New ionic structures observed in the equatorial Ring Current during CLUSTER perigee passes

Cluster's highly eccentric polar orbit at 4 Re perigee permits sampling of the ring current, the radiation belts and the plasmasphere. The CIS (Cluster Ion Spectrometry) experiment onboard Cluster is capable of obtaining full three-dimensional ion distributions (from about 5eV/q to 40 keV/q) with one spacecraft spin time resolution (4 sec) and with mass-per-charge composition determination. The ion distribution functions obtained in situ by CIS during numerous perigee passes reveal new and very interesting structures on the ion spectrograms: the presence of narrow and stable energy bands. These bands are observed for all the major ion species. The pitch-angle distributions, which are highly structured and energy dependent, confirm the presence of distinct populations. Furthermore, the four point measurements of the magnetic field, obtained by the FGM experiment, allow to calculate the ring current density profile along the spacecraft orbit, using the curlometer technique. This is the first time that in situ multi-measurements of the ring current are made (ions and magnetic field), and this permits to study its latitudinal profile. Our statistical analysis, including various magnetospheric conditions and Cluster trajectories (with a wide range in local time and positioning with respect to the plasmapause), shows the dependence of these ion energy bands on the magnetospheric activity. The combination of high time and energy resolution, good mass per charge discrimination, and favorable orbits has enabled Cluster to discern inner magnetospheric populations that have not been well-studied and characterized in previous observations.