Accumulation of charge on spacecraft surfaces leads to changes in their potentials. In most cases these potential changes tend to reduce the flow of attracted charged particles and increase the flow of repelled charged particles. Ultimately, an equilibrium can be reached corresponding to a net current to the system equal to zero. Such an equilibrium potential in a plasma is called the floating potential.
When the dominant charging process is collection of ambient ions and electrons the floating potential is negative. This is because even when ions have a similar temperature to the electrons, the electrons are faster and impact uncharged spacecraft surfaces in higher numbers than the ions. In general, the floating potential of conductive structures is negative in the high density ionosphere and in eclipse with value in volts of the order of magnitude of the electron temperature in eV. This can mean charging levels of over 10kV in the outer magnetosphere.
It is the balance of all currents to the surface that determines the surface charging level. Usually this concerns the negative current from the impact of primary electrons and the positive currents from ion impact, secondary electron emission and photoemission. Conductive currents also occur and may be positive or negative.
Because secondary electron emission and photo-emission currents are the major currents that mitigate against strong negative charging, the secondary and photo-electron yield characteristics of surface materials are crucial in determining whether a surface is able to charge to hazardous levels. Selecting coatings, such as indium-tin-oxide, where the yield curve crosses unity at a high energy is a means of reducing the likelihood of hazardous charging. Floating spacecraft surfaces exposed to sunlight are often positively charged. When photo-emission is the dominant charging process, the floating potential can vary from the order of one volt positive in the topside ionosphere and the plasmasphere to several tens of volts positive in the depleted regions of the magnetosphere.
All the surface processes can strongly vary from one material to another which can explain difference of potential values achieved by different materials in similar environments.
Hazardous charging occurs when differential potentials greater than around 100 V occur. Such levels may be exceeded commonly in the high-altitude plasma-sheet region of the magnetosphere and during low-altitude crossings of auroral arcs. Charging in the magnetospheres of other planetary magnetospheres, such as Jupiter and Saturn, may also reach hazardous levels.