Large amplitude (~30 nT), high frequency (~0.14 Hz) geomagnetic field oscillations were observed superimposed on lower frequency ones (~0.008 Hz) at L~3 during a large magnetic storm. These are the largest amplitude oscillations ever reported in the Pc2 frequency band (0.1-0.5 Hz). Measurements at two stations spaced in azimuth by ~240 km show the higher frequency waves to occur at the same time at both locations. The higher frequency oscillations, which have larger amplitudes at the station closer to local midnight, are consistent with a wave excited by the bounce motion of protons with a peak in the energy distribution at ~100 keV. The longer-period variations, which are highly non-correlated between the two locations, could be evidence of a surface ''wave'' on the plasmapause.