Characteristics of hard X-ray spectra of impulsive solar flares

The typical characteristics of the hard X-ray emission of impulsive solar flares are examined. At times of hard X-ray peaks, spectra that break downward are the rule rather than the exception. The break energy is typically at about 100 keV and rarely exceed 150 keV. There is little or no dependence of spectral slopes or of the break energy on the hard X-ray fluxes. During the burst decay phases, there is a strong tendency for the spectra to evolve to either a single power law or to cross over to one that breaks upward. The break energy is usually lower after the crossover, but in about 30 percent of the cases it is higher. During the rise phase of many fast bursts, the rise in flux at high energies occurs later than that at lower energies. In most cases the high-energy flux catches up by the time of the burst peak and the lag is rarely or never observed in bursts whose rise time is more than about 10 s.