Hα and Hβ Emission in a C3.3 Solar Flare: Comparison between Observations and Simulations

The hydrogen Balmer series is a basic radiative loss channel from the flaring solar chromosphere. We report here on the analysis of an extremely rare set of simultaneous observations of a solar flare in the H α and H β lines, at high spatial and temporal resolutions, that were acquired at the Dunn Solar Telescope. Images of the C3.3 flare (SOL2014-04-22T15:22) made at various wavelengths along the H α line profile by the Interferometric Bidimensional Spectrometer (IBIS) and in the H β with the Rapid Oscillations in the Solar Atmosphere (ROSA) broadband imager are analyzed to obtain the intensity evolution. The H α and H β intensity excesses in three identified flare footpoints are well-correlated in time. We examine the ratio of H α to H β flare excess, which was proposed by previous authors as a possible diagnostic of the level of electron-beam energy input. In the stronger footpoints, the typical value of the the H α /H β intensity ratio observed is ∼0.4–0.5, in broad agreement with values obtained from a RADYN non-LTE simulation driven by an electron beam with parameters constrained (as far as possible) by observation. The weaker footpoint has a larger H α /H β ratio, again consistent with a RADYN simulation, but with a smaller energy flux. The H α line profiles observed have a less prominent central reversal than is predicted by the RADYN results, but can be brought into agreement if the H α -emitting material has a filling factor of around 0.2–0.3.