Chang'E-6 reveals solar wind-dependent H^- ions on the Moon.

Abstract

Apart from positive ions and electrons, negative ions are expected in various astrophysical environments. However, they have never been detected on the Moon until the Chang'E-6 mission. The NILS instrument onboard Chang'E-6 lander is the first dedicated instrument for detecting negative ions beyond Earth and has successfully obtained H^- spectra on the lunar surface, providing an unprecedented opportunity to investigate their origin and distribution. Here, we present a positive correlation between the H^- spectra and solar wind parameters, which provides direct evidence for the generation of negative ions from solar wind-surface interaction. Combined with Monte Carlo simulations, we predict a thin dayside H^- layer and a long nightside H^- tail, which can contribute to the lunar plasma environment, especially during an extreme solar wind density event. These findings greatly improve our understanding of the generation and distribution of negative ions on the Moon and other airless bodies.