The effects of lunar surface plasma absorption and solar wind temperature anisotropies on the solar wind proton velocity space distributions in the low‐altitude lunar plasma wake

Abstract

We study the solar wind proton velocity space distribution functions on the lunar nightside at low altitudes (∼100 km) above the lunar surface using a three‐dimensional hybrid plasma solver, when the Moon is in the unperturbed solar wind. When the solar wind encounters a passive obstacle, such as the Moon, without any strong magnetic field and no atmosphere, solar wind protons that impact the obstacle's surface are absorbed and removed from the velocity space distribution functions. We show first that a hybrid model of plasma is applicable to study the low‐altitude lunar plasma wake by comparing the simulation results with observations. Then we examine the effects of a solar wind bi‐Maxwellian velocity space distribution function and the lunar surface plasma absorption on the solar wind protons' velocity space distribution functions and their entry in the direction parallel to the interplanetary magnetic field lines into the low‐altitude lunar wake. We present a backward Liouville method for particle‐in‐cell solvers that improves velocity space resolution. The results show that the lunar surface plasma absorption and anisotropic solar wind velocity space distributions result in substantial changes in the solar wind proton distribution functions in the low‐altitude lunar plasma wake, modifying proton number density, velocity, and temperature there. Additionally, a large temperature anisotropy is found at close distances to the Moon on the lunar nightside as a consequence of the lunar surface plasma absorption effect.