An electric field instrument for lunar surface missions

As part of an ESA contract the University of Oslo (UiO) is, under the leadership of the Swedish Institute of Space Physics (IRF) and in collaboration with the Czech Academy of Science (CAS) and the University of Stuttgart (UoS), developing an instrument package to measure important parameters near the lunar surface. The overall system design including a boom package is managed by IRF, while CAS is designing an eBox that handles power and data/command distribution. The package includes two scientific instruments: (1) a dust detector designed by UoS, and (2) an electric field instrument designed by UiO. Here we present the progress made on the electric field instrument design, from measurement requirements to instrument concept selection. As part of the measurement requirements, we present a review of the current knowledge of near-surface lunar electric fields (including the DC and AC components) and plasma parameters derived from models of lunar electrodynamics and previous observations. Based on these expectations, the current instrument comprises a number of spherical probes, mounted on booms, that can dynamically be used in a double probe mode (with current biasing), or in a Langmuir probe mode. We outline the expected mode of operation, and briefly touch upon the expected science return of the instrument package.