Ring Current Structure and Dynamics: Local Measurements (CLUSTER/CIS and FGM)
and Global Observations (IMAGE/HENA)

Claire Vallat (CESR, Toulouse cedex 4, France, 31028), Iannis Dandouras, 
Pontus C:son Brandt, Malcolm Dunlop, George K. Parks, Mark Wilber


ClusterÕs highly eccentric polar orbit at around 4 Re perigee permits sampling
of the ring current population, and gives its latitudinal profile evolution. 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 CIS data reveal new and very interesting
structures on the ion spectrograms: the presence, for all the major ion species,
of different features in energy bands, all along the CLUSTER pass inside the
diffuse auroral zone. The highly structured pitch-angle distributions, which are
energy dependent, confirm the presence of distinct populations. The latitudinal
profile of the ring current is also analysed. The magnetic field measurements on
board the 4 CLUSTER spacecraft by the FGM (Flux Gate Magnetometer) instrument
allow for the first time to calculate the ring current density profile along the
spacecraft orbit, using the curlometer technique. 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-characterized in previous observations. Events have also been selected
for which the CLUSTER spacecraft are within the field of view of the HENA (High
Energy Neutral Atom) imager onboard IMAGE. HENA provides energetic neutral atom
images with a high geometric factor and with a 120 degree x 360 degree field of 
view over the spin. The H+ ion distribution functions obtained in situ by CIS 
are then compared to the ones deduced by inverting the HENA hydrogen neutral 
atom images for the overlapping energy range of the two instruments (27-39 keV). 
The results show the complementarity of the two approaches, i.e. local 
measurements providing the full distribution function and giving the 'ground 
truth', and ENA imaging which allows to put the local measurements into a global 
context. 

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Presented at the August 2003 AGU Chapman Conference, Helsinki, Finland