The Solar XUV Spectrum.

The successful performance of instrumentation aboard OSO-3 and OSO-4 has advanced solar x-ray and ultraviolet astronomy to a new level of sophistication. As background for the new observations, a considerable amount of valuable information has been collected from the series of NRL SOLRAD satellites, the British Ariel satellite, NASA'S OGO-1, -2 and -4, the Air Force's OV-1 the USSR Electron 2 and 4, and many rocket flights. From these observations it is possible to develop a broad picture of the maximum to minimum variations of the "quiet" sun spectrum, the nature of transient variations under "quiet" conditions, the time-dependent growth and decay of flare spectra, and evidence of the degree of departure from thermal equilibrium under various conditions of solar activity.