A survey of spacecraft charging events on the DMSP spacecraft in LEO

The DMSP satellites orbit the Earth at similar to840 km and carry various instruments for sensing the space environment. They provide the ideal platform from which to investigate spacecraft charging in low-earth orbit (LEO). We have searched 12 years of DMSP satellite data, 1989 - 2001, for events in which the spacecraft frame charged to more than 100 V negative. We found 1253 events, all associated with an intense energetic electron precipitation event (an auroral arc) within a region of very low plasma density in the auroral zone. The occurrence frequency of events was highly correlated with solar activity with the preponderance of events occurring near solar minimums indeed, no events were found during the peak the last solar cycle in 1989 - 1990. There was a strong hemispherical asymmetry with almost 80% of the events occurring in the southern hemisphere due to the seasonal relationship of the spacecraft orbital plane and the Earth's spin axis. There was a strong seasonal dependence with all of the events centered around winter solstice and none occurring at summer solstice due to solar illumination. During solar minimum, the plasma density was three orders of magnitude or more less than the density at solar maximum. During solstice, the density was up to three orders of magnitude less in the winter hemisphere than in the summer hemisphere. The currents associated with precipitating electrons in an auroral arcs are usually much smaller that the currents from the in situ plasma. However, the plasma density can be low enough at DMSP altitudes that the currents from precipitating electrons dominate, driving the spacecraft potential negative. We have shown that these conditions exist a significant fraction of the time, particularly in the southern hemisphere at winter solstice, such that high-level charging events occur almost daily on the DMSP spacecraft.