On the fast fluctuations in solar flare Hα blue wing emission

Fine temporal structures in hard X-ray and microwave emissions of solar flares have been known for many years. Recent observations with high time and spatial resolution revealed that emissions in the wings of H alpha could also exhibit fast (subsecond) fluctuations. We argue that such fluctuations are physically related to the small-scale injection of high-energy electrons. We explore this through numerical calculations. The energy equation and the equations for energy-level populations in hydrogen, in particular including the nonthermal collisional excitation and ionization rates, are solved simultaneously for an atmosphere impacted by a short-lived electron beam. We determine the temporal evolution of the atmospheric temperature, the atomic level populations, and the Ha line intensity. We find that although the background H alpha wing emission is mainly formed in the photosphere, the fast fluctuations are probably produced in the chromosphere, which is penetrated by similar to 20 keV electrons. To yield H alpha wing fluctuations of amplitude comparable to the observations, a mean energy flux of similar to (1-2) x 10(11) ergs cm(-2) s(-1) is required for the electron beam, if one adopts a Gaussian macrovelocity of 25 km s(-1). Such a burst contains a total energy of 10(25)-10(26) ergs. These parameters are compatible with elementary flare bursts.