Statistical Study of the Lunar Plasma Wake Outer Boundary

The Moon does not have an intrinsic magnetic field and lacks the conductivity necessary to develop an induced magnetosphere. Therefore, the interaction of the Moon with the solar wind is dominated by impact absorption of solar wind particles on the day side and the generation of a plasma wake on the night side. A plasma density gradient forms between the flowing solar wind and the plasma wake, causing solar wind plasma to gradually refill the wake region. Electrons fill the wake first, pulling ions in after them via ambi-polar diffusion. Despite the existence of comprehensive new plasma measurements of the lunar wake region, relatively little attention has been devoted to the shape and variability in location of its outer boundary. Improved knowledge of this boundary condition for the physical processes associated with wake refilling would provide useful tests for simulations and theoretical models of the lunar plasma interaction. The ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) spacecraft mission is a two-probe lunar mission derived from the THEMIS (Time History of Events and Macroscale Interactions During Substorms) mission, repurposed to study the lunar space and planetary environment. Over the course of the mission there have been numerous passes of the ARTEMIS spacecraft through the lunar wake, at distances of up to seven lunar radii from the Moon. They have occurred for a variety of external conditions. We present a statistical study of tens of selected wake-crossing events of the ARTEMIS probes in 2011, using data primarily from the ARTEMIS fluxgate magnetometers (FGMs) and electrostatic analyzers (ESAs) to identify when the spacecraft entered and exited the wake. We study the shape of the outer wake boundary and its response to external conditions using two different techniques: one defines the wake boundary by a sharp decrease in ion density, the other by a decrease in magnetic field magnitude. We investigate how the wake boundary changes in response to solar wind parameters such as plasma beta, ion velocity, ion temperature, and magnetic field cone and clock angles. These results are compared with earlier wake crossing studies and computational modeling.