Plasma sheath around sunlit moon: monotonic & non-monotonic structures

Abstract

An analytical investigation of the sheath formation over sunlit Moon under the influence of observed solar ultraviolet/ extreme ultraviolet radiation and solar wind plasma has been presented. Poisson's equation is coupled with the latitude-dependent Fermionic photoelectrons, non-Maxwellian solar wind electrons and cold ions to derive the electric potential, electric field, and population density profiles within the photoelectron sheath. We notice that the high energy photons of the solar spectra in addition to Lyman- α, significantly contribute in determining the surface charge and subsequent sheath structure. A traditional Debye (Type C) sheath forms around the terminator due to dominant plasma electron accretion and marginal photoemission. While moving towards the equator, the photoelectron contribution increases, yielding monotonic (Type B) and non-monotonic (Type A) sheath solutions over sunlit locations. The calculations show that the non-monotonic potential structures are more stable near the terminator region, while both types of potential structures are probable near the equator region for the nominal solar wind plasma. A vertical sheath extension of ∼60 m and ∼12 m is predicted for typical solar wind plasma at the equator corresponding to Type A and Type B sheath, respectively. Under exotic plasma conditions (e.g. Earth's magnetospheric tail lobe, plasma sheet), non-monotonic solutions become stable for the photoemission-dominated region, and the entire sunlit lunar surface may acquire negative potential.