Experimental evidence on the role of the large spatial scale electric field in creating the ring current

This paper presents the first simultaneous in situ measurements of the larg e-scale convection electric field and the ring current induced magnetic fie ld perturbations in the equatorial plane of the inner magnetosphere and com pares them to the evolution of major geomagnetic storms as characterized by Dst. The measurements were obtained from the University of California, Ber keley double-probe electric field experiment and the Air Force Geophysics L aboratory fluxgate magnetometer on the CRRES spacecraft. This spacecraft ha d an apogee near geosynchronous orbit and a perigee near 300 km altitude. W e focus on the major geomagnetic storm on March 24, 1991, for which the max imum negative excursion of Dst was about -300 nT. During the main phase of the storm, the large-scale electric field repeatedly penetrated earthward, maximizing between L=2 and L=4 with magnitudes of 6 mV/m. These magnitudes were larger than quiet time values of the electric field by a factor of 60 or more. Electric potential drops across the dusk region from L=2 to L=4 ra nged up to 50-70 kV in concert with increases in Kp up to 9 and dDst/dt (an indicator of the net ring current injection rate) which ranged up to -50 n T/hr. These electric fields lasted for time periods of the order of an hour or more and were capable of injecting ring current ions from L=8 to L=2.4 and energizing particles from initial plasma sheet energies of 1-5 keV up t o 300 keV through conservation of the first adiabatic invariant. The data o btained during the recovery phase of this storm provide the first direct ex perimental evidence in the equatorial plane that the electric field is syst ematically diminished or shielded earthward of the inner edge of the ring c urrent during this phase of the geomagnetic storm. Also observed during the 2-week recovery phase were episodic enhancements in the electric field whi ch coincided and were colocated with enhancements of in situ ring current i ntensity and which also coincided with decreases in Dst. These enhancements in the electric field and in the ring current magnetic field perturbation occurred at progressively larger radial positions as the recovery phase con tinued. Evidence for regions of reversed convection near midnight during th e recovery phase is provided. An unexpected and important feature of this d ata set, during both main and recovery phases, near 1800-2100 MLT, is that electric fields are often much stronger earthward of L=4 or L=5 than at pos itions more distant than L=6. This suggests important features of the inter action between the hot ring current plasma and the large-scale electric fie ld in the inner magnetosphere ate not yet understood.