Cluster multipoint observations of ionic structures in the plasmasphere by CIS and comparison with image-EUV observations and with model simulations
Cluster Ion Spectrometry (CIS)
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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 analyze Cluster crossings of the 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, 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. Low-energy O+ is observed mainly as upwelling ion, on auroral field lines. Detached plasmasphere events are also observed during some of the passes. The bi-directional distribution functions of these detached plasmaspheric populations allow us to distinguish them from upwelling ion populations. The CIS-RPA observations of the plasmapause position have been simulated with an interchange instability numerical model for the plasmapause deformations, and the model reproduces in a very satisfactory way the CIS observations. 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 that the difference observed between two Cluster spacecraft was temporal (boundary motion). 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.
CitationDandouras, I.; Pierrard, V.; Goldstein, J.; Vallat, C.; Parks, G.K.; Rème, H.; McCarthy, M.; Kistler, L.M.; Klecker, B.; Korth, A.; Bavassano-Cattaneo, M.-B.; Escoubet, P.; Masson, A. (2006). Cluster multipoint observations of ionic structures in the plasmasphere by CIS and comparison with image-EUV observations and with model simulations. , Proceedings Cluster and Double Star Symposium: 5th Anniversary of Cluster in Space, Issue 598,