Analysis of Balmer Series Emission in the Solar Flare SOL2017-04-21

Abstract

We observed a B6.2-class solar flare in active region NOAA 12651 using the HSFA spectrograph at the Ondřejov Observatory in hydrogen spectral lines. After processing the spectra, the integral emission fluxes in the H\(\alpha\), H\(\beta\), and H\(\varepsilon\) lines were determined. Within the heated-gas approach, a theoretical reconstruction of the plasma parameters was performed, taking into account the physical conditions in the chromosphere, including self-absorption in the spectral lines. Treating the observed fluxes requires the assumption of inhomogeneous gas. Agreement between the theoretical and observed fluxes is achieved in a model involving the superposition of two gas layers. Behind lies a dense gas with concentration \(N\) ranging from \(3\times 10^{12}\) to \(3\times 10^{13}\) cm\({}^{-3}\), and between it and the observer is a rarefied layer where \(N=(3{-}6)\times 10^{10}\) cm\({}^{-3}\). The layer thickness is in the range from 600 to 3000 km, the temperature—between 4000 and 7200 K, and the turbulent velocity—from 0 to 90 km/s. The presence of dense regions indicates that the source of the observed emission originates from the middle chromosphere, not higher than 1000 km.