Local Time Asymmetries in the Main Phase of Two Geomagnetic Storms as a Function of Ion Energy D.G. Mitchell, J.P. McFadden, P. Brandt, and C.J. Pollock Since the launch of the IMAGE spacecraft in March, 2000, the High Energy Neutral Atom imager (HENA) and the Medium Energy Neutral Atom imager (MENA) have imaged several large magnetic storms in energetic neutral atom (ENA) emission. The largest of these were the Bastille day storm (July 15-16, Dst -300nT) and the August 12 storm (Dst -240 nT). The August 12 storm was preceded by a weaker storm on August 11 (Dst = -100nT). Both phases of the August storm were driven by long lived negative Bz structures in the solar wind; in the second, stronger phase, Bz reached about -30nT. In both phases of this storm, By remained positive throughout most of the recovery, more strongly positive during the stronger, second phase. HENA and MENA images reveal that the maximum ENA emission in the ring current remained close to Earth on the dawn side, or in the dawn-midnight quadrant for all energies between about 10 and 70 keV during the driven portions of the storm. Later in the recovery, the ring current became nearly symmetric. This behavior, which has been reported for lower energies based on FAST data, is in contrast to the Bastille Day storm, for which the emission was quite symmetric at all energies at the time the recovery began. These observations can be explained to some extent by the details in the solar wind electric field time profile, but the observation of the dawn side enhancement to energies as high as 70 keV on August 12 was unexpected. FAST spacecraft data from the August 12 event confirming and constraining the HENA and MENA observations will also be presented. _______________ Submitted to the Spring 2001 AGU Meeting, Boston, Massachusetts