Signatures of storm sudden commencements in the electric field measured at geostationary orbit (GEOS-2)

GEOS-2 carried a long wire boom antenna to measure the ambient electric field. In view of the long successful operation of the spacecraft a vast amount of electric field data was accumulated between 1978 and 1983. A survey of the data set revealed about 90 sudden occurrences of large amplitude Pc5 type waves. It turned out that about half of these events were associated with SSC's recorded on ground. We describe how the electric field and the computed E × B/B2 plasma drift changed from the undistrubed pre-SSC periods to the initial tens of seconds after the sudden compression occurred. The typical picture obtained from these events is that SSC's detected by GEOS-2 near local noon show a clear compressional plasma drift pattern, i.e., E × B/B2 points tailward. Close to the flanks or in the nightside these signatures contain not only a tailward flow proper but also a radially inward component. The magnetic field signatures show that the compressional component is strongest around local noon with a remarkable decrease via the flanks toward local midnight. Our data indicates that SSC's are hardly detectable in the geostationary orbit near local midnight. Based on simple geometrical considerations and within the accuracy of the ground recording times of the SSC's (±30s) we derive propagation velocities for the radially inward travelling compressive wave of 950 km/s and for azimuthal directions in the outer magnetosphere 1100 km/s.