First comparisons of local ion measurements in the inner magnetosphere with 
energetic neutral atom magnetospheric image inversions: Cluster-CIS and 
IMAGE-HENA observations

Claire Vallat, Iannis Dandouras, Pontus C:son Brandt, R. DeMajistre, 
Donald G. Mitchell, Edmond C. Roelof, Henri Reme, Jean-Andre Sauvaud, 
Lynn Kistler, Christopher Mouikis, Malcolm Dunlop, Andre Balogh


Data provided by the CIS (Cluster Ion Spectrometry) instruments on board 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 on board IMAGE. HENA provides
energetic neutral atom images with a high geometric factor and with a 120 degree
- 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 (27Ð39 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., 9 August 2001 event). The results show
the consistency between the ion fluxes deduced from energetic neutral atom (ENA)
image inversions and the fluxes measured locally. They thus show the
complementarity of the two approaches. 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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Journal of Geophysical Research, 109, A04213, doi:10.1029/2003JA010224, 2004