Plasmaspheric helium ion distribution from satellite observations of Hel: 304 A

High sensitivity and spatial resolution observations of the He(2)II-304 A emission line intensity in the Earth's nightglow have been carried out by the extreme ultraviolet telescope on the Apollo-Soyuz mission in July 1975. The data, obtained over a wide range of parameters, are compared with the predictions of a kinetic equilibrium model of plasmaspheric ion density. Excellent overall agreement observation and theory is found using as inputs a temperature model, solar flux, and H+ and O+ number densities determined by extrapolating nearly simultaneous Atmospheric Explorer C measurements at 300 km. The observations in the Northern Hemisphere are well fit by a model having 285 He+ ions cm-3 at 500 km independent of latitude or longitude while those in the south require 430 He+ ions cm -3 at the same altitude. This result is consistent with available information on the interhemispheric asymmetry of He+ observed by a mass spectrometer on Explorer 32 and on the winter neutral helium bulge.