Outward expansion of the lunar wake: ARTEMIS observations

Magnetohydrodynamics (MHD) predicts that lunar wake expands outward at magnetosonic velocities in all directions perpendicular to background solar wind; however, fluid theories emphasize that lunar wake expands outward at sound speeds mainly along the interplanetary magnetic field (IMF). Early observations supported the MHD predictions in the near-moon region despite lack of solar wind and IMF observations. Thanks to the special orbit design of the ARTEMIS mission, the solar wind conditions are well determined at the time of concurrent observations in the lunar wake. 164 wake crossings made by ARTEMIS are statistically studied in this paper. Observations indicated that, in either distant or near-Moon regions, the lunar wake expands outward at the fast MHD wave velocities. This simple model provides a powerful way to determine wake boundaries, particularly at large distances where the boundary signatures are indistinct, thus allowing further studies on the Moon-solar wind/crustal field-solar wind interactions. Citation: Zhang, H., et al. (2012), Outward expansion of the lunar wake: ARTEMIS observations, Geophys. Res. Lett., 39, L18104.