Influence of the Ambient Electric Field on Measurements of the Actively Controlled Spacecraft Potential by MMS

Space missions with sophisticated plasma instrumentation such as Magnetospheric Multiscale, which employs four satellites to explore near‐Earth space benefit from a low electric potential of the spacecraft, to improve the plasma measurements and therefore carry instruments to actively control the potential by means of ion beams. Without control, the potential varies in anticorrelation with plasma density and temperature to maintain an equilibrium between the plasma current and the one of photoelectrons produced at the surface and overcoming the potential barrier. A drawback of the controlled, almost constant potential is the difficulty to use it as convenient estimator for plasma density. This paper identifies a correlation between the spacecraft potential and the ambient electric field, both measured by double probes mounted at the end of wire booms, as the main responsible for artifacts in the potential data besides the known effect of the variable photoelectron production due to changing illumination of the surface. It is shown that the effect of density variations is too weak to explain the observed correlation with the electric field and that a correction of the artifacts can be achieved to enable the reconstruction of the uncontrolled potential and plasma density in turn. Two possible mechanisms are discussed: the asymmetry of the current‐voltage characteristic determining the probe to plasma potential and the fact that a large equipotential structure embedded in an electric field results in asymmetries of both the emission and spatial distribution of photoelectrons, which results in an increase of the spacecraft potential. Key Points The controlled spacecraft potential is correlated with the electric field, masking variations due to plasma density and temperature Artifacts in the potential data can be removed to facilitate the derivation of plasma density and temperature Two physical mechanisms are proposed as tentative explanations for the observed correlation