A spacecraft embedded in a plasma will become charged due to the collection and emission of electrons to and from the surface. There are typically two main sources of current: the first being due to the ambient electron density collected by the surface of the spacecraft, and the photoelectron current where ultraviolet radiation from the Sun can cause electrons to be stripped from the spacecraft surface. The relative importance of these currents as well as other minor currents determine the value of the spacecraft potential.
The spacecraft potential can be a useful quantity for analysis in some cases, but it can also have undesirable affects in others. Under certain conditions it can be calibrated to measure the ambient density fluctuations with the advantage of higher time resolution when compared with particle detectors. The density fluctuations which are obtained in this way are useful for the study of turbulent fluctuations in the solar wind.
In different circumstances a spacecraft with a large positive potential can be problematic. Discharges can occur presenting a danger to instrumentation, furthermore spacecraft will interact with the ambient plasma causing ions to be repelled from the spacecraft before they can reach the detectors, hindering the measurements. In these circumstances the potential can be regulated to a low level by the Active Spacecraft Potential Control (ASPOC) instrument. This instrument fires positive Indium ions from the spacecraft causing the potential to be reduced, but can also affect the plasma environment.
ASPOC was developed at IWF and has flown on Double Star, Cluster and most recently the Magnetospheric MultiScale mission (MMS). Members of IWF are engaged in the analysis of spacecraft potential data for density estimation, as well as understanding the effects of the spacecraft and ASPOC on the immediate plasma environment. Tools for the calibration of spacecraft potential and calibrated solar wind data are available here.