The effects of solar wind velocity distributions on the refilling of the lunar wake: ARTEMIS observations and comparisons to one-dimensional theory

The lunar plasma wake refills from all directions, with processes operating both parallel and perpendicular to the magnetic field. The resulting wake structure depends sensitively on the properties of the flowing plasma, including the form of the ion and electron velocity distributions. In this manuscript, we discuss theoretical approximations for the refilling of the lunar wake along the magnetic field. While an often-used treatment for the parallel refilling assumes cold ions, one can derive solutions for arbitrary ion velocity distributions. Similarly, though the most tractable theory utilizes Maxwellian electrons, one can derive solutions for other types of distributions. We discuss the theoretical framework for various one-dimensional solutions, spanning the full range from cold-ion theories to gas-dynamic solutions, and utilizing both Maxwellian and kappa electron distributions. We compare these solutions to ARTEMIS observations of the lunar wake, for time periods with appropriate plasma parameters. We also present cases that reveal the inherent limitations of one-dimensional approximations, including those related to electron anisotropies and those related to perpendicular processes associated with both fluid flow and ion gyro-motion.