Energetic helium isotopes trapped in the magnetosphere

Helium nuclei have been measured within the magnetosphere (L less than or equal to 6) during the 1990/1991 CRRES mission over an energy range of similar to 40-100 MeV nucleon(-1). The Office of Naval Research 604 instrument resolves helium isotopes with a mass resolution of similar to 0.1 amu. Each helium isotope can be represented by a power law energy spectrum, and the energy spectrum of the He-3 is different from that of He-4; that is, the He-3/He-4 ratio is energy dependent. In the energy range 51-86 MeV/nucleon the He-3/He-4 ratio is 7.4+/-2.6 for L = 1.15-1.3 and 2.2+/-0.6 for L = 1.8-2.15. All observed helium events at L < 2.65 show pitch angle distributions consistent with equatorially trapped ions, whereas the distributions at larger L values are more uniform, within a factor of 2. For 2.15 < L < 6, as L decreases, the He-3/He-4 ratio increases and the helium energy spectrum softens. A possible origin of the geomagnetically trapped helium isotopes at L <2.15 is proton interactions in the residual atmosphere. The CRRES satellite was operational during the large geomagnetic storm on March 24, 1991, and we have divided the analysis into prestorm and poststorm time periods. Following the storm, the magnetosphere became more dynamic, and at L similar to 2.3 an enhanced helium flux was observed. Over the energy range of 51-86 MeV nucleon(-1) both the average He-3 and He-4 fluxes at L = 2.15-2.65 increased by a factor of 6 after the storm relative to the prestorm period, while at L = 1.8-2.15 the average 3He and 4He fluxes decreased by factors of 3 and 11, respectively, following the storm. The implications of these observed results are discussed.