Photoelectron Sheath on Lunar Sunlit Regolith and Dust Levitation

Abstract

Photoelectron sheath formation and subsequent fine dust levitation over the sunlit lunar regolith have been investigated by consistently accounting for the continuous interaction of the solar wind and solar radiation with the Moon. In deriving the photoelectron sheath, the Poisson equation is coupled with the latitude-dependent population density of the Fermionic photoelectrons. The altitude and latitude profiles of the electric potential, electric field, and electron density within the photoelectron sheath have been derived. A larger sheath is predicted near the terminator compared to the subsolar point. Accounting for the sheath features, the charging of levitating particles under the kinetic balance of anisotropic photoelectron flux, solar radiation, and solar wind plasma has been calculated. The dust charge is coupled with a characteristic sheath field to evaluate the altitude profile of the particle size, displaying levitation under its electrostatic equilibrium with the lunar gravity. Our analysis suggests that in equilibrium, the submicron particles may levitate up to a couple of meters above the lunar surface; for instance, at the subsolar point (0° latitude) 200 and 50 nm particles may float up to an altitude of ∼64 and ∼194 cm, respectively, while at 70° latitude near the terminator these particles are estimated to levitate at an altitude of ∼18 and ∼227 cm, respectively. The floating charged submicron dust may electrostatically interact with the functioning of experiments and can significantly affect the instrument operation.