Brightness distribution of soft X-rays on the Sun, inferred from ionospheric E-layer variations during an eclipse

Solar eclipse variations of the E-layer critical frequency at two stations near the central line of the eclipse of 20 May 1966, can be explained by quasi-equilibrium (constant effective recombination coefficient alpha eff>or=8*10-8 cm3 sec-1) between the electron density and the ionizing soft X-ray radiation emitted non-uniformly from the Sun. The ionospheric data are fitted well by a brightness distribution model attributing 27 per cent of the solar emission to the limb corona (including two localized bright parts of the limb), and 70 percent to a uniform disk radiation, in fair agreement with Elwert's (1958) theoretical estimations for soft X-rays. A source of hard X-ray emission near the central meridian, as found in the SOLRAD-satellite measurements during the eclipse, had only a very small influence on the E-layer variation. This leads to the assumption that the soft X-rays from local sources, in contrast to the hard X-rays, exceed the relatively high uniform-disk background level only when the sources approach the solar limb.