Electric fields in the plasma sheet and plasma sheet boundary layer

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Abstract
Data from the spherical double probe electric field experiment on ISEE 1 have been used to study a number of plasma sheet/lobe boundary crossings during intervals selected for the Coordinated Data Analysis Workshop 6 on March 22 and 31, 1979. These crossings took place during periods of substorm occurrence. They could be identified by keV plasma measurements and by using the electric field probes as a reference for measurements of the spacecraft potential. Typical spacecraft potentials in the plasma sheet are +10 V to +20 V. Because of decreased plasma density in the lobes the satellite potential increases here to +20 V to +30 V. Strong electric fields, with a dominant dawn‐to‐dusk component, are observed throughout the boundary layer outside the plasma sheet both for contracting and expanding motions of the plasma sheet and for different magnetic field directions. Characteristic amplitudes and durations are 5–10 mV m−1 and 5–15 min. The corresponding E × B vectors are always toward the plasma sheet. The spacecraft were situated both north and south of the plasma sheet edge, in such boundary layers, and in two or three cases the spacecraft were most likely on the earthward side of an X line. These latter cases coincided with the maximum of the expansion phase of substorms. Prior to these substorms, during plasma sheet thinning, onsets of sunward convection were observed by an electric field experiment on GEOS 2 in the geostationary orbit.