Effects of Solar UV on Spacecraft Charging in Sunlight

摘要

Spacecraft surface charging is determined by the balance of currents. Photoelectron currents from spacecraft surfaces greatly exceed the ambient electron or ion currents and therefore are often of prime importance for charging in sunlight. The authors present a brief overview of several aspects of spacecraft charging in sunlight. For a conducting spacecraft at geosynchronous altitudes, charging in sunlight is usually up to a few positive volts only. If the spacecraft is in areas where the solar ultraviolet radiation is strong and the ambient electron density is low, the spacecraft can charge to a few tens of positive volts. For a non-conducting spacecraft at geosynchronous altitudes, the dark side can charge to hundreds or thousands of negative volts as a result of the collection of ambient electrons. There exists a critical electron temperature governing the onset of negative voltage charging. The sunlit side initially tends to charge to low positive volts. The high negative voltage of the dark side may wrap around the sunlit side forming a potential barrier blocking the photoelectrons emitted from the sunlit surfaces. As a result, the sunlit side may also charge to negative voltages. The critical temperature for this differential charging to occur is approximately the same as for eclipse charging. Depending on the spin axis with respect to the sun direction, monopole-dipole or monopole-quadrupole potential distributions may occur. For spacecraft with high surface reflectance, the photons do not deposit enough energy to generate photoelectrons. As a result, the surface can charge to high negative voltages in sunlight without invoking differential charging. In this case, the critical temperature is changed, depending on the reflectance and the photo-emissivity of the surface.