Simulating the Reiner Gamma Swirl and Magnetic Anomaly: The Long-term Effect of Solar Wind Standoff

The Moon is scattered with visually distinctive, high albedo surface features called lunar swirls. Measurement campaigns have shown that all known swirls are co-located with lunar magnetic anomalies. The opposite, however, does not hold and the evolutionary scenario of these enigmatic features has been debated since the Apollo era.

Possibly the most well-known swirl formation is Reiner Gamma, a tadpole-shaped albedo marking with an overall length of about 70 km, located west of the crater Reiner on the Oceanus Procellarum. Reiner Gamma's magnetic topology produces a mini-magnetosphere. Assuming that the energy flux the surface receives is inversely proportional to the amount of nanophase iron formed in the regolith grains, solar wind standoff has been proven to be at the very least a necessary precursor for the formation of the lunar albedo differences.

Using the fully kinetic particle-in-cell code, iPIC3D, coupled with a surface vector mapping magnetic field model based on Kaguya and Lunar Prospector observations, we model the solar wind interaction with the Reiner Gamma magnetic topology for all plasma regimes the region is exposed to along a typical lunar orbit, including various solar wind conditions and the Moon's crossing through the terrestrial magnetosphere.

The resulting proton and He²⁺ energy flux profile, a representative measure for the long-term effect of regolith weathering by impinging plasma, reveals all large-scale signatures of the Reiner Gamma albedo pattern only when integrating over the full lunar orbit. Intriguingly, evolving He²⁺ as a self-consistent plasma species in the simulation is crucial to reproduce the optical brightness ratios between the inner/outer bright lobes and the dark lanes as observed by the Lunar Reconnaissance Orbiter Camera.

To conclude, as Reiner Gamma may be a prime subject for one of our next landers or low-orbiting missions to the Moon, understanding better the integrated plasma flux profile to the surface is imperative to estimate the long-term effects both on the lunar regolith and on future robotic/human exploration activities.

J.D. and M.H. gratefully acknowledge support from NASA-LDAP, #80NSSC17K0420. This work was supported by NASA-SSERVI's IMPACT. HPC Resources were provided by NASA-HEC/NAS at Ames Research Center and TGCC/CINES-DARI #A0050400295.