DEPENDENCE OF THE LARGE-SCALE, INNER MAGNETOSPHERIC ELECTRIC-FIELD ONGEOMAGNETIC-ACTIVITY

Measurements made with the University of California, Berkeley/Air Forc e Geophysics Laboratory electric field instrument on the CRRES spacecr aft are examined to determine the average structure of the inner magne tospheric electric field for different geomagnetic activity levels. Da ta 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 magneti c local time. The average dawn-dusk component of the electric field wa s nearly always duskward, scaling in magnitude from 0.05 mV/m to 1.5 m V/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 d istance starting at about L=5.0 for Kp=1 and moving 0.5 R-E earthward for. every unit increase in Kp. There is evidence that the electric fi eld penetrates below L=2.5 for Kp greater than or equal to 5. The most interesting aspect of this statistical study is the development for m oderate 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 no t 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 la rger at this location than at higher L values. These results are discu ssed in the context of large-scale flows' and the effects of hot plasm a in the inner magnetosphere.