The IPDP pulsations are assumed to be generated in the region where the low-density energetic protons (10-100 keV) injected during substorms encounter the high-density plasmasphere. The increase in the wave frequency during the IPDP events is proposed to be caused by the enhanced dispersion of the wave velocity due to the presence of the helium ions in the plasmasphere. In the presence of the helium ions, the variation in the traveling time of the unstable waves is found to be typically 5--20 min, which is comparable to the duration of some of the observed IPDP events.