Stratification of physical parameters in a C-class solar flare using multiline observations

Institute for Solar Physics, Dept. of Astronomy, Stockholm University, AlbaNova University Centre, 10691 Stockholm, Sweden
e-mail: rahul.yadav@astro.su.se

Received: 5 November 2020
Accepted: 8 March 2021

Abstract

We present high-resolution and multiline observations of a C2-class solar flare (SOL2019-05-06T08:47), which occurred in NOAA AR 12740 on May 6, 2019. The rise, peak, and decay phases of the flare were recorded continuously and quasi-simultaneously in the Ca II K line with the CHROMIS instrument and in the Ca II 8542 Å and Fe I 6173 Å lines with the CRISP instrument at the Swedish 1 m Solar Telescope. The observations in the chromospheric Ca II lines exhibit intense brightening near the flare footpoints. At these locations, a nonlocal thermodynamic equilibrium inversion code was employed to infer the temperature, magnetic field, line-of-sight (LOS) velocity, and microturbulent velocity stratification in the flaring atmosphere. The temporal analysis of the inferred temperature at the flare footpoints shows that the flaring atmosphere from log τ₅₀₀ ∼ −2.5 to −3.5 is heated up to 7 kK, whereas from log τ₅₀₀ ∼ −3.5 to −5 the inferred temperature ranges between ∼7.5 kK and ∼11 kK. During the flare peak time, the LOS velocity shows both upflows and downflows around the flare footpoints in the upper chromosphere and lower chromosphere, respectively. Moreover, the temporal analysis of the LOS magnetic field at the flare points exhibits a maximum change of ∼600 G. After the flare, the LOS magnetic field decreases to the non-flaring value, exhibiting no permanent or step-wise change. The analysis of response functions to the temperature, LOS magnetic field, and velocity shows that the Ca II lines exhibit enhanced sensitivity to the deeper layers (i.e., log τ₅₀₀ ∼ −3) of the flaring atmosphere, whereas for the non-flaring atmosphere they are mainly sensitive around log τ₅₀₀ ∼ −4. We suggest that a fraction of the apparent increase in the LOS magnetic field at the flare footpoints may be due to the increase in the sensitivity of the Ca II 8542 Å line in the deeper layers, where the field strength is relatively strong. The rest may be due to magnetic field reconfiguration during the flare. In the photosphere, we do not notice significant changes in the physical parameters during the flare or non-flare times. Our observations illustrate that even a less intense C-class flare can heat the deeper layers of the solar chromosphere, mainly at the flare footpoints, without affecting the photosphere.