On the fast fluctuations in solar flare H alpha blue wing emission

Fine temporal structures in hard X-ray and microwave emissions of solar fla res have been known for many years. Recent observations with high time and spatial resolution revealed that emissions in the wings of H alpha could al so exhibit fast (subsecond) fluctuations. We argue that such fluctuations a re physically related to the small-scale injection of high-energy electrons . We explore this through numerical calculations. The energy equation and t he equations for energy-level populations in hydrogen, in particular includ ing the nonthermal collisional excitation and ionization rates, are solved simultaneously for an atmosphere impacted by a short-lived electron beam. W e determine the temporal evolution of the atmospheric temperature, the atom ic level populations, and the Ha line intensity. We find that although the background H alpha wing emission is mainly formed in the photosphere, the f ast fluctuations are probably produced in the chromosphere, which is penetr ated 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 tot al energy of 10(25)-10(26) ergs. These parameters are compatible with eleme ntary flare bursts.