Understanding the ring current with IMAGE/HENA

P. C. Brandt, D. G. Mitchell, Y. Ebihara, E. C. Roelof

The energetic neutral atom (ENA) imager HENA onboard IMAGE has provided global ENA images 
of the Earth's ring-current region since May 2000.  HENA measures ENAs in the energy range 
~10-200 (hydrogen assumed) keV. We will focus on an event just before the commencement of 
a moderate storm the 24 May 2000 where HENA observed an increase of ENAs (~15-60 keV) 
originating from the afternoon sector associated with the gradual increase in the 
northward component of the IMF. The ENA fluxes then suddenly dropped a factor of five 
during approximately 10 min. During the increase the in ENA flux IMF Bz was approximately 
15 nT. During the sudden decrease of ENA flux, the IMF Bz dropped to -20 nT and turned 
northward short thereafter. We have sought to understand this event by comparing the ENA 
images with a time-dependent ring-current model driven by the IMF [Ebihara and Ejiri, JGR, 
105, p. 15843, 2000] and found that the sudden decrease of the dayside ion fluxes can be 
explained by a rapid change in the global electric field when the IMF Bz flips southward. 
This results in a reconfiguration of the drift trajectories of pre-existing ions, implying 
the global magnetospheric electric field plays an important role in governing the 
ring-current formation and deformation. IMAGE/HENA data will be shown as movies from other 
events as well.

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Presented at the Fall American Geophysical Union Meeting, 
San Francisco, CA., December 15-19, 2000