Simultaneous quiet time observations of energetic radiation belt protons and helium ions: the equatorial α/p ratio near 1 MeV

Abstract

Simultaneous monitoring of energetic helium ions and protons in the earth's radiation belts has been carried out with the satellite Explorer 45 in the immediate vicinity of the equatorial plane. Protons were measured from < 1 keV to 1.6 MeV in differential energy detector channels and above 3.3 MeV in a quasi‐integral channel responsive up to 22 MeV; helium ions were monitored in three differential energy passbands in the range 910 keV to 3.15 MeV and in a differential channel with double passband: 590–910 keV and 2.0–3.99 MeV. The measurements permit determination of the equatorial helium ion to proton flux ratios (α/p) both at equal energy per nucleon and equal energy per particle in the heart of the radiation belts on L shells below the spacecraft apogee at L ∼ 5.25. The flux ratios are found to vary significantly with energy and location in the radiation belts. At equal energy per nucleon a range of variability for α/p from 10−4 to well above 10−3 is found, and at equal energy per ion the corresponding variability is from 10−3 to above 10. The latter findings emphasize the relative importance of the very energetic helium ions in the overall radiation belt ion populations. Comparison is made with theoretical equatorial predictions of the α/p ratios that were made by Cornwall (1972) based on the radial diffusion, charge exchange, and Coulomb interactions; the observational findings support many of the theoretical results.