Interaction of Interplanetary Shocks with the Earth’s Moon

Abstract:

Using data from two ARTEMIS spacecraft and electromagnetic hybrid simulations (kinetic ions, fluid electrons), this paper describes the nature of the interaction between interplanetary shocks and the Earth’s Moon when it is in the solar wind. During shock encounters, ARTEMIS observations show a clear jump in the magnetic field strength associated with the passage of the shock regardless of the position in the tail. Compared to the shock front observed in the solar wind, the magnetic field strength in the tail is stronger both upstream and downstream of the shock ramp, which is consistent with the expectations of larger field strengths in the tail in general. However, the ramp is broadened and no longer a step-function-type discontinuity. In contrast, plasma observations show that density and velocity enhancements in association with the ramp may or may not be observed, depending on the position of the spacecraft in the tail. Using the observed solar wind conditions, we have used hybrid simulations to examine the interaction of interplanetary shocks with the Moon. The results indicate that by virtue of IMF passage through the lunar body, the magnetic field shock ramp also passes through the Moon. As such a jump in the magnetic field strength is observed throughout the lunar tail in association with the passage of the shock. As expected, the field strength on both sides of the ramp in the tail are larger than the corresponding values in the solar wind. The passage of the shock through the lunar tail is broadened as seen in observations. In addition, the absorption of the core solar wind protons on the dayside introduces a density hole in the shock front as it passes through the Moon and its wake and, as such, the shock front as a whole is disrupted. This hole is gradually filled with the ambient plasma while the hole travels further down the tail until eventually the shock front is fully restored several lunar radii away from the Moon. The simulation results are well consistent with ARTEMIS observations and provide a continuous 3D evolution of a ruptured interplanetary shock. Here we also discuss the impacts of shock Mach number on the interaction. These results depict the lunar environment under transient solar wind conditions, which provide helpful information for the NASA’s plan to return humans to the Moon.