H$α$ and H$β$ emission in a C3.3 solar flare: comparison between observations and simulations

Abstract

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 resolution, which 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 $\sim 0.4-0.5$, 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.

14 pages, 17 figures