The powerful transient energy releases from the Sun occur in the solar flares ( ergs in s). The X-ray radiation during a flare is thought to be due to electron-ion bremsstrahlung: the electrons are accelerated until they emit X-rays when interacting with the plasma ions of a relatively cold solar atmosphere (). In particular, during the impulsive phase of the flare, the electron acceleration to 20 keV takes place ([Dennis, 1985]); in this non-thermal situation, only of the energy lost by such electrons is released in bremsstrahlung X-rays, the remaining being lost to Coulomb collisions. In many hard X-ray flares, the fluences observed imply that a significant fraction ( 10-50%) of the total energy released comes from the 20 keV electrons. Recently, observations of flare gamma-ray lines suggest that also accelerated ions down to 1 MeV may carry a similar amount of energy ([Ramaty et al., 1995]). Particle acceleration anyhow seems to play a key role in the energy release of solar flares.
The solar hard X-ray flares are definitely softer than cosmic GRBs: high-sensitivity observations of solar hard X-ray bursts down to 8 keV performed with BATSE show no 25 keV signal when X-ray flares are detected in the lower band ([Li et al., 2001]).
The spectra of hard X-ray flares are definitely non-thermal and are fitted with power laws ( ph cms keV), with typically ranging from 3 to 10 (10-30 keV; [Dennis, 1985]). This shows that typical solar hard X-ray flares are indeed softer than GRBs.