Solar wind access to lunar polar craters: Feedback between surface charging and plasma expansion

Determining the plasma environment within permanently shadowed lunar craters is critical to understanding local processes such as surface charging, electrostatic dust transport, volatile sequestration, and space weathering. In order to investigate the nature of this plasma environment, the first two‐dimensional kinetic simulations of solar wind expansion into a lunar crater with a self‐consistent plasma‐surface interaction have been undertaken. The present results reveal how the plasma expansion into a crater couples with the electrically‐charged lunar surface to produce a quasi‐steady wake structure. In particular, there is a negative feedback between surface charging and ambipolar wake potential that allows an equilibrium to be achieved, with secondary electron emission strongly moderating the process. A range of secondary electron yields is explored, and two distinct limits are highlighted in which either surface charging or ambipolar expansion is responsible for determining the overall wake structure. Key Points The lunar crater‐solar wind interaction is self‐consistently simulated in 2D The surface‐plasma interaction is critical to the local wake environment Protons are drawn into the crater by the wake electric field