Interaction Between Solar Wind Particles and the Reiner Gamma Magnetic Anomaly: Observations and Test-Particle Simulations

Some lunar crustal magnetic anomalies are associated with albedo markings known as swirls; however, the processes governing their formation remain unclear. In this study, we focus on Reiner Gamma, a well-studied lunar anomaly and a key site for investigating the relationship between albedo patterns and magnetic anomalies. We perform test-particle simulations to examine how the Reiner Gamma swirl interacts with the local magnetic field, employing incident solar wind particles with energies of 0.5–1.0 keV and both line and disk magnetization models. The simulated magnetic fields are comparable to observations from previous lunar orbiters at altitudes of approximately 20 km and 40 km. Their maximum and minimum intensities, corresponding respectively to bright lobes and dark cusps on the lunar surface, align with the optical albedo patterns observed at Reiner Gamma. Our simulations show that the reflection area of solar wind particles above Reiner Gamma increases as the incident solar wind energy decreases. In the bright lobes, solar wind particle reflection exhibits a clear dependence on strong horizontal magnetic fields and dominant perpendicular energies. In contrast, reflection in the cusps is less definitive, being additionally governed by the interplay between relative perpendicular energy and magnetic configuration. We discuss the necessary conditions under which incident solar wind particles are absorbed at the surface or reflected above Reiner Gamma.