A large number of time-varying electron and ion features, such as those observed in energy-time spectrograms of data obtained near geosynchronous orbit, have been previously explained through the use of an injection boundary model. This model describes such features in terms of the dispersion signatures of fresh, hot plasma which is ''injected'' into an extended region tailward of an injection boundary at the time of substorm onset. Using data from the Lockheed ion composition instrument and the University of California, San Diego, electron and ion analyzers on SCATHA, we present two events indicating that the innermost edge of the injection region is an especially important location for the substorm input of ionospheric ions. The injection signatures are identified by comparison of electron and ion data with the dispersion features predicted by various electric and magnetic field models. In both events the ion composition data show intense, narrow-energy, field-aligned O+ fluxes on the limiting edge of the dispersing ions. This feature corresponds to ions which can be backtracked to localized positions in the vicinity of the injection boundary at the onset time. We conclude that the injection boundary, which corresponds statistically to the equatorward edge of the auroral oval, is at times the location of direct ionospheric input into the trapped, energetic particle population.