The substorm injection boundary model has enjoyed considerable success in explaining plasma signatures in the near geosynchronous region. However, the injection boundary has remained primarily a phenomenological model. In this paper we examine 167 dispersionless energetic ion injections which were observed by AMPIE CCF. The radial and local time distribution of the events as a function of Kp is qualitatively similar to that envisioned in the injection boundary model of Mauk and McIlwain (1974). We will argue that particles observed during dispersionless injections are locally energized during the disruption of the cross tail current sheet. Therefore we identify the injection boundary, as derived from the spatial distribution of dispersionless injections, with the earthward edge of the region of the magnetotail which undergoes current sheet disruption during the substorm expansion phase. We will show that this qualitiative model for the generation of the injection boundary can provide an explanation for the dispersionless nature, the double spiral shape, and the Kp dependence of the boundary. ¿ American Geophysical Union 1990