Intervals of pulsations of dimminishing periods (IPDP) magnetic pulsation data have been analyzed from several recording compaigns and from stations that are widely spaced in both meridional and azimuthal directions. On the basis of their local time occurrence it seems useful to divide the events into at least two main categories: 1400-1. There are necessarily no remarkable changes in frequency-time characteristics of IPDP pulsations. In the radial direction, IPDP wave frequency seems to be related to the He+ gyrofrequency in the equatorial plane. Observations are shown to be in general agreement with the evolution of the ion cyclotron instability in the plasma convection system of the magnetosphere. Several mechanisms seem to contribute simultaneously to the frequency shift of pulsations during IPDP events.