Temporal Analysis of the SOL2015-08-21 Flare Radiation, Theoretical Calculation of Observed Fluxes in the Lines of Calcium, Hydrogen, and Helium. Inhomogeneity of the Flare Structure and the Role of Plasmoids

We observed an M1.4-class flare in active region NOAA 12403 using the horizontal spectrograph for solar observations (HSFA-2) at the Ondřejov Observatory in the spectral lines of calcium, helium, and hydrogen, as well as using two radio telescopes. The active region remained unchanged over several days, being located in a region of strong magnetic field. It was a typical bipolar region consisting of a group of spots, pores, and bright flocculi. After processing the spectra, the integral radiation fluxes in the H\(\alpha\), H\(\beta\), H\(\varepsilon\), H CaII, IR CaII (\(\lambda=8542\) Å), D3 HeI lines were determined. Then, using the model of heated homogeneous gas layers, a theoretical calculation of the plasma parameters was performed, taking into account the physical conditions in the chromosphere, including self-absorption in the spectral lines. To explain the observed fluxes, three gas layers oriented perpendicular to the line of sight, each with its own individual parameters, had to be considered. One of them is very extended, while the other two are thin, indicating that the flare plasma is highly inhomogeneous. The radio observations carried out at frequencies of 0.8–2.0 GHz revealed drifting pulsating structure with frequency decreasing, which evidences the upward rise of the radio-emission source.