We report that intervals of pulsations of diminishing periods (IPDP) type plasma events which are associated with magnetospheric substorms are less frequent in the years of high solar acitivity than in years of low activity. This new result is based on two independent continuous data sets from 1974 to 1985. We show also that IPDP events occur throughout the recurrent magnetic storms in 1976, whereas they appear more sporadically in 1980, i.e., during high solar activity. It is concluded that several factors such as changes in plasmapause position, attenuation of horizontal ionospheric wave propagation, and heavy ion densities in the magnetosphere can contribute to the observed long-term variability. ¿ American Geophysical Union 1988