Kinetic simulations of kilometer-scale mini-magnetosphere formation on the Moon

Abstract

Kinetic simulations are used to examine the solar wind's interaction with a 3 km wide region of strong crustal dipole magnetization on the Moon. In contrast with recent hybrid and implicit particle-in-cell simulations of magnetic anomalies that have aimed to resolve electric fields over several tens of kilometers, kinetic simulations reveal a much smaller scale regime in which magnetically driven ion-electron separation can generate a kV potential difference over a height of less than 200 m. The resulting electric field structure varies considerably between dawn and noon (when the solar wind flows, respectively, horizontally across the surface and vertically down from above) and is strong enough to reflect some ions back into space, consistent with spacecraft observations. Ion velocity and energy distributions are extracted near the surface and are used to derive maps of ion flux and impact energy, and the effects on sputtering and defect formation within the regolith are discussed. However, considerable uncertainty remains in how the surface ion flux evolves throughout a lunar day and how the plasma-surface-magnetic field interaction changes with respect to different magnetic topologies.

The statistical mechanics of solar wind hydroxylation at the Moon, within lunar magnetic anomalies, and at Phobos

“…These swirls are associated with lunar magnetic anomalies, and it was suggested therein that the change in OH content was connected to the magnetic deflection of solar wind inflow into the region. Specifically, the magnetized regions within the swirls that possessed the lower OH content were found to have higher albedo and overlying horizontally directed magnetic fields that have been shown to retard incoming solar wind flow—both reducing the solar wind influx and reducing the ion energy by over 80% [ Fatemi et al ., ; Zimmerman et al ., ; Poppe et al ., ]. Regions with open field lines pointing vertically out to into the solar wind allowing ions to directly access the surface, and such regions appear similar in the visible and NIR to the nonmagnetic terrain.…”

“…Zimmerman et al . [] modeled the effect of a decelerated and flux‐reduced solar wind in a simulated magnetic anomaly and found significantly decreased sputtering rates from the surface (see their Figure 7). In anomaly regions, TRIM code indicates that there is less vacancy production from the decelerated proton. In unshielded regions, TRIM results indicate that each 1 keV proton creates two crystal lattice vacancies for each incident ion and over a few hundred years have the ability to create an amorphous crystal structure in the top tens of nanometers like that reported by Noble et al .…”

“…For the magnetic anomalies, the solar wind ions have lost energy due to the retarding ambipolar and Hall E fields [Fatemi et al, 2015;Zimmerman et al, 2015;Poppe et al, 2016]-leading to shallower penetration into the regolith. In essence, we run both statistical mechanics and Monte Carlo modeling in the case of a less potent solar wind of lower overall flux and energy.…”

“…The solar wind shielding efficiency in excess of 50% has been measured in the anomaly-rich south-pole Aitken (SPA) basin region [Vorburger et al, 2012]. Models of this electrostatic deflection of solar wind indicate that the 1 keV solar wind energy should be reduced at ground level by these polarization E fields to values as low as 200 eV [Fatemi et al, 2015;Zimmerman et al, 2015].…”

ARTEMIS Observations of Solar Wind Proton Scattering off the Lunar Surface

“…The padding is added to allow a certain uncertainty in the back tracing, as well as an uncertainty in the effective area‐of‐influence of the LMAs relative to their crustal location (the area shielded by an LMA may, e.g., be shifted downstream relative to the solar wind flow; c.f. Deca et al, ; Lue et al, ; Zimmerman et al, ). The resulting regions are comparable to maps of LMA‐reflected H + observations (Lue et al, ; Poppe et al, , ).…”