Loss of solar wind plasma neutrality and affect on surface potentials near the lunar terminator and shadowed polar regions

Abstract

As the solar wind is absorbed on the lunar dayside, a clear and obvious plasma void is created in the anti‐solar direction that extends many lunar radii behind the Moon. Plasma adjacent to this void will expand into the depleted region and this process is modeled here using a particle‐in‐cell code. It is found that thermal electrons will diffuse into the void ahead of the ions, creating a break in plasma quasi‐neutrality. In essence, immediately trailing behind the lunar terminator/polar regions, there is a magnetic‐field aligned E‐field peaking near 400 mV/m associated with a standing double‐layer, this layer consisting of fast thermal electrons (electron cloud) moving into the central void ahead of an ion‐enhanced layer. We demonstrate that the surface locations that are incident with the electron cloud (just nightside of the terminator) will develop enhanced negative surface potentials due to the dominance of electron currents. Such regions of large negative potentials near the lunar poles are of interest, especially given NASA's recent announcement of a lunar outpost at the south polar Aitken Basin.