For a dielectric spacecraft charging in sunlight, the potentials are different on the sunlit and dark sides. Differential charging of spacecraft surfaces can trap low-energy electrons by means of potential wells and barriers. The low-energy electrons are mostly photoelectrons and secondary electrons. Motivated by the recent interest in trapped photoelectrons measured by the Van Allen Probes in the radiation belts, we calculate the extent of the trapped photoelectron area and the potential barrier as a function of the dipole strength and sun angle using the monopole- dipole model. We find that the dipole strength is an important parameter in controlling the behavior of the potential wells and barriers. The usual inequality, 1/2 ≤ A ≤ 1 where A is the dipole strength, used in the monopole-dipole model can be relaxed and amended for finite sun angles. We then use a simple method to estimate the density of the trapped low-energy electrons in these areas. In sunlight charging, the low-energy electron population around the spacecraft is enhanced by the photoelectrons trapped inside the potential barrier.
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