Experimental evidence for the presence of helium ions based on Explorer VIII satellite data

Values of positive-ion concentration (N+ = 1.3 ± 0.1 × 104 ions/cm3), electron temperature (Te = 1750° ± 200°K), and the ratio of atomic helium to oxygen ions (He+/O+ = 1.3 ± 0.3), measured by three separate experiments at an altitude of 1630 km on the Explorer VIII satellite, are presented. These together with ionosonde data are shown to be consistent with a model of an isothermal upper ionosphere in diffusive equilibrium. The model infers that hydrogen ions are less important than either helium or oxygen ions at altitudes below 1600 km. The measured ratio of helium to oxygen ions is consistent with that postulated by Nicolet.