Observations of hard X-ray emission from solar flares

Spectral measurements of solar X rays with photon energy greater than 20 kev were carried out in 1959 during portions of three class 2+ solar flares. Two of the flares showed strong fluxes with spectra equivalent to that expected from a thermal plasma with temperature ∼108 degrees. The emission spectra obtained on September 1, 1959, were indicative of a rapidly cooling plasma with initial temperature of about 285 million degrees. It is argued that a similar plasma could have explained the observations of Peterson and Winckler in 1958 and that there currently exists no definitive observational evidence for nonthermal solar X-ray emission. Spectra in the 2- to 20-kev spectral region during solar microflare activity are suggestive of the existence of a peaking in the solar X-ray spectrum in the 8- to 15-kev region, presumably caused by Fe line and recombination continuum emissions.