Observations of Electric Fields Near the Plasmapause at Midnight

We report here static electric field measurements in the equatorial plane from the 100-m double floating probe experiment carried on the SCATHA (P-78-2) satellite. Eclipse measurements on auroral L shells are studied to better understand the change in magnetospheric convection at the plasmapause. The use of eclipse data allows the electric fields to be measured directly without contamination from the spacecraft photoelectron sheath; and it allows the thermal ion population in the outer plasmasphere to be observed, identifying the satellite location with respect to the plasmapause. (Such measurements are limited to near local midnight.) We find that in the corotating frame there is generally an electric field component in the antisolar direction during periods of low magnetic activity which begins at the edge of the plasmasphere and builds in amplitude as the satellite enters the plasma sheet. The amplitude of this electric field varies from 0.05 mV/m to 0.2 mV/m with increasing magnetic activity; the electric field switches to radially inward during periods of high activity with magnitudes up to 2 mV/m. The amplitude of the observed electric field increases with the amount of contribution from east-west currents in the earth's plasma sheet-magnetotail current system. Since the electric fields observed on these eclipse passes are radially outward in the midnight meridian during periods of low activity and since they are observed on a near-synchronous satellite which generally corotates with the earth, they are not included in most modeling studies of the formation of the plasmasphere.