Kinetic modeling of the dynamic nature of Lunar Mini-Magnetospheres

As the solar wind plasma collides with the moon, particles are absorbed causing a deep density void on the downstream side. Due to the lack of a global magnetic dipole, the interplanetary magnetic field largely passes through the moon with only weak perturbation due to pressure balance across the wake. However, this simple picture is complicated by the presence of localized crustal magnetic anomalies that are scattered over the surface of the moon. Spacecraft orbiting closely over the surface have found shock-like compressions and electromagnetic activity both upstream of these anomalies and downstream along the edges of the wake. The basic plasma physics of these processes is interesting as the stand-off distance of the crustal anomalies is often less than the ion skin depth and gyro-radius.

We show results of 2D kinetic particle-in-cell simulations of these Lunar Mini-Magnetospheres under different solar wind driving conditions. In contrast to previous simulation studies, we find that these processes can be highly dynamic. Depending on the orientation of the interplanetary magnetic field, simulations show time-dependent features such as mini flux-transfer events, as well as electromagnetic foreshock waves that can extend a significant distance upstream of the moon.