We studied the frequency versus time slopes of IPDP (intervals of pulsations of diminishing periods) pulsation events occurring in the midnight sector, utilizing 60 events that were seen at Fairbanks, Macquarie Island, Anchorage, or Nurmij¿rvi. The slopes were found to have a significant correlation to the AE index, especially for times of weak to moderate substorms. One explanation is that the IPDP source region convects inward under the substorm electric field in midnight sector, the radial velocity of convection being correlated to the IPDP rate of frequency drift and to the AE index. Electric field intensities of 0.1--2 mV/m were implied by the IPDP slope measurements. The data are consistent with the proton cyclotron resonance instability as the source mechanism for IPDP's and with a source region near the plasmapause.