The existence of a threshold energy dependence is shown for the charging of geostationary spacecraft in eclipse. Applied Technology Satellite 6 and P78-2 (SCATHA) data show that plasma sheet fluxes must extend above 10 keV for these satellites to charge in eclipse. The existence of a threshold energy is significant because magnetospheric convection boundaries produce relatively sharp phase space boundaries, with electron fluxes falling rapidly above the energy associated with the local Alfven layer. The threshold effect is due to the shape of the normal secondary yield curve, in particular the high energy crossover, where the secondary yield drops below 1. A large portion of the ambient electron flux must exceed this energy for a negative current to exist.