The substorm injection boundary model proposed by McIlwain (1974) and since adopted by many researchers describes the phenomenology of plasma introduction into the middle magnetosphere during a magnetic substorm: at the time of the substorm onset, both electrons and ions of all energies share a common Earthward boundary. Such an injection should leave a clear signature in the auroral precipitation data. We used the Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites to search for such signatures in the ≤30-keV auroral electron and ion data. Clear examples of an equatorward leap of the previously energy dependent equatorward cutoff in auroral precipitation to a cutoff boundary common to ions of all energies measured (i.e., dispersionless up to 30 keV) and electrons up to several keV (usually less than 10 keV) were observed in the near-midnight sector at a time corresponding to substorm onset. A collection of 10 such events occurring over a 1-month interval was studied, providing unusually direct confirmation that the phenomenological model of an initially dispersionless boundary accurately describes at least some injections. The DMSP data show the injection boundary to be dispersionless over a wide longitudinal range, but often only a dispersive injection is seen at dawn. ¿American Geophysical Union 1987