High sensitivity and spatial resolution observations of the He II 304-¿ 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.