Abstract
All lunar swirls are known to be co-located with crustal magnetic anomalies (LMAs). Not all LMAs can be associated with albedo markings, making swirls, and their possible connection with the former, an intriguing puzzle yet to be solved. By coupling fully kinetic simulations with a Surface Vector Mapping model, we show that solar wind standoff, an ion–electron kinetic interaction mechanism that locally prevents weathering by solar wind ions, reproduces the shape of the Reiner Gamma albedo pattern. Our method reveals why not every magnetic anomaly forms a distinct albedo marking. A qualitative match between optical remote observations and in situ particle measurements of the back-scattered ions is simultaneously achieved, demonstrating the importance of a kinetic approach to describe the solar wind interaction with LMAs. The anti-correlation between the predicted amount of surface weathering and the surface reflectance is strongest when evaluating the proton energy flux.
Highlights
All lunar swirls are known to be co-located with crustal magnetic anomalies (LMAs)
If the surface weathering pattern generated by the solar wind interaction with the magnetic topology matches the observed albedo markings, it supports the formation of lunar swirls by solar wind standoff
We show that solar wind standoff explains the correlation between the lunar surface albedo patterns and LMAs
Summary
All lunar swirls are known to be co-located with crustal magnetic anomalies (LMAs). Not all LMAs can be associated with albedo markings, making swirls, and their possible connection with the former, an intriguing puzzle yet to be solved. By coupling fully kinetic simulations with a Surface Vector Mapping model, we show that solar wind standoff, an ion–electron kinetic interaction mechanism that locally prevents weathering by solar wind ions, reproduces the shape of the Reiner Gamma albedo pattern. If the surface weathering pattern generated by the solar wind interaction with the magnetic topology matches the observed albedo markings, it supports the formation of lunar swirls by solar wind standoff. A fluid (magnetohydrodynamic) or hybrid (using a kinetic description for the ions but describing the electrons as a mass-less fluid) approach requires surface magnetic fields and/or spatial scales of at least an order of magnitude greater than what is at present day inferred from in-orbit observations to shield the underlying surface, form Reiner Gamma’s three bright lobes, and focus the solar wind plasma into its dark lanes[8,22]. The reflected proton fluxes predicted by our fully kinetic simulations are in excellent agreement with in-orbit flux measurements from the Sub-keV Atom Reflecting Analyzer–Solar Wind Monitor (SARA-SWIM) ion sensor onboard the Chandrayaan-1 mission, reassuring that a kinetic approach to describe the solar wind interaction with LMAs is vital
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