Equatorially confined warm trapped ions at around 100 eV near the plasmapause

[1] Near the equatorial plasmapause at around 4-5 Earth radius (R-E) geocentric distance, pancake distributed tens of eV ions are sometimes found, as previously reported by Olsen et al. (1987). Cluster CIS data during perigee traversals in 2001-2002 (nearly 200 traversals) revealed new features of these equatorially-trapped warm ions. (1) The characteristic energy of He+ is often higher than that of H+. (2) Some events show non-thermal ring distribution for He+ rather than superthermal pancake distribution. H+ can also have the ring distribution in such events. (3) While majority of the events are dispersion-free, some events show energy-time dispersion, indicating drifts from different local times. (4) The time scale of the development is about an hour, which is much shorter than the drifting time of these ions around the Earth. Cluster statistics also confirmed some results from the previous studies: (5) These ions are confined within a few degrees of latitudinal range near the equator, and have nearly 90 degrees pitch angles. (6) At a geocentric distance of about 4-4.5 R-E where Cluster traversed the equator during its perigee, the probability of observing clear events is about 40-45% in the noon and dusk sectors and about 20-25% in the night-to-dawn sector. (7) They are dominated by H+ with variable content of He+. The He+/H+ ratio is much less than 5% for the majority of the cases.