The low-energy outflowing and bouncing ion distributions displayed by a statistical study of the retarding ion mass spectrometer data on DE 1 have been investigated using three-dimensional simulations of ion trajectories in the magnetosphere. Because of the simultaneous azimuthal and latitudinal drifts imposed by the dawn-dusk convection electric field, it is shown that the statistically observed ionospheric outflow evolves into the observed trapped ion population. The drift paths of these ions as well as the location of their ''source'' region strongly depend on their mass. It is demonstrated that the morning sector auroral zone in the nightside magnetosphere is the major source of low-energy trapped ions. A comparison between the computed and observed magnitudes has been made for both light (H+) and heavy (O+) ions that reveals a better agreement at short flight times (about 2 hours) than at longer ones. This emphasizes the role of loss mechanisms and the dynamic aspects of the magnetospheric convection. ¿ American Geophysical Union 1988