SOLAR-CYCLE DEPENDENCE OF SPACECRAFT CHARGING IN LOW EARTH ORBIT

Recent experimental evidence has shown that Defense Meteorological Satellite Program (DMSP) polar orbiting spacecraft at 840 km can develop electric potentials as severe as -1430 V while at high magnetic latitudes. To explore this charging region, an analysis of DMSP F6, F7, F8, and F9 satellite precipitating particle and ambient plasma measurements taken during periods of high, medium, and low solar flux is performed. One hundred eighty-four charging events ranging from -46 to -1430 V are identified, and an extreme solar cycle dependence is found as charging is most frequent and severe during solar minimum. Satellite measurements and time-dependent ionospheric model (TDIM) output are used to determine the cause of the solar cycle dependence and to characterize the environments which generate and inhibit these potentials. The electron precipitation associated with various DMSP charging levels is analyzed; it is suggested that precipitating electrons as low as 2 to 3 keV may contribute to charging though higher-energy electrons make greater contributions. Secondary electron production due to incident electrons below 1 keV is shown to inhibit charging. The energetic electron fluxes shown to generate charging do not vary significantly over the solar cycle. Instead, DMSP ambient plasma data and TDIM generated results identify a variation in plasma density over 1 or more orders of magnitude as the cause of the solar cycle dependence, and an ambient plasma density of less than 10(4) cm-3 is found necessary for significant negative charging (greater-than-or-equal-to 100 V) to occur.