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

arXiv comment5 pages, 3 figures, accepted for publication in ApJ Letters

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• Three-dimensional magnetic field structure of a flux emerging region in the solar atmosphere
• Capabilities of bisector analysis of the Si I 10827 A line for estimating line-of-sight velocities in the quiet Sun
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• Flare-induced changes of the photospheric magnetic field in a $\delta$-spot deduced from ground-based observations
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• The effects of stellar activity cycles on planetary atmospheric escape and the HeI 1083nm transit signature
• Infrared spectropolarimetry of a C-class solar flare footpoint plasma -- I. Spectral features and forward modelling