL2015-09-27T10:40 UT using high-resolution full-Stokes imaging spectropolarimetry of the Ca ii 8542 {\AA} line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semi-empirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and non-flaring areas reveals strong heating of the flare ribbon during the flare peak. The polarization signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post- flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and non-flaring atmospheres shows that during the flare the Ca ii 8542 {\AA} line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation which led to results similar to those obtained with the NICOLE inversions.