Theoretical considerations and observational evidence suggest that the polar cap (which marks the boundary between closed and open field lines) can expand rapidly enough during the growth phase of a substorm to release geomagnetically trapped particles from previously closed drift shells and thus enable them to escape into the tail. This scenario requires induced electric fields sufficient to yield E⋅J0 there at all times. In support of our concept we cite observations which suggest that the moving separatrix can overtake convecting nightside plasma during intervals of polar-cap expansion, and we illustrate the occurrence of this phenomenon with a simple model which suggests that closed nightside field lines containing trapped particles can become open and thus be evacuated of their particle populations. Such evacuation seems to account for the occurrence of energetic-particle bursts in the tail (for which impulsive energization is therefore unnecessary) and for some of the plasma flows that occur in the region adjacent to the plasma-sheet boundary layer. Expansion of the dayside portion of the polar cap should contribute to the transition of particles from closed to open field lines there and thus should increase the fluxes of magnetospheric ions in the magnetosheath. ¿ American Geophysical Union 1989