Ionic Structures Observed in the Plasmasphere by CLUSTER

Data provided by the Cluster Ion Spectrometry (CIS) instruments are used to analyze Cluster crossings of the plasmasphere. The Cluster spacecraft orbit the Earth in a highly eccentric polar orbit at 4 Re perigee, and this permits them to sample the ring current, the radiation belts and the outer plasmasphere. CIS is capable of obtaining full three-dimensional ion distributions (about 0 to 40 keV/q) with a time resolution of one spacecraft spin (4 sec) and with mass-per-charge composition determination. In addition the CIS Retarding Potential Analyzer (RPA) allows more accurate measurements in the about 0 - 25 eV/q energy range, covering the plasmasphere energy domain. The low-energy ion distribution functions, obtained by CIS-RPA during the perigee passes, reveal new and interesting features, not reported by previous missions. The ion discrimination capability of CIS reveals how the density profile is different for each of the main ion species (H+, He+, O+): H+ and He+ present mostly similar profiles; O+, however, is not observed as trapped plasmaspheric population at the Cluster orbit altitudes (R greater than 4 Re). Low-energy O+ is observed only as upwelling ion, on auroral field lines. Detached plasmasphere events, that are observed by CIS during some of the passes at about 0.5 Re outside the plasmapause, are also present. The bi-directional distribution functions of these detached plasmaspheric populations allow us to distinguish them from upwelling ion populations. The final new results we show concern the structure of the plasmasphere. The CIS multi-spacecraft measurements reveal very sharp boundaries at the plasmapause, less than 200 km thickness structures.