Observational evidence from a latitudinal array of four ground-based magnetometer stations near L=4 suggests that magnetic disturbances at latitudes L>4.4 can excite damped sinusoidal oscillations at plasma density gradients at or inside the plasmasphere. The observations, together with the theoretical interpretations of Chen and Hasegawa (1974a), suggest that the oscillations can be attributed to a magnetohydrodynamic surface eigenmode excited at the density gradients. The wave frequency, damping rate, and latitudinal dependence of the amplitude for each event can be used to infer the steepness of the plasma density gradient as well as the plasma density inside the gradient region.