Plasmaspheric helium ion distribution from satellite observations of He II 304‐Å

Abstract

High sensitivity and spatial resolution observations of the He II 304‐Å 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 plasmasphere parameters, are compared to the predictions of a kinetic equilibrium model of plasmaspheric ion density. Excellent agreement between 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.