Multipoint Observations of Ionic Structures in the Plasmasphere by CLUSTER -
CIS and Comparisons with IMAGE-EUV Observations and with Model Simulations

I. Dandouras (Centre d'Etude Spatiale des Rayonnements, Toulouse, France), 
V. Pierrard, J. Goldstein, C. Vallat, G.K. Parks, C. Gouillart, F. Sevestre, 
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 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 Analyser (RPA) allows more accurate meas-urements
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 (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
popula-tions 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, for this event, that the difference observed between two Cluster
spacecraft was temporal (boundary motion): the radial density profile of the
plas-masphere 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. 

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Presentation at the Yosemite Conference of Inner Magnetospheric Interactions,
3 - 6 February 2004, Yosemite, California, USA