Electrostatic charging of crater's surface over sunlit moon

Abstract

Under direct exposure to solar UV radiation and solar wind plasma collection, the sunlit lunar surface undergoes electrostatic charging. The presence of surface relief results in non-trivial effects on surface charging and gives rise to complex electric potential and field structures. An analytical formulation describing surface charging within a simple crater is given using adequate solar radiation spectrum, solar wind plasma, and Fermi-Dirac distribution of the photoelectrons. Lunar surface potential is determined by balancing the photoelectrons and solar wind plasma collection currents. The electric potential structures on the surface within a fully illuminated crater have been derived for different solar inclinations and solar wind plasma conditions. Two specific scenarios, viz. (i) normal photon incidence and (ii) oblique photon incidence, have been discussed. For normal photon incidence, the craters acquire positive potential. A finite region over the crater's surface acquires negative potential for oblique photon incidence.