Magnetic and dynamical photospheric disturbances observed during an M3.2 solar flare

TL;DR

This study presents spectropolarimetric observations of an M3.2 solar flare, revealing magnetic field weakening and dynamic plasma flows in the photosphere during the flare, with rapid magnetic recovery post-flare.

Contribution

It provides detailed measurements of photospheric magnetic and velocity changes during a solar flare using full-Stokes spectropolarimetry in the near-infrared He I 10830 A line.

Findings

Magnetic field strength decreases during the flare and recovers within 30 minutes.

Upflows dominate during the flare, indicating plasma evaporation.

Photospheric density increases significantly during the flare.

Abstract

This letter reports on a set of full-Stokes spectropolarimetric observations in the near infrared He I 10830 A spectral region covering the pre-, flare, and post-flare phases of an M3.2 class solar flare. The flare originated on 2013 May 17 and belonged to active region NOAA 11748. We detected strong He I 10830 A emission in the flare. The red component of the He I triplet peaks at an intensity ratio to the continuum of about 1.86. During the flare, He I Stokes V is substantially larger and appears reversed compared to the usually larger Si I Stokes V profile. The photospheric Si I inversions of the four Stokes profiles reveal the following: (1) the magnetic field strength in the photosphere decreases or is even absent during the flare phase, as compared to the pre-flare phase. However, this decrease is not permanent. After the flare the magnetic field recovers its pre-flare configuration in a short time (i.e., in 30 minutes after the flare). (2) In the photosphere, the line-of-sight velocities show a regular granular up- and down-flow pattern before the flare erupts. During the flare, upflows (blueshifts) dominate the area where the flare is produced. Evaporation rates of ~ $10^{-3}$ and ~ $10^{-4}$ g cm$^{-2}$ s$^{-1}$ have been derived in the deep and high photosphere, respectively, capable of increasing the chromospheric density by a factor of two in about 400 seconds.