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

Abstract

This paper presents the first simultaneous in situ measurements of the large‐scale convection electric field and the ring current induced magnetic field perturbations in the equatorial plane of the inner magnetosphere and compares them to the evolution of major geomagnetic storms as characterized by Dst. The measurements were obtained from the University of California, Berkeley double‐probe electric field experiment and the Air Force Geophysics Laboratory fluxgate magnetometer on the CRRES spacecraft. This spacecraft had an apogee near geosynchronous orbit and a perigee near 300 km altitude. We focus on the major geomagnetic storm on March 24, 1991, for which the maximum 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 ranged 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 nT/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 to 300 keV through conservation of the first adiabatic invariant. The data obtained during the recovery phase of this storm provide the first direct experimental evidence in the equatorial plane that the electric field is systematically diminished or shielded earthward of the inner edge of the ring current during this phase of the geomagnetic storm. Also observed during the 2‐week recovery phase were episodic enhancements in the electric field which coincided and were colocated with enhancements of in situ ring current intensity 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 continued. Evidence for regions of reversed convection near midnight during the recovery phase is provided. An unexpected and important feature of this data 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 positions more distant than L = 6. This suggests important features of the interaction between the hot ring current plasma and the large‐scale electric field in the inner magnetosphere are not yet understood.