Counterstreaming ion beams originating from the ionosphere are a natural product of interhemispheric plasma flow in empty plasmaspheric flux tubes after geomagnetic storms. Owing to the prevailing collisionless conditions during the early stage of the flow, trapping of the plasma in the flow at relatively high altitudes has remained a mystery. Using 2.5-D particle-in-cell simulations, a study of the ion-beam driven plasma instabilities indicates that the fastest growing wave mode is an effective source for the pitch angle scattering of the ions transferring part of the parallel energy in the magnetic field-aligned flows into the ions' perpendicular energy. The pitch angle scattering is facilitated by the anomalous cyclotron resonance of the beam ions with the fastest growing waves driven by the interaction between the Doppler-shifted cyclotron modes of the two counterstreaming ion beams; the interaction generates purely growing waves at nearly zero frequency. The Landau resonance of the Doppler-shifted cyclotron mode of one of the beams with the other beam, and vice versa, also generates waves which are relatively slow growing at frequencies &ohgr;≅n&OHgr;i/2, where n is an integer and &OHgr;i is the ion cyclotron frequency. Since these waves appear at &ohgr;≅&OHgr;i in the rest frame of the beams, they transversely accelerate the ions. Both the pitch angle scattering and the transverse acceleration contribute to the trapping of the flow in the flux tubes, but the former being a much faster and more effective process than the latter. This collisionless trapping at high altitudes, facilitated by the self-generated waves, can be a major process in plasmaspheric refilling. ¿ 1998 American Geophysical Union