Note on solar hard X-ray bursts

Observationally solar X bursts fall into three different categories : soft X bursts (E < 10 keV), deka-keV bursts (10–150 keV), and very hard X bursts or deci-MeV bursts (200–1000 keV). The first kind is quasi-thermal, the last kind is non-thermal. The real existence of the third kind of burst looks probable but has not yet been proved by direct observations. The difference between deci-MeV and deka-keV bursts may mainly be a matter of geometry of the emitting plasma.