Geosynchronous environment for severe spacecraft charging

Abstract

A survey of data from the first year of the P78-2 (SCATHA) satellite operations showed that severe spacecraft frame charging (fy) both in sunlight (-340 V) and in eclipse (> -8 kV) occurred on 24 April 1979. Analysis of the data indicates that if the sunlight charging environment had been present during eclipse, the vehicle would have charged in excess of 15 kV, which is above any known charging level observed to date for the SCATHA satellite. Therefore, the environment at the peak of the sunlight charging at -0650 UT 24 April 1979 was chosen for this worst case study. The environment at this time is characterized by an injection of high-energy (30-335 keV) electron fluxes whose combined current correlates with 0y with a correlation coefficient of 0.95. The fluxes were highly anisotropic, maximizing perpendicular to the magnetic field. The low-energy (<4 keV) electron population had a density <1 cm~ 3 and the low-energy ions were near background. The measured electron distribution functions, when fitted to double Maxwellians by a least-squares technique, show that throughout the sunlight charging period the high and low temperatures remained nearly the same, while the density of the high-energy component followed the charging levels. The injection occurred simultaneously with the rapid return of the magnetospheric magnetic field to a more dipolelike configuration.