Relationship between electrostatic discharges on spacecraft P78-2 and the electron environment

Abstract

Abstract : The relationship between the energetic electron environment and electrostatic discharges on the P78-2 (SCATHA) spacecraft has been examined. Internal discharges occur near perigee while surface discharges occur about uniformly over the radial range covered by SCATHA, 5.5 to 8 Re. The surface discharges peak at local midnight and decrease toward local morning while the internal discharges have a broad occurrence maximum centered on local noon. Both types are far more likely to occur when the Earth's magnetic field is disturbed. Although few surface discharges occur when the Planetary Magnetic Index, Kp, is less than 4, a modest number of internal discharges occur under these quiet to normal conditions. In all cases where internal discharges occurred under quiet to normal conditions, a period of very disturbed conditions had occurred a few days earlier. Surface discharges have a strong tendency to occur when the flux of electrons with energies of 10's of keV is high while internal discharges occur when the flux of electrons with energies of 100's of keV is high, and the flux of 10's of keV electrons is low. A significant correlation has been found between the occurrence of discharges on SCATHA and an estimate of the energetic electron fluence at geosynchronous orbit obtained from a neural network model of the relativistic electron flux at geosynchronous orbit. At predicted daily fluences greater than 1010 elec/sq cm almost all discharges result from internal charging. The incidence of internal discharges at predicted daily fluences above 1010 elec/sq cm is sufficiently high to warrant the use of this predictor to issue warnings for real time satellite operations.