Dependences of Energetic Neutral Atoms Energy on the Solar Wind Energy and Solar Zenith Angle Observed by the Chang’E-4 Rover

Abstract

Solar wind can directly interact with the lunar surface and bring a space weathering effect. Some solar wind protons can be scattered as energetic neutral atoms (ENAs), which include rich information of the solar wind–surface interaction. However, people still know little about the ENA truth on the lunar ground due to the lack of in situ measurements. Different from the previous in-orbit measurements, here we present the first ground-based ENA measurements by the Chang’E-4 rover and find a good correlation between the mean ENA energy and the solar wind energy. Moreover, the loss rate of ENA energy can strongly depend on both the solar wind energy and the solar zenith angle (SZA), in which the energy loss rate can be enhanced by 73% when the solar wind energy increases from 400 to 1400 eV and can be reduced by 32% when the SZA increases from 57° to 71°. Combined with numerical simulations by SDTrimSP code, we propose that the solar wind protons can penetrate deeper into the lunar surface with a longer path length when the solar wind energy is higher or the SZA is lower, which results in a larger energy loss rate for the scattered ENAs. Our results provide an important constraint for the solar wind–surface research and have general implications in studying the surficial space weathering of the Moon and other airless bodies.