D.E. Rowland, J.R. Wygant (School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455)
Measurements made with the UCB/AFGL electric field instrument on the CRRES spacecraft are examined to determine the average structure of the inner magnetospheric electric field for different geomagnetic activity levels. Data were gathered between L=2.5 and L=8 over the period from January to October 1991, in the local time sector between 1200 and 0400 local time. The average dawn-dusk component of the electric field was nearly always duskward, scaling in magnitude from 0.05 mV/m to 1.5 mV/m as Kp increased from 0 to 9-. The data indicate that the electric field is shielded out of the inner magnetosphere, with the shielding distance starting at about L=5.0 for Kp=1 and moving 0.5 RE earthward for every unit increase in Kp. There is evidence that the electric field penetrates below L=2.5 for Kp > 5. The most interesting aspect of this statistical study is the development for moderate to high Kp of a region of enhanced electric field between L=3.5 and L=6. For moderately active conditions the electric field does not decrease monotonically as one moves earthward. Instead, it increases to a broad local maximum near the position where the ring current is typically observed to be the strongest, falling off earthward of that position. The electric field magnitude can be a factor of 2 or more larger at this location than at higher L values. These results are discussed in the context of large-scale flows and the effects of hot plasma in the inner magnetosphere.