DEKA-KEV X-RAY OBSERVATIONS OF SOLAR-BURSTS WITH WATCH GRANAT - FREQUENCY-DISTRIBUTIONS OF BURST PARAMETERS/

Solar flare observations in the deka-keV range are performed by the WA TCH experiment on board the GRANAT satellite. The WATCH experiment is presented, including the energy calibration as applied in the present work. The creation of the solar burst catalogue covering two years of observation is described and some examples of solar observations are g iven. The estimated energy releases in the flares presented here are f ound to extend below the range of hard X-ray flares which were previou sly studied by ISEE-3 and HXRBS/SMM detectors. The X-ray emitting comp onent cannot be exclusively explained by contributions from a thermal plasma around a few keV. Either a hotter component or a non-thermal po pulation of particles must also be present to produce the observed dek a-keV emission. The WATCH data furthermore shows that the relative con tributions of these components may change during an event or from even t to event and that the injection of energy contained in suprathermal electrons may occur throughout an event and not only during the rise p hase. For the most energetic WATCH flares simultaneous observations pe rformed by other experiments at higher energies further indicate that non-thermal emission can be observed as low as 10 keV. A statistical s tudy is performed on the total WATCH solar database and frequency dist ributions are built on measured X-ray flare parameters. It is also inv estigated how the properties of these frequency distributions behave w hen subgroups of events defined by different ranges of parameters are considered. No correlation is found between the elapsed time interval between successive flares arising from the same active region and the peak intensity of the flare.