Electrostatic Charging of Lunar Cavities Governed by the Flow-to-Thermal Speed Ratio: 3D PIC Simulations and a Free-Fall Model

Abstract

We use Particle-In-Cell (PIC) simulations to investigate the charging characteristics inside deep cavities on the lunar surface under the solar wind plasma conditions. Specifically, we systematically study the dependence of the cavity bottom potential on plasma flow velocity and cavity aspect ratio. In light of prior results indicating that the charging characteristics are predominantly determined by the cavity aspect ratio, the present analysis employs a rectangular shape for the cavity with a width smaller than the local Debye length. Three flow regimes are then defined according to the ordering among the bulk flow velocity ($v_{\mathrm{flow}}$) and the ion thermal velocity ($v_{\text{ti}}$) and the electron thermal velocity ($v_{\text{te}}$); low (), medium (), and high (). Accordingly, the charging characteristics differ across the regimes. In the low-flow velocity regime, the influx of electrons dominates over the influx of ions, resulting in the development of a negative potential at the bottom. In the medium-flow velocity regime, most electrons are lost to the side walls, while ions tend to reach the bottom. This results in the development of a positive potential at the cavity bottom. In the high-flow velocity regime, the influx of ions saturates while the influx of electrons becomes comparable to that of ions, resulting in the bottom potential converging to zero. These results provide significant insight into the electrostatic environment related to the complex morphology of the lunar surface, formed as a result of interactions with the space plasma.