We show that hydromagnetic wave electric fields could cause the fluctuations in cold plasma number density that have been observed as spacecraft cross the plasmapause. To explore the effects of a hydromagnetic wave electric field on the particles in the vicinity of the plasmapause we constructed a simple computer model of the flow of cold plasma in the equatorial plane of the magnetosphere. A hydromagnetic wave electric field which is localized in space and time is introduced. Particles are perturbed by the wave electric field. The resulting fluctuations in cold plasma number density are most evident in the dusk region and occur over both large (∼1 R_E) and small (∼0.1 R_E) distance scales. As these variations in the number density are similar to those observed, hydromagnetic wave electric fields could very well account for the density variations observed in the vicinity of the plasmapause boundary.