Energetic ionized helium in the quiet time radiation belts: Theory and comparison with observation

Abstract

The nonadiabatic behavior of energetic α particles and singly ionized helium ions in the earth's inner magnetosphere is modeled for average quiet time conditions. A quasi‐equilibrium balance among inward radial diffusive transport of both kinds of ions, classical collisional Coulomb interactions, and charge exchange processes is assumed. Ion flux spectra are modeled at the outer boundary of the trapping region of the radiation belts which for this study was taken at L = 7. Radial profiles and energy spectra are computed at all lower L values, and the theory removes one degree of freedom compared to earlier work by predicting absolute intensities of the differential fluxes. The theoretical quiet time predictions are compared with direct observation of energetic helium ions in the lower MeV range obtained from the satellites ATS 6 and Explorer 45 at and below the geostationary orbit, respectively. To the extent of this data it is found that the theory simulates the most important characteristics of the radiation belt helium ion population. A number of spectral features of helium ions are predicted also below 1 MeV, and these can only be observationally tested on future satellites with more sophisticated ion detector systems.