Aspects of Spacecraft Charging in Sunlight

Abstract

This paper is an overview of spacecraft charging in sunlight. The daylight photoelectron flux emitted from spacecraft surfaces normally exceeds the ambient electron flux. As a result, charging of spacecraft surfaces to positive voltage is expected to occur in sunlight. Indeed, spacecraft are often observed to charge to low positive voltages in sunlight. However, spacecraft can charge to high-level (kiloelectronvolts) negative voltages in sunlight. Why do spacecraft charge negatively in sunlight? One chief reason concerns differential charging between the sunlit and dark sides. For a satellite with dielectric surfaces, an electric field builds up on the shaded surfaces and then wraps around to the sunlit side to form a potential barrier that suppresses the photoemission. A monopole-dipole (for zero spin) or monopole-quadrupole model (for fast spin) describes the differential charging potential distribution due to blocked photoelectrons. It is shown that these cases are similar to a more general multipole potential field in that the surface node potentials satisfy an approximate linear relation. These cases are all driven by the shade side charging so that the onset for charging is approximately the same in sunlight or eclipse if conduction currents through the spacecraft can be neglected. If conduction currents are important, potential barriers can develop on the dark side, leading to suppression of the secondary emission currents and modification of charging onset. The results were briefly compared with observations. Another important reason for negative charging concerns reflectance. Highly reflective mirrors generate substantially reduced photoemission so that current balance can be achieved without barrier formation. The onset for charging in this case depends strongly on the reflectivity. The critical temperature for charging of surface materials under space substorm conditions with different ratios of photoemission current to electron ambient current, corresponding to varying satellite surface reflectivity values, was calculated. Numerical results, which show that with substantially reduced photoemission, highly reflective surfaces charge in sunlight with the critical temperature for onset decreasing with increasing reflectivity, are presented