Surface charging occurs because electric charges (electrons and ions) of the plasma are free to move and eventually get trapped on material surfaces when they hit them. Electrons and ions provide negative and positive current respectively. Both electrons and ions can also expel electrons when they hit the surface, providing an additional positive current. UV and soft X-ray photons coming from the Sun also have enough energy to expel electrons from materials, thereby providing a positive current. The accumulation of charge (positive or negative) leads to the creation of a potential that eventually prevents further charge accumulation by repelling charges of a given polarity and attracting charges of opposite polarity. Different surfaces on the same spacecraft can charge to very different potentials, resulting in strong local electric fields. Photo-emission and secondary yield are important factors in determining occurrence of high level differential charging. Charging effect is highly dynamic as the plasma populations can change rapidly.
Internal charging occurs because higher energy electrons are able to penetrate thin surfaces and deposit charge in or on materials within the spacecraft. Internal charging involves lower currents but potentials can build up over longer periods on materials with low resistivity because current through the structure is the only mechanism by which charge can be removed. Strong electric fields again are the result.
An important hazard related to charging (surface and internal) is sudden electrostatic discharge (ESD). ESDs are most likely initiated by field effect emission and avalanche process at so-called ‘triple-points’, where metal, dielectric and vacuum are found together. This can eventually result in the injection of high current electrical transients into the spacecraft electrical system. This can cause transient state changes in electronics or permanent damage of material coating or electronic components. In the vicinity of high potential differences, plasma released in an ESD can trigger a secondary discharge or even a sustained arc if the potential is maintained e.g. by the solar array.