Geomagnetically trapped alpha particles: 3. Low-altitude outer zone alpha-proton comparisons

Abstract

The outer zone population of trapped α particles and protons was measured off the equator [B/B0 = 1.3–2.3] in the L interval 2 ≤ L ≤ 4 during 1969. The α particle spectra are best represented by a power law with spectral indices between 2.4 and 4.0 over the energy range 0.85 ≤ Eα ≤ 9.0 MeV. The α particle and proton spectra have the same slope at the same total energy. The distribution functions for a particles and protons have similar shapes, and they suggest that the location of the source of these particles is at L > 4. The proton pitch angle distribution at small pitch angles is flatter than near the equator. The α/p ratio is, at best, very weakly dependent upon pitch angle over the range of small pitch angles covered [18°–37°]. The α/p ratio peaks near L = 2.5 for Eα 2.8 at all E/A. The L and energy dependence of the ratios can be explained in terms of the observed proton and α particle energy spectra. The observed α/p ratios are in agreement with previous measurements. It is suggested that pitch angle scattering may be the primary source of the low-altitude ions. The measurements are compared with recent theoretical predictions.