Research On Interaction Between Lunar Surface And Space Plasma Environment

Due to the tenuous atmosphere and lacking global magnetic field,the Moon provides a natural laboratory to investigate the interaction between space plasma and celestial bodies without a dense atmosphere and global magnetic field.Studying the particle transport and energy transport in the lunar space environment and associated processes,can lead to a better understanding of the lunar space environment,improve the accuracy of the lunar space weather forecast,and guarantee the implementation of the lunar exploration program.The exploration and study of lunar water is one of the necessary conditions for the establishment of a lunar base,and also help us understand the formation and evolution of the Moon and other celestial bodies.The bombardment of the plasma on the lunar surface produces energetic neutral atoms(ENA)and water outside and inside the lunar regolith,respectively,which are closely related.Therefore,this work aims to study the surface ENA and water produced by interaction between the space plasma and lunar surface:(1)For ENA,previous studies showed that the ENA vary with solar incident angle,plasma environment,and magnetic anomalies,which promotes our understanding of the interactions between the space plasma and the lunar surface.However,the diurnal variation of ENA remains unclear as lacking continuous observations focused on the same area.In addition,the low spatial resolution of orbital observations makes it impossible to study the effect of lunar surface topography and composition on ENA.Based on the Advanced Small Analyzer for Neutrals(ASAN)on board the Chang’E-4 Yutu-2 rover,we examine the ENA energy spectra obtained in the 23 lunar days from January 11,2019 to October 12,2020 and find a higher ENA differential flux on the lunar dawnside than on the duskside.Combined with ARTEMIS(Acceleration,Reconnection,Turbulence,and Electrodynamics of the Moon’s Interaction with the Sun)data,we find the ENA differential flux is positively correlated with solar wind flux,density,and dynamic pressure.However,the ENA cutoff energy and temperature are lower on the duskside than on the dawnside at the same solar wind energy.The difference between the ENA-solar wind observation on the dawnside and duskside is possibly caused by solar wind deflection and deceleration on the duskside,which can be attributed to the interaction between the solar wind and the lunar magnetic anomalies located nearby in the northwestern direction of the Chang’E-4 landing site.Furthermore,we tried to study the influence of lunar surface topography and composition on ENA combined with the dataset obtained by Visible and Near-infrared Imaging Spectrometers aboard the Yutu-2 rover.Preliminary results showed that ENA flux is lower in the region where npFe0(nanophase iron)abundance is higher,but its microscopic physical mechanism needs further studies.(2)The solar wind is traditionally considered to be the major source of lunar surface water.However,the species and energy of particles vary significantly during one lunation period,especially when the Moon is in the Earth’s magnetosphere,and the influence of these variations on lunar water remains unclear.The mechanisms of lunar water formation have been studied previously based on remote sensing observations or laboratory lunar samples studies,although never been studied with multi-satellite simultaneous observations.Here we report the temporal and spatial distribution of polar surficial OH/H2O abundance,using the Chandrayaan-1 Moon Mineralogy Mapper(M3)data,which covers the regions inside/outside the Earth’s magnetosphere.The results shows that polar surficial OH/H2O abundance increases with latitude and that the probability of polar surficial OH/H2O abundance remains at the same level when the Moon is in the solar wind and in the magnetosphere by controlling latitude,composition,and lunar local time.This indicates that the OH/H2O abundance in the polar regions may be saturated,or supplemented from other possible sources,such as Earth wind(particles from the magnetosphere,distinct from the solar wind),which may compensate for thermal diffusion losses while the Moon is inside the Earth’s magnetosphere.This work provides some clues for studies of the planet-moon systems,whereby the planetary wind serves as a bridge connecting the planet with its moons.In addition,we also study the variation of lunar water abundance when the plasma environment in the magnetosphere changes using Kaguya data.Preliminary results show that the plasma particles in the magnetosphere may promote the production of lunar water,but the physical mechanism of which needs to be further studied.We also try to study the influence of the earthward flux and tailward flux in the magnetosphere on the formation of lunar water using ARTEMIS data combined with a water molecules migration model based on Monte Carlo simulation.Preliminary results showed that lunar water produced by the solar wind,Earth wind,and earthward/tailward flux will accumulate at the morning terminator.The water produced by earthward flux will also accumulate at the night terminator.However,compared to the solar wind,the contribution of earthward and tailward flux to lunar water is smaller due to its lower flux.In conclusion,we studied two substances produced by the interaction between space plasma and the lunar surface.The continuous observations of lunar surface ENA in the Chang’E-4 landing region found dawn-dusk asymmetry,which was caused by the interaction between the solar wind and magnetic anomalies neighboring the landing region.The electrostatic potential on the lunar surface was calculated for the first time.Lunar water observations within the magnetosphere found that Earth wind also is one of the sources of the lunar water,which can provide the reference not only for the numerical simulation and laboratory simulated lunar water production processes,and can also further add the knowledge of magnetosphere particle material cycle and energy transport process to promote the understanding of Earth-Moon system interactions.The results of this paper promote the understanding of the microphysical mechanisms of the interactions between space plasma and the lunar surface,serving as a reference for the further study of the interaction between celestial bodies without atmosphere and global magnetic field and space plasma,and provide theoretical reference for China’s forthcoming lunar exploration missions such as the International Lunar Scientific Research Station.