A new model is proposed for the ion beams frequently observed in the plasma sheet boundary layer. The model assumes injection of magnetosheath plasma onto closed tail field lines. The plasma then expands along the field lines and is convected towards the earth. Particle simulations of the Expansion process are used to calculate properties of these beams. The expansion produces beams of ions at energies of up to a few tens of KBTe, which are consistent with the observed energies of the beams. ¿ American Geophysical Union 1987