Geomagnetically trapped energetic helium nuclei

Abstract

Geomagnetically trapped helium nuclei, at high energy (∼40–100 MeV/nucleon), have been measured by the ONR‐604 instrument during the 1990/1991 CRRES mission. The ONR‐604 instrument resolved the isotopes of helium with a mass resolution of 0.1 amu. The energetic helium observed at L<2.3 have a pitch angle distribution peaking perpendicular to the local magnetic field, which is characteristic of a trapped population. Both the trapped 3He and 4He show two peaks at L=1.2 and 1.9. Each isotope’s flux, in each peak, can be characterized by a power law energy spectrum. The energy spectrum of the 3He is different from that of 4He, indicating that the 3He/4He ratio is energy dependent. Over the energy range of 51–86 MeV/nucleon, the 3He/4He ratio is 8.7±3.1 at L=1.1–1.5 and is 2.4±0.6 at L=1.5–2.3. The trapped helium counting rates decrease gradually with time during the CRRES mission, when the anomalous component is excluded from the inner heliosphere, indicating that these high energy ions were not injected by flares during this time period. The decrease in intensity is attributed mainly to the events around L=1.9. The helium around L=1.2, dominated by 3He, does not show a significant temporal evolution, which implies a long‐term energetic trapped 3He population. Two possible origins of the geomagnetically trapped helium isotopes are the interactions of energetic protons with the upper atmosphere and/or the inward diffusion and acceleration of helium ions due to electric‐field fluctuations.