First Comparisons of Local Ion Measurements in the Inner Magnetosphere with
ENA Magnetospheric Image Inversions: Cluster- CIS and IMAGE- HENA Observations

C. Vallat (Centre d'Etude Spatiale des Rayonnements, Toulouse, France),
P.C:son Brandt, I. Dandouras, R. DeMajistre, D.G. Mitchell, E.C. Roelof, 
H. Reme, J.-A. Sauvaud, L. Kistler, C. Mouikis, M. Dunlop, and A. Balogh

Data provided by the CIS (Cluster Ion Spectrometry) instruments onboard the
CLUSTER spacecraft are used to survey recent crossings of the inner
magnetosphere and ring current. CIS is capable of obtaining full
three-dimensional ion distributions (about 0 to 40 keV/q) with one spacecraft
spin time resolution and with mass-per-charge composition determination.
Events are 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). This analysis concerns events 
obtained both during well-developed ring current conditions (e.g. 18 April 
2002 event) and during quiet magnetospheric conditions (e.g. 09 August 2001 
event). 

The results show the consistency between the ion fluxes deduced from ENA
image inversions, and the fluxes measured locally. They thus show the
complementarity of the two ap-proaches. The locally measured fluxes provide
the "ground truth", and they give the detailed ion distributions. ENA images
allow to situate local measurements into a global context, and to position
them with respect to the ring current large scale structure. Our results also
show the limitations of the ion fluxes deduced from the ENA image inversions,
for images taken from a single vantage point, with a substantial scatter of
the inversion fluxes with respect to the in-situ measured ones, and a more
limited dynamic range. 

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