TRAPPED ANOMALOUS COSMIC-RAYS NEAR THE GEOMAGNETIC CUTOFF

Observations made with the mass spectrometer telescope (MAST) instrume nt aboard the SAMPEX spacecraft have previously been used to map the b elt of geomagnetically trapped anomalous cosmic rays at L approximate to 2. Here we use the heavy ion large telescope (HILT) instrument to e xtend these observations to lower energies, closer to the peak of the interplanetary spectrum at 1 AU, for oxygen and neon, At lower energie s than observed by MAST we see a distinct peak in the oxygen spectrum at the energy corresponding to the geomagnetic cutoff for singly charg ed incident ions arriving from the west, at each L where that is in th e HILT energy range. This is due to the suppression of the particle so urce at energies below the cutoff, and the subsequent filling-in of lo wer energies via slowing down of ions that had energies above the cuto ff when originally trapped. In addition, our observations of oxygen co nfirm the MAST discovery that the pitch angle distribution at SAMPEX a ltitudes is nearly isotropic outside the loss cone, and we see similar variations with time, trends with L, and general level of trapped flu x at the energies where the two instruments' responses overlap. The no rmalization of the trapped oxygen and neon fluxes as functions of L, w hen compared with previously published models, suggests more efficient trapping toward low L and/or additional losses at high L.