Hybrid simulation of the interaction of solar wind protons with a concentrated lunar magnetic anomaly

Abstract

Using a two-dimensional hybrid simulation, we study the physics of the interaction of the solar wind with a localized magnetic field concentration, or "magcon," on the Moon. Our simulation treats the solar wind protons kinetically and the electrons as a charge-neutralizing fluid. This approach is necessary because the characteristic scale of the magcon is of the same order or smaller than the proton inertial length-the characteristic scale in the hybrid simulation. Specifically, we consider a case in which the incident solar wind flows exactly normal to the lunar surface, and the magcon is represented by a simple dipole whose moment is parallel to the surface, with a center just below it. We find that while the magcon causes the solar wind to be deflected and decelerated, it does not completely shield the lunar surface anywhere. However, protons which impact the surface in the center of the magnetic anomaly have energies well below the solar wind ram energy. Thus, in this region, any backscattered neutral particles resulting from the interaction of solar wind protons with the lunar regolith would have energies lower than that of the solar wind. Moreover, very few neutrals, if any, would emanate from within the magcon with energies comparable to the solar wind energy. This may explain recent observations of lunar energetic neutral atoms associated with a strong crustal magnetic anomaly. Our study also finds that a significant fraction of the incoming solar wind protons are reflected back into space before reaching the surface. These particles are reflected by a strong electrostatic field which results from the difference in the proton and electron inertia. The reflected particles are seen at very high altitudes above the Moon, over 200 km, and over a much broader spatial scale than the magcon, several hundred kilometers at least. Our simulation also revealed a second population of reflected particles which originate from the side of the magcon where the interplanetary and magcon magnetic fields are directed opposite to one another, leading to a magnetic topology much like magnetic reconnection. As previously reflected particles move through this region, they are deflected upward, away from the surface, forming a second component. Our simulation has a number of similarities to recent in situ spacecraft observations of reflected ions above and around magcons.

Scattering characteristics and imaging of energetic neutral atoms from the Moon in the terrestrial magnetosheath

“…There is no significant shift of the observed spectra toward lower energies compared to the model results. Thus, we do not see any indication of a deceleration by hundreds of eV of the incident ions by electric fields such as those observed at the Moon in the solar wind by e.g., Saito et al [2012] and Futaana et al [2013], modeled by several recent studies [e.g., Poppe et al, 2012;Kallio et al, 2012;Jarvinen et al, 2014;Deca et al, 2014Deca et al, , 2015Fatemi et al, 2015;Giacalone and Hood, 2015;Zimmerman et al, 2015], and observed in laboratory [e.g., Bamford et al, 2012;Howes et al, 2015;Wang et al, 2012Wang et al, , 2013. Notably, most of the cited studies concern primarily the low solar zenith angle case, while the case we observe in P1 is at high solar zenith angles.…”

Lunar Swirl Morphology Constrains the Geometry, Magnetization, and Origins of Lunar Magnetic Anomalies

“…Roughly speaking, portions of the surface with open magnetic field lines may be expected to experience normal or even accelerated solar wind‐related space weathering, leading to darkening, whereas portions of the surface beneath closed magnetic field lines should experience greater protection from solar wind weathering, thus remaining relatively bright (Hemingway & Garrick‐Bethell, ). Indeed, magnetometer‐based studies of magnetic field structure (Hemingway & Garrick‐Bethell, ; Shibuya et al, ; Tsunakawa et al, ), as well as hybrid and kinetic plasma simulations (e.g., Bamford et al, , ; Deca et al, , ; Fatemi et al, ; Giacalone & Hood, ; Jarvinen et al, ; Poppe et al, , ; Zimmerman et al, ), indicate that swirl morphology may be dictated by magnetic field topology in precisely this way. It has also been proposed that swirls may be the result of electrostatic (Garrick‐Bethell et al, ) or magnetic (Pieters et al, ) sorting of fine‐grained materials, rather than of deflection of solar wind.…”

ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies

“…Numerous investigations have explored the potential mechanisms at work within crustal magnetic anomalies with laboratory experiments [ Wang et al , , ] and numerical simulations, including magnetohydrodynamic (MHD) [e.g., Harnett and Winglee , , ; Xie et al , ], hybrid [ Fatemi et al , , ; Jarvinen et al , ; Giacalone and Hood , ; Poppe et al , ], and kinetic/particle‐in‐cell methodologies [e.g., Poppe et al , ; Deca et al , , ; Zimmerman et al , ; Bamford et al , ]. These simulations have suggested that large‐scale ambipolar and/or Hall electrostatic fields may be the primary mechanism of proton reflection from lunar crustal magnetic fields, rather than magnetic reflection.…”