SCATHA survey of high‐level spacecraft charging in sunlight

High-time-resolution (0.5 s) measurements of the vehicle frame to ambient plasma potential were made with a 50-m antenna experiment on the SCATHA satellite. Accurate measurements were limited to sunlight conditions or to low-level eclipse charging periods. Significant variations in the spacecraft potential in sunlight occurred which depended on the orientation of the satellite with respect to the sun. Using the SCATHA data, statistical occurrence of charging at near-geosynchronous orbit in daylight is studied. Charging greater than −10 V (in the negative sense) occurs only between 1900 LT and 0900 LT but at all altitudes and latitudes of the SCATHA orbit. High-level (> −100 V) charging occurs only for magnetic activity, as measured by Kp, of 2+ or greater. Three “worst case” daylight charging event periods (−340 V to −740 V) show that the electron population that directly drives the vehicle potential on the SCATHA satellite has an energy typically greater than 30 keV. The vehicle potential (in the negative sense) is directly proportional to the electron flux carried by the population above 30 keV, although the linear regression coefficients change from case to case. The vehicle potential is insensitive to changes in the electron flux below 30 keV, even though the fluxes in this range are considerably more intense. To explain the data, we suggest that the low-energy electron fluxes are essentially self-balanced by the combination of their own secondary and backscattered emissions and that a substantial portion of the photoelectron emissions are returned to the satellite by the action of the surrounding magnetic field.