Asymmetric Lunar Magnetic Perturbations Produced by Reflected Solar Wind Particles

Magnetic perturbations characterize the solar wind interaction of the Moon. The solar wind plasma absorption on the dayside surface produces large-scale field perturbations behind, i.e., the field enhancement in the central wake and reduction on the wake boundary. The solar wind repellence over local lunar magnetic anomalies (LMAs) leads to small-scale magnetic compressions ahead. In this study, the magnetic perturbations around the Moon are examined by using the observations from a near-Moon satellite mission, the Lunar Prospector, and they exhibit a clear left–right asymmetry in a coordinate system related to the solar wind convection electric field (${{\boldsymbol{E}}}_{\mathrm{SW}}$). The magnetic field is observed to enhance before the left terminator that ${{\boldsymbol{E}}}_{\mathrm{SW}}$ points to, while on the opposite side, it is not. The test particle simulations show that ${{\boldsymbol{E}}}_{\mathrm{SW}}$ can divert the particles reflected over the LMAs to the left and then the solar wind pickup of these particles leads to the field enhancement observed before the left terminator. Behind the lunar terminator, the wake field reduction is also asymmetric. On the left, the field reduction is more remarkable and located closer to the central wake. The denser plasma, consisting of the background as well as the reflected solar wind particles, may produce a stronger diamagnetic current and thus more significant field reduction there. The asymmetric plasma and magnetic perturbations associated with the reflected particles may be a common and nonnegligible element during the solar wind interaction of a small-scale magnetic field, such as that of an asteroid or a comet.