Lunar plasma and electrostatic environment: numerical approach and its future prospects

Mission preparation for lunar exploration using landers has been rapidly increasing, and strong demand should arise toward precise understanding of the electrostatic environment. The lunar surface, which has neither a dense atmosphere nor a global magnetic field, gets charged electrically by the collection of surrounding charged particles of the solar wind or the Earth's magnetosphere. As a result of the charging processes, the surface regolith particles behave as "charged dust grains". Dust particles have been suggested to have adverse effects on exploration instruments and living organisms during the lunar landing missions, and their safety evaluation is an issue to be solved for the realization of sustainable manned lunar explorations. It is necessary to develop comprehensive and organized understanding of lunar charging phenomena and the electrodynamic characteristics of charged dust particles.

It is widely accepted that the surface potential of the lunar dayside is, "on average" several to 10 V positive due to photoelectron emission in addition to the solar wind plasma precipitation. Recent studies, however, have shown that insulating and rugged surfaces of the Moon tend to make positive and negative charges separated and irregularly distributed, and intense and structured electric fields can be formed around them. This strong electric field lies in the innermost part of the photoelectron sheath and may contribute to mobilizations of the charged dust particles. Since this strong electric field develops on a spatial scale of less than the Debye length and can take various states depending on the lunar surface geometry, it is necessary to update the research approach. In this paper, we will discuss the direction of the near-surface plasma, electrostatic, and dust environment for upcoming lunar landing missions.