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

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.