We present results from a model developed to study the motion of impulsively injected energetic particles which become trapped in the Earth's magnetic field at geosynchronous orbit. The model is based on numerically solving the analytic expressions for the bounce average gradient and curvature drift in a model magnetic field. The predicted motion of trapped geosynchronous particles in the Tsyganenko and Usmanov (1982) model (TU-82) has characteristics which are not predicted by less sophisticated models. We investigate the motion of drifting particles predicted by the model under various conditions such as: Kp level, pitch angle, orientation of the magnetic field, and location of the origin of the drift shell. As a test of the predictions of the model, we apply it to observations of a substorm injection event which took place on October 16, 1983, and was observed by three geosynchronous satellites. The injection region for this event is found to span approximately 45¿ from near midnight to near 2100 LT. We also report the existence of a ''periphery'' outside the central injection where either injected ions or electrons, but not both, are observed with dispersionless signatures. The internal consistency of the predictions of the structure of the injection region using remote observations from the three geostationary satellites and the TU-82 field model is found to be quite good. ¿American Geophysical Union 1991