Electrostatic potential distribution of the sunlit lunar surface

The steady-state lunar surface charge and potential distributions are determined by the condition that the net current to a small surface area vanish, where the dominant currents are due to photoemission of electrons and collection of solar wind particles. The lunar crust and photoelectron cloud are too resistive to carry a significant flux. A calculation similar to one used in collisionless electrostatic probe theory shows that the current from the solar wind is predominantly due to the electrons, is independent of potential, and is weakly dependent upon the polar angle θ measured from the moon-sun line. The calculation of the photoelectron current, which takes into account the spread in energies of electrons emitted by monoenergetic photons, determines the surface potential as a function of θ. The solution is insensitive to the detailed structure of the solar spectrum and depends parametrically on the photoemissive properties of the lunar surface. For a work function of 5.0 volts and quantum yield of 0.01, the electrostatic potential during solar minimum decreases from 3.0 volts at the subsolar point to less than a volt near the limb. Plausible ranges for the lunar quantum yield and work function are 0.001 to 0.1 and 4 to 6 volts, respectively, which correspond to a range of potentials at the subsolar point from 0.6 to 10.2 volts. These values assume a solar wind electron number density and temperature of 5 cm−3 and 105°K, respectively.