A study of the spectrum beyond 200 keV of second-stage solar flare X-ray emission

We have studied the spectra of three solar flares with second-stage hard X-ray emission observed by instruments on board the OSO 5 spacecraft. Using a Monte Carlo program, we have calculated the expected flux of solar X-rays reflected from the Earth's atmosphere for the observed solar X-ray spectra. The results have been compared with the observed reflected fluxes, which were also detected by the above instruments. We show that for the events considered here, which are necessarily intense and have hard spectra, the simple extrapolation of the observed solar spectrum to higher energies cannot explain the observed flux of the reflected radiation. Instead, we find the need to postulate the existence of a solar X-ray flux at energies above 200 keV larger than that obtained from simple extrapolation. In particular, we find evidence against a thermal spectrum in at least one of the events considered.