ARTEMIS Observations of Lunar Crustal Field‐Solar Wind Interaction and Impact on Reflected Plasma Under Weak Radial IMF

Abstract Magnetic anomalies or crustal fields are frequently observed close to the lunar surface, yet their interaction with the ambient plasma is not fully understood. Utilizing data from the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft, we perform case studies of such interactions under radial, weak interplanetary magnetic field. Our analysis shows that while the lunar crustal field direction did not directly affect the velocity and density of reflected ions, it locally increased their pitch angles, favoring growth of ultralow‐frequency waves. Additionally, we find that while the solar wind ions were demagnetized, retaining a nearly constant velocity, the magnetized electrons acquired significant perpendicular drift velocity due to the crustal field, resulting in a Hall current. This current altered the field strength near the lunar surface. The calculated loss cone angles of electrons reflected by the near‐surface field estimated from the Hall current are consistent with observations, suggesting that the crustal field was dominated by the Hall magnetic field. Our study advances our understanding of how lunar crustal fields interact with the solar wind and modify the nearby reflected plasma.