CLUSTER MULTIPOINT OBSERVATIONS OF IONIC STRUCTURESIN THE PLASMASPHERE BY CIS AND COMPARISON WITHIMAGE-EUV OBSERVATIONS AND WITH MODEL SIMULATIONS

摘要

The 4 Cluster spacecraft orbit the Earth in a highlyrneccentric polar orbit at 4 RE perigee, and this permitsrnthem to sample the ring current, the radiation belts andrnthe outer plasmasphere. Data provided by the ClusterrnIon Spectrometry (CIS) instruments are used to analyzernCluster crossings of the plasmasphere. CIS is capable ofrnobtaining full three-dimensional ion distributions (aboutrn0 to 40 keV/q) with a time resolution of one spacecraftrnspin (4 sec) and with mass-per-charge compositionrndetermination. In addition the CIS Retarding PotentialrnAnalyzer (RPA) allows more accurate measurements inrnthe about 0 - 25 eV/q energy range, covering thernplasmasphere energy domain. The low-energy ionrndistribution functions, obtained by CIS-RPA during thernperigee passes, allow to reconstruct statistically thernplasmapause morphology and dynamics, but they alsornreveal new and interesting features. The ionrndiscrimination capability of CIS reveals how the densityrnprofile is different for each of the main ion species (H+,rnHe+, O+): H+ and He+ present mostly similar profiles;rnO+, however, is not observed as trapped plasmasphericrnpopulation at the Cluster orbit altitudes. Low-energy O+rnis observed mainly as upwelling ion, on auroral fieldrnlines. Detached plasmasphere events are also observedrnduring some of the passes. The bi-directionalrndistribution functions of these detached plasmasphericrnpopulations allow us to distinguish them from upwellingrnion populations. The CIS-RPA observations of thernplasmapause position have been simulated with anrninterchange instability numerical model for thernplasmapause deformations, and the model reproduces inrna very satisfactory way the CIS observations. The CISrnlocal ion measurements have also been correlated withrnglobal images of the plasmasphere, obtained by the EUV instrument onboard Image, for an event where thernCluster spacecraft were within the field-of-view ofrnEUV. The EUV images show that the differencernobserved between two Cluster spacecraft was temporalrn(boundary motion). They thus show the necessity forrncorrelating local measurements with global images, andrnthe complementarity of the two approaches; localrnmeasurements giving the “ground truth” (includingrnplasma composition, distribution functions etc.) andrnglobal images allowing to put local measurements into arnglobal context, and to deconvolve spatial from temporalrneffects.