High time resolution observations of solar Hα flares.: I.

We present here the first results of a search for fast changes of solar flare Ha emission correlated in time with variations of hard X-ray fluxes recorded with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Using the Large Coronagraph (LC), Multi-Channel Subtractive Double Pass (MSDP) spectrograph and the Solar Eclipse Coronal Imaging System (SECIS) at the Bialkow Observatory, we collected a total of 23 sets of high time resolution (0.04-0.075 s) observations of Ha bright flare kernels during two observational seasons (2003 and 2004). For the first time, several-minute-long high-resolution observations of the H-alpha line profiles as well as simultaneous two-dimensional images have been obtained with time resolution much better than 1 s. Detailed observations of four flares recorded on 2003 July 16 and 2004 April 23 are described. We found examples of good time correlations between X-ray flux variations and variations of the Ha emission of the selected bright flaring kernels. In some events particularly, small but impulsive variations of the hard X-ray (20-50 keV) flux and of the Ha emission are well correlated. The Ha emission follows the 20-50 keV X-ray emission by similar to 2 s and similar to 3 s for two of the flares, longer for a third. The 3-10 keV and 10-20 keV X-ray emission, generally characterizing the thermal emission, follow these impulsive changes. There are, however, several examples of impulsive increases in 10-20 keV emission which appear to be of nonthermal origin. These results are consistent with previous observations and with theoretical studies in which the chromospheric response to energetic beamed electrons is calculated, provided that the chromospheric hydrogen densities are relatively high(similar to 10(14)-10(15) cm(-3)).