ENERGETIC HELIUM PARTICLES TRAPPED IN THE MAGNETOSPHERE

High energy (approximately 40-100 MeV/nucleon). geomagnetically trappe d helium nuclei have been measured, for the first time, by the ONR-604 instrument during the 1990/1991 CRRES mission. The helium events obse rved 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 wa s active during solar maximum when the anomalous component is excluded from the inner heliosphere, making it unlikely that the observed even ts derived from the anomalous component. The trapped helium counting r ates decrease gradually with time indicating that these high energy io ns were not injected by flares during the 1990/91 mission. Flare injec tion prior to mid-1990 may account for the highest energy particles, w hile solar wind injection during magnetic storms and subsequent accele ration could account for the helium at lower energies.