Diamagnetic Plasma Clouds in the Near Lunar Wake Observed by ARTEMIS

Abstract

Abstract Because of the plasma pressure gradient across the lunar wake boundary, the interplanetary magnetic field is enhanced in the lunar wake. In previous works, the solar wind ions that enter into the near lunar wake are found to have an unimportant influence on the magnetic field within the lunar wake. In this study, two cases of a 22%–70% reduction of the magnetic field in the near lunar wake are first observed by the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun mission (ARTEMIS). The magnetic field depressions are caused by the refilling of dense plasma clouds (with densities of 0.20–0.47 cm−3) into the near lunar wake. The ions of the plasma clouds that originate from the reflected solar wind ions in the lunar dayside are accelerated into the near lunar wake by the solar wind convection electric field. The source regions of the reflected ions can be traced back to the lunar magnetic anomalies over the sunlit surface. Pressure (magnetic pressure + thermal pressure) balances are roughly maintained for both cases at the boundary between the plasma cloud and the ambient plasma. Our results imply that the interaction between the solar wind and lunar magnetic anomalies drastically disturbs the near-Moon electromagnetic environment.