Geomagnetic pulsations and magnetospheric substorms are often treated as separate topics in the literature. These phenomena do, however, share common properties including spatial localization and characteristic propagation velocities. In this paper we discuss the possibility that both phenomena originate from Kelvin-Helmholtz instabilities at the interface between the low latitude boundary layer/boundary layer plasma sheet and the central plasma sheet. The variations in horizontal scale size and azimuthal velocities found in ground-based observations of Pc 4,5, Ps 6 and westward travelling surges can be explained by variaitons in the shear flow and width of the regions where the instability occurs.