Evidence that Solar X-Ray Emission is of Purely Thermal Origin (Also Observation of Far UV Flash During 28 August 1966 Proton Flare)

Abstract

X-ray emission from the sun, as thus far observed, is fully interpretable as thermal plasma emission from sets of hot plasmas at different temperatures. Solar minimum conditions are characterized by the presence of only a few plage regions, which dominate X-ray emission below 20 Å. Activity events under these conditions can cause a large increase in flux below 20 Å in coincidence with a decrease in emission between 44 and 60 Å, as the silicon and magnesium ions that dominate 44–60 Å emission move to higher stages of ionization. Under flare conditions temperatures rise to the 10–30 million degree regime, with the higher temperature portions of the plasma dominating the shorter wavelength portions of the spectrum. Thus, flare temperatures calculated from the ratio of Sxv and Sxvi lines are slightly higher than values calculated from the Si xiii and Si xiv lines, and no major discrepancy exists between these temperatures and temperatures calculated from X-ray continuum emission in the 4–6 Å part of the spectrum. The emission from most flares in the 1–6 Å region is from plasmas that are too cool to contain observable numbers of one-electron iron and calcium ions. The short bursts of very high energy X-rays accompanying strong explosive type flares also show a multithermal spectral character. None of the hard (> 20keV) X-ray spectra show evidence of monoenergetic electron swarms.

A rare burst of far ultraviolet radiation was observed during the flare of 28 August 1966. The burst lasted four minutes and was coincident with emission of hard (> 80 keV) X-rays and with the visible brightening and disappearance of a small subregion of the flare, as seen in Hα.