Energetic He+ ions from the radiation belt at low altitudes near the geomagnetic equator

Energetic helium ions trapped in the radiation belt can get lost through charge exchange by collisions with neutral exospheric hydrogen. Since the cross section for conversion of singly ionized He to neutral He is much greater than the cross section for conversion of doubly ionized He to neutral He, the energetic He flux can be considered to originate from the He+ population of the radiation belt. At low altitudes the energetic neutral helium atoms are again converted to ionized helium by collisions with the neutral oxygen atoms of the atmosphere. When a model for the flux distribution based on all available measurements of equatorially mirroring α particles is assumed, the expected flux of ionized He at low altitudes near the geomagnetic equator is computed according to an extension of the equilibrium model by Moritz (1972). It is assumed for the model calculations that the total ionized He particle distribution consists of singly charged He ions. A comparison of the results of the model calculations with measurements made in two different time periods with a proton-α particle telescope on board the low-altitude satellite Esro 4 below the radiation belt yields values of 6 and 23% for the percentage of singly charged helium in the trapped ionized helium at energies between 2.5 and 8 MeV. Accuracy of the results from the model calculations is limited by uncertainties in the trapped α particle characteristics, in the charge exchange cross sections above 1.5 MeV, and in the exospheric neutral hydrogen density. The different values obtained for the two periods is an indication of the variability of the He+ radiation belt content and/or of the exospheric neutral hydrogen density.