The importance of monitoring particle injection into the inner magnetosphere from the plasma sheet is exceeded perhaps only by its difficulty. The recent progress in using ground‐based riometer data [e.g., Spanswick et al., 2007] to detect by proxy particle injection has raised much hope that this important aspect of substorms can be more consistently monitored. In this paper we develop a theoretical model for explaining the observed dispersionless injection events reported by Spanswick et al. The substorm event on 3 October 1998 is analyzed to give the empirical context for riometer responses during a typical substorm. Our simulation shows that riometer electrons produce a clean and strong signal that can be uniquely and easily related to magnetic field dipolarization that normally gives rise to injection.