The response of the magnetosphere to pressure pulses at the magnetopause has been studied using a three-dimensional model of ULF waves in a dipole geometry. Pressure pulses at the magnetosphere directly excite compressional waves, which then convert to shear mode Alfv¿n waves due to inhomogeneity. The behavior of the system depends on the frequency of the source at the magnetopause, with vortex structures tending to form on field lines resonant with the source frequency. The perturbations between the vortices are skewed toward noon, in agreement with observations. ¿ American Geophysical Union 1992