Energetic helium particles trapped in the magnetosphere
High energy (∼40‐100 MeV/nucleon) geomagnetically trapped helium nuclei have been measured, for the first time, by the ONR‐604 instrument during the 1990/1991 CRRES mission. The helium events observed at L < 2.3 have a pitch angle distribution peaking perpendicular to the local magnetic field and are contained in peaks located at L = 1.2 and 1.9. The events in each peak can be characterized by power law energy spectra with indices of 10.0±0.7 for L = 1.9‐2.3 and 6.8±1.0 for L = 1.15‐1.3, before the large storm of 24 March 1991. CRRES was active during solar maximum when the anomalous component is excluded from the inner heliosphere, making it unlikely that the observed events derived from the anomalous component. The trapped helium counting rates decrease gradually with time indicating that these high energy ions were not injected by flares during the 1990/91 mission. Flare injection prior to mid‐1990 may account for the highest energy particles, while solar wind injection during magnetic storms and subsequent acceleration could account for the helium at lower energies. Copyright 1994 by the American Geophysical Union.