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.
