Global magnetospheric ULF pulsations with frequencies in the Pc 5 range (f = 1.7--6.7 mHz) and below have been observed for decades in space and on the Earth. Recent work has shown that in some cases these pulsations appear at discrete frequencies. Global cavity and waveguide modes have been offered as possible sources of such waves. In these models the magnetosphere is presumed to resonate globally at frequencies determined solely by its internal properties such as size, shape, field topology, mass density distribution, etc. We show in this work that upstream solar wind number density and dynamic pressure variations precede and drive compressional magnetic field variations at geosynchronous orbit. Furthermore, spectral analysis shows that wave power spectra in both the solar wind and magnetosphere contain peaks at the same discrete frequencies. Therefore, in contrast to the cavity mode hypothesis, we suggest that discrete ULF pulsations observed within the magnetosphere are at least sometimes directly driven by density oscillations present in the ambient solar wind. Finally, we comment on possible sources for such pulsations observed in the solar wind.