Pitch angle distributions of geomagnetically trapped MeV helium ions during quiet times

Abstract

During geomagnetically quiet conditions energetic radiation belt helium ion fluxes at MeV energies have been found to exhibit characteristic radial profiles and large pitch angle anisotropies. Compiling data from many experiments we deduce a systematic dependence of this anisotropy with helium ion energy. When the observed pitch angle distributions are approximated by j(α0) = j⊥ sinn α0, α0 being the equatorial pitch angle, an empirical relation n = 7 + 9.1 log10(E), with the helium ion energy E, in MeV, is deduced over the energy range 0.59–9 MeV total ion energy (148–2250 keV per nucleon). These values are obtained for L shells in the range L ∼ 2 to 5, and possible radial variations of n are averaged over. The results are compared with theoretical expectations, and we suggest a qualitative explanation for the observed trend.