Dependence of lunar surface charging on solar wind plasma conditions and solar irradiation

摘要

The surface of the Moon is electrically charged by exposure to solar radiation on its dayside, as well as by the continuous flux of charged particles from the various plasma environments that surround it. An electric potential develops between the lunar surface and ambient plasma, which manifests itself in a near-surface plasma sheath with a scale height of order the Debye length. This study investigates surface charging on the lunar dayside and near-terminator regions in the solar wind, for which the dominant current sources are usually from the photoemission of electrons J_p, and the collection of plasma electrons J_e, and ions J_i. These currents are dependent on the following six parameters: plasma concentration n_O, electron temperature T_e, ion temperature T_i, bulk flow velocity V, photoemission current at normal incidence J_(PO), and photoelectron temperature T_p. Using a numerical model, derived from a set of eleven basic assumptions, the influence of these six parameters on surface charging - characterized by the equilibrium surface potential, Debye length, and surface electric field - is investigated as a function of solar zenith angle. Overall, T_e is the most important parameter, especially near the terminator, while J_(PO) and T_p dominate over most of the dayside. In contrast, V and T_i are found to be the least effective parameters. Typically, lunar surface charging in the solar wind can be reduced to a two-current problem: on the dayside in sunlight, J_p+J_e = 0, since |J_p|＞|J_e|＞|J_j|, while near the terminator in shadow, J_e+J_i = 0. However, situations can arise that result in a truly three-current problem with some important consequences; e.g., very cold T_e and/or very fast V can result in |J_p|＞|J_e|＞|J_j| on the dayside. The influence of surface charging pervades the environments of the Moon and other airless bodies, and the investigation presented here provides insights into the physical processes involved, as well as being useful for interpreting and understanding more complicated simulations.