Multipoint Observations of Ionic Structures in the Plasmasphere by CLUSTER-CIS and Comparisons With IMAGE-EUV Observations I. Dandouras, C. Vallat, J. Goldstein, G.K. Parks, M. Wilber, C. Gouillart, F. Sevestre, M. McCarthy, L.M. Kistler, P.M. Escoubet, H. Reme The 4 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. Data provided by the Cluster Ion Spectrometry (CIS) instruments are used to analyse Cluster crossings of the plasmasphere. CIS is capable of obtaining full three-dimensional ion distributions (about 0 to 40 keV) with a time resolution of one spacecraft spin (4 sec) and with mass-per-charge composition determination. In addition the CIS Retarding Potential Analyser (RPA) allows more accurate measurements in the about 0 - 25 eV energy range, covering the plasmasphere energy domain. The low-energy ion distribution functions, obtained by CIS-RPA during the perigee passes, allow to reconstruct statistically the plasmapause morphology and dynamics, but they also reveal new and interesting features. 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 > 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 CIS local ion measurements have also been correlated with global images of the plasmasphere, obtained by the EUV instrument onboard Image, for an event where the Cluster spacecraft were within the field-of-view of EUV. The EUV images show, for this event, that the difference observed between two Cluster spacecraft was temporal (boundary motion): the radial density profile of the plasmasphere varies with MLT, and a more extended radial profile "rotated" into between the two Cluster spacecraft perigee passes. They thus show the necessity for correlating local measurements with global images, and the complementarity of the two approaches, local measurements giving the "ground truth" (including plasma composition, distribution functions etc.) and global images allowing to put local measurements into a global context, and to deconvolve spatial from temporal effects. _______________ Presentation, Fall Meeting, American Geophysical Union, San Francisco, USA, 8-12 December 2003