Global electrodynamics observed during the initial and main phases of the July 1991 magnetic storm

Data acquired by seven fortuitously positioned satellites in the interplanetary medium, the magnetosphere, and the topside ionosphere, as well as from numerous ground magnetometers have allowed us to follow the evolution of global currents and electric fields during the geomagnetic storm on July 8 - 9, 1991. An interplanetary disturbance collided with the magnetosphere at similar to 1636 UT on July 8th, compressing the magnetopause inside of geostationary orbit for about 3 hours, as observed by magnetometers on two GOES satellites. The resulting storm developed in four segments with a 7 hour lull separating the initial and main phases. The initial phase was characterized by (1) an extensive, DP 2 current system that produced an AE perturbation of similar to 3500 nT, with reporting stations on the dayside, (2) a twenty-fold increase in auroral electron fluxes, and (3) the immediate appearance of electric fields in the inner magnetosphere. The main phase intensification and earthward movement of the ring current are associated with southward turnings of the interplanetary magnetic field (IMF) and a polar cap potential increase above 200 kV. A significant fraction of the ring current growth was observed prior to the first of several substorms that occurred during the main and recovery phases. The beginning of recovery was characterized by a diminution of the southward IMF component, the cross polar cap potential and magnetospheric electric fields. It was also marked by a substorm onset. Considering the timing of Dst modulations relative to substorm occurrence, we conclude that during this magnetic storm, the electric fields responsible for ring current transport/energization were mostly associated with the DP 2 current system. Finally, in the evening-sector penetration electric fields of the initial and main phases led to the formation of upward moving equatorial plasma bubbles that were detected by two DMSP satellites in the topside ionosphere.