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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 $\sim$ 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 ($\sim$ 15-60 keV) originating from the afternoon
sector associated with the gradual increas 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
$B_z$ was approximately 15 nT. During the sudden decrease of ENA flux,
the IMF $B_z$ 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, {\it 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 $B_z$ 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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