Solar wind interaction with lunar crustal magnetic fields: The Reiner Gamma magnetic anomaly

The Moon, unlike Earth, does not have a global dipolar magnetic field. Instead, there are localized patches of crustal magnetization scattered across its surface. The general morphology of crustal magnetic fields has been obtained from low-altitude measurements taken by the Lunar Prospector and Kaguya missions, and surface maps have been generated using both forward and inverse models based on these orbital data, although we are still far from a precise description. Strong crustal magnetic fields can deflect or reflect the solar wind away from the surface, while unprotected regions of the surface undergo a darkening and reddening process in visible to near-infrared wavelengths known as space weathering. Recent satellite observations and advanced numerical simulations, however, showed that the interaction between the solar wind and lunar crustal fields is surprisingly complex, and we still do not fully understand the exact mechanisms behind the solar wind-crustal fields interaction. In this study, we utilize a hybrid-kinetic plasma model, Amitis (https://www.amitiscode.com), to simulate the effects of the lunar crustal magnetic fields on the plasma environment of the Moon. We investigate the interaction between the solar wind and the lunar crustal fields and its dependence on the IMF orientation. We focus on the Reiner Gamma magnetic anomaly and on how the morphology of the crustal fields over it shields the surface from the incoming solar wind and contributes to surface weathering. We further examine the factors controlling surface shielding by strong crustal fields. We find that the lunar crustal fields can either partially or fully shield the surface in the vicinity of their location from solar wind protons depending, in addition to the strength of the magnetic field, on the angle of the incident solar wind.