Differential photoelectric charging of nonconducting surfaces in space

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Abstract
Results of numerical calculations of photoelectric charging are presented for an infinitely long strip of sunlight across a nonconducting plane, the simplest geometry for which differential charging occurs. The model contains an electrical cutoff radius and therefore simulates charging of a sunlit area with dimensions equal to the strip's width, exposed to a plasma with a comparatively large Debye length. Uniform potential is quickly established on a uniformly sunlit strip as a result of charge redistribution by low‐energy photoelectrons. Results are consistent with a theoretical surface conductivity derived for photoelectron sheaths above highly charged sunlit areas. Conductorlike distribution of charge persists throughout electrification of the strip, accretion of photoelectrons by the dark plane, and contraction of the strip (simulation of solar occultation or sunset). The surface potential drops sharply across the sunlight‐shadow boundary, and if the boundary is diffuse, the potential in the penumbra falls substantially below the maximum strip potential only very near the penumbra's dark edge. These results demonstrate the generality of conductorlike behavior of sunlit areas and the consequent intensification of electrostatic fields at the sunlight‐shadow boundary.