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

Abstract

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° × 360° 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., 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.