Comparison of energetic ions in cusp and outer radiation belt

Recently, large diamagnetic cavities with a size of as large as 6 Re have been observed in the dayside high-altitude cusp regions. Associated with these cavities are charged particles with energies from 20 keV up to 10 MeV. Their seed population is a mixture of ionospheric and solar wind particles. The energetic ion intensity, charge composition, energy spectrum, and phase space density observed in the high-altitude dayside cusp have been compared with that measured in the outer radiation belt over an energy range of 1-200 keV/e for protons and He ++ ions and of 41-200 keV/e for O + ions. It is found that (1) the shape of ion energy spectra in the cusp is different from that in the outer radiation belt, (2) the ion phase space densities in the outer radiation belt are organized by magnetic moment, and (3) the ion phase space density of both He ++ and O + in the cusp are higher than that in the outer radiation belt at a given magnetic moment. These results suggest that (1) the measured cusp energetic ions cannot be explained by simple transport from an outer radiation belt (or ring current) source; rather, (2) a nonadiabatic energization process would be required to relate the cusp energetic ion population to the belt ion population, and (3) cusp energetic particles are potentially an additional independent source of the charged particles in the outer radiation belt.