A body in a plasma is subject to a flux of electrons and ions. Although by definition a plasma contains equal number densities of positive and negative charges, the electrons are much more mobile and the flux of electrons to an "uncharged" surface normally exceeds the flux of ions. The net effect, seen in laboratory plasmas and often in space, is that surfaces charge negatively with respect to the local plasma. The surface continues to attempt to acquire the amount of charge necessary to establish a current balance: to reduce the flux of electrons and enhance the flux of ions, until equilibrium is reached at the floating potential. In space, under solar UV and energetic particle irradiation, the current balance also include secondary and photo-emitted electrons. As a consequence, the charging of spacecraft surfaces is a function of the energy distribution of the plasma (e.g. temperature), the area of the sunlit surfaces and the properties of the materials on the spacecraft surfaces.
Last update: 15 September 2010