Dependence of the large-scale, inner magnetospheric electric field on geomagnetic activity

Measurements made with the University of California, Berkeley/Air Force Geophysics Laboratory 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=8.5 over the period from January to October 1991 in the local time sector between 1200 and 0400 magnetic 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.