Abstract
The lunar surface is continuously under the impact of solar wind plasma, which breaks the chemical bonds of the surface material resulting in weathering of the surface and a modified chemical composition. Ion impact also sputters the surface material, affecting the composition of the lunar exosphere, and it controls the electrical properties of both the lunar surface and the near surface space.We have studied the lunar farside-nearside (FN) hemispherical asymmetry of the solar wind proton impact on the lunar surface along the orbit of the Moon during fast solar wind conditions. The analysis is based on a 3D hybrid model where ions are accelerated by the macroscopic j × B and pressure gradient forces.The derived proton impact surface map shows that the highest cumulative solar wind proton addition on the lunar surface is located on the farside while the most energetic protons precipitate on the nearside. The total ion impact rate was found to be smallest when the Moon is deep in the magnetotail. The total ion impact rate on the lunar surface varies while the Moon orbits the Earth and these longitudinal variations are caused by the magnetosphere and lunar tidal locking.
Highlights
Lunar surface is continuously bombarded by the solar wind plasma because the Moon has neither an atmosphere nor a global intrinsic magnetic field that could deflect the plasma flow around it
We have investigated the properties of the solar wind protons impacting on the lunar surface by the 3D j  B hybrid model
The j  B hybrid model was built in the simulation platform that has been used earlier to study the interaction of solar wind plasma with the lunar surface in 3D (Kallio, 2005), the Debye layer near the lunar surface (Dyadechkin et al, 2015), the properties of plasma near lunar magnetic anomalies (Jarvinen et al, 2014) and the motion of charged dust particles near the lunar surface and around the Moon (Dyadechkin et al, 2015)
Summary
Lunar surface is continuously bombarded by the solar wind plasma because the Moon has neither an atmosphere nor a global intrinsic magnetic field that could deflect the plasma flow around it. Recent observations of the optical properties of walls of lunar craters has shown, that there is an East-West asymmetry inside craters, which has been suggested to be consequence of systematic change of the properties of plasma when the Moon is in the magnetosphere (Sim et al, 2017). In addition to these longitudinal space weather effect variations, there are latitudinal variations, which has been suggesting to be related the decreasing intensity of precipitating solar wind particles with increasing latitude (Hemingway et al, 2015). Because of the tidal locking, the cumulative total proton precipitation rate on the lunar surface is different on opposite sides of the Moon and the instantaneous plasma precipitation depends on the position of the Moon on its orbit
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