In sudden flux increases at synchronous altitude the lower energy channels often show progressively more delay or dispersion. It is usually assumed that the dispersion is caused by a simultaneous injection of particles of all energies at some location, and by the subsequent drift of these particles to the synchronous altitude measurement site "downstream" of the injection event. In this paper we present a method for timing and locating the injections from proton auroral precipitation inferred by the Lyman α emission data from the IMAGE FUV instrument. We compare the timing of the proton flux increases observed by the Los Alamos National Laboratory synchronous altitude satellites to the time delay predicted by a model describing the longitudinal drift of particles in the magnetosphere. We present comparisons for eleven substorm particle injections and find that the observed azimuthal drift times are reasonably consistent with those calculated by a simple model using the Tsyganenko 89 magnetic and Volland electric field models as input. This consistency supports the concept that the proton auroral intensification at substorm onset and the proton injection in the magnetosphere occur at the same magnetic local time (longitude).