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 Hα and Hβ 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 Hα line profile by the Interferometric Bidimensional Spectrometer (IBIS) and in the Hβ with the Rapid Oscillations in the Solar Atmosphere (ROSA) broadband imager are analyzed to obtain the intensity evolution. The Hα and Hβ intensity excesses in three identified flare footpoints are well-correlated in time. We examine the ratio of Hα to Hβ 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 Hα/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 Ha/H β ratio, again consistent with a RADYN simulation, but with a smaller energy flux. The Hα line profiles observed have a less prominent central reversal than is predicted by the RADYN results, but can be brought into agreement if the Hα-emitting material has a filling factor of around 0.2-0.3.