Local Time Asymmetries in the Main Phase of Two Geomagnetic Storms as a
Function of Ion Energy

D. G. Mitchell, and P. Brandt

Since the launch of the IMAGE spacecraft in March 2000, the High Energy
Neutral Atom imager (HENA) has 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), the August 12 storm (Dst
-240 nT), and the March 31, 2001 storm (Dst = -360).  We review these
three, as well as several weaker storms, to investigate the global scale
symmetry of the storm time ring current.  We find that the degree of
symmetry of the ring current depends more on the interplanetary magnetic
field (IMF), than on the "phase" of the storm.  In particular, the ring
current may remain quite asymmetric well into the recovery phase, if the
IMF stays southward, whereas it may become quite symmetric even at the
end of the main phase if the IMF turns northward.  The March 31 storm
demonstrates the first effect quite dramatically, remaining very
asymmetric to the main phase, then quickly filling all local times when
the IMF abruptly switches polarity at the beginning of recovery.  Much
later, the ring current in this storm becomes markedly asymmetric again
when the IMF returns to a southward orientation.