Spacecraft charging at synchronous orbit.

Observation that ATS 5 can charge to potentials as high as 10,000 volts during eclipse, and as high as 200 volts in sunlight. This charging has been used to infer the presence of the plasmasphere at synchronous altitude in the midnight region, and its prompt disappearance when hot plasma is injected in association with substorms. Although injected plasma has a density of 1.0 per cu cm the remaining cold plasma (less than 50 eV) has a density of 0.01 per cu cm. A mathematical model of ATS 5 has been constructed that can be used to predict spacecraft potentials. In addition, parts of ATS 5 can charge to several hundred volts without affecting the overall spacecraft potential appreciably.