A solid surface that is in contact with a plasma collects electric charges of impinging electrons and ions. In general, due to the higher differential influx of electrons than that of ions, the surface gets charged negatively, that is, acquires a negative potential with respect to the environment (Whipple, 1981). This potential field exerts a repulsive force for approaching electrons and restrains further inflow of thermal electrons. The equilibrium (or floating) potential is eventually reached when the positive and negative charge inflows are balanced; that is, the net current flow into the surface becomes zero. This concept of current balance holds also in the presence of additional current components that are caused by emission processes of charged particles. The charging process (e.g., Garrett, 1981) and its feedback to plasma environment, such as the formation of sheaths and presheaths (e.g., Robertson, 2013;Scheiner et al., 2015), have been investigated extensively. In space applications, the understanding of charging processes has been developed through long-standing studies of spacecraft charging as well as probe measurements (e.g., Fahleson, 1967;Garrett & Whittlesey, 2000).The surface charging is a significant research area also for the physics of airless, solid planetary bodies in the solar system, such as the Moon and asteroids (Manka, 1973;Stubbs et al., 2014;Zimmerman et al., 2014). This is because charging possibly leads to electrostatic transport of charged dust grains on their surfaces (Nitter et al., 1998;. Such natural bodies covered with nonconducting regolith layers exhibit manifested differential charging depending on both space-environmental and surface conditions. On-orbit observations and theoretical predictions have shown that the darkside and terminator

Unconventional current balance is established between solar wind ion and electron at the bottom of deep cavities on the Moon and asteroids • Cavity bottom surface could have significant positive potentials that are comparable to the kinetic energies of solar wind ions • Photoelectrons no longer promote positive charging by themselves but rather relax the positive potentials at the cavity bottom

<p>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.</p><p>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.</p>