Measurements of the solar spectrum in the wavelength band 4 to 14 Å

Abstract

Over 150 low -resolution X -ray spectra obtained during the first 3-week period of satellite Ariel have now been reduced. These measurements illustrate the variations of the shortwavelength ‘tail* of the solar X -ray spectrum in both intensity and spectral slope, under non-flare and flare-active conditions. Typical non-flare and flare-enhanced spectra are given in the paper. It is shown that the difficulty in describing the measured intensity of non-flare X-rays in this short-wavelength band, by the emission from a thermalized coronal plasma, may be rem oved by consideration of both line and continuum radiations from one or more discrete hot regions in the corona. An electron temperature of 5 million degrees is consistent with the non-flare emission spectrum typical of 26-27 April 1962. The X -ray enhancement associated writh some chromospheric flares may also be interpreted as the thermal emission of a localized hot region, though the variability in the X-ray emission from one flare to another is quite marked. Three flares are described in detail in the paper, and of these, two appear to be of this ‘simple’, thermal type. In the third flare an unusually rapid enhancement or X -ray ‘burst’ occurs about 20 min after the flare commencement. The spectral and time development of this ‘burst’ are described and the excellent correlation with an impulsive microwave ‘burst’ and with the onset of the ionospheric effects are noted. The possibility of both microwave and X-ray bursts having a non-thermal origin is discussed.