Measurements of geomagnetically trapped alpha particles, 1968-1970, 1. Quiet time distributions

Abstract

Observations of geomagnetically trapped α particles over the energy range 1.18 ≤ Eα ≲ 8 Mev obtained with the Injun 5 polar-orbiting satellite during the period September 1968 to May 1970 are presented. The time history at L ≃ 3 and B ≃ 0.19 is dominated by the large intensity increase following the magnetic storm on October 31, 1968, during which the α particle intensity reached 2 × 10³ (cm² sec ster)−1 and the intensity ratio of α particles to protons (jα/jp) attained values of ≳2 × 10−3. The short-term decay of both α particles and protons in the same energy per nucleon range is similar, decay constants being ∼40 days. The α particle fluxes and the jα/jp ratio attained minimum values in mid-1969 and in early 1970 of ∼20 (cm² sec ster)−1 and ∼2 × 10−4, respectively, indicating that α particles are not replenished as readily as protons. The period February 11–28, 1970, was selected for detailed study, as representing the ‘quiet time’ trapped α particle population. During this period, it was found that (1) the α particle intensity versus L profile is most similar to the proton profile at the same energy per nucleon interval, (2) the jα/jp ratio is nearly constant as a function of L in the same energy per nucleon representation but rises sharply with L when it is computed in the same total energy interval, (3) the variation of α particle intensity with B suggests a steep (j(α) ∝ sinn α, n ≳ 10) angular distribution at small (∼10°–15°) equatorial pitch angles but the jα/jp ratio shows little dependence on B, and (4) the α particle spectral parameter E0 shows a markedly different dependence on LL from the equivalent one for protons. It is concluded on the basis of the observations that the source of the observed α particles during quiet times must be the solar wind or the upper ionosphere (via the polar wind) or some combination of the two but not directly captured solar particles. Further, the particle velocity appears to be the most physically plausible parameter in organizing the data for geomagnetically trapped nuclei. The data are compared with the theoretical model of Cornwall (1972), and it is found that reasonable agreement exists between the predictions and the observations.