Photoemission Driven Charging in Tenuous Plasma

In the cold, tenuous plasma commonly encountered in magnetospheric and interplanetary orbits, and in the case of scientific satellites with nearly all surfaces effectively conducting, surface charging is driven by photoemission current. The differential potential of the few insulating surfaces, such as lenses or insulating grout between solar cells, can be positive or negative depending on the photoemissivity of the material. We analyze this effect for spacecraft like those of the Magnetospheric MultiScale and the Radiation Belt Storm Probes missions. This paper develops a simple theory for the potentials of sunlit insulators, focusing on insulators that make up a small part of a large conductive surface. The shape of the photoemission spectrum places an absolute limit of about 12 V of positive differential charging on sunlit insulators. For small insulating surfaces, the conventional assumption-that the photoelectronsthat cannot energetically escape return to their surface of origin-is not valid because the photoelectron trajectory path length is large compared with the surface dimension. We describe a theory that accounts for photoelectron transport between small insulators and the surrounding conductive area. These calculations are done both for the case that the insulator has photoemission similar to the conductive area and for the case that the photoemission is far less, as is the case for many insulators. If the insulator has photoemission current density similar to that of a conductor, we predict positive differential potentials of about 2 V at low chassis potential and negligible differential at high chassis potential. In the opposite case that the insulator photoemission is low, we predict no differential at low chassis potentials and negative differential potentials of up to several volts at high chassis potential.