First satellite observations of the He+304-Å radiation and its interpretation

Abstract

Beginning in 1967, several investigators have measured radiation in the 170- to 400-A or 170- to 800-A wavelength range in the day and night skies using rocket-borne broad band photometers [Young et al., 1968, 1971; Weller et al., 1970, 1971; Donahue and Kumer, 1971; Ogawa and Tohmatsu, 1971; Meier and Weller, 1972; Paresce et al., 1972, 1973; Kumar et al., 1973]. The observed radiation was presumed to be the 304-A resonance line of the helium ion and/or the 584-A resonance line of neutral helium. In 1972 the night sky observation in the 170- to 800-A wavelength range of Young et al. [1971] was quantitatively analyzed by Meier and Weller [1972] and found to be basically consistent with a computed radiation intensity distribution in which the solar 304-A emission line is resonantly scattered from the helium ions contained within the earth's plasmasphere. The key observations and correlation with the intensity model were that the emission maximum is markedly displaced from the solar azimuth, as would be expected from a nonspherical distribution of helium ions, and that an abrupt shadow was present in the antisolar direction.