Large amplitude (~5--30 nT), high-frequency (~0.14 Hz) geomagnetic field oscillations were observed across eastern North America and in the conjugate region in Antarctica during a large magnetic storm. These are the largest amplitude oscillations reported to data in the Pc 2 frequency band (0.1--0.5 Hz). The waves are first seen on the east coast, with the onset delayed by about 4 min at the midcontinent stations. The oscillations are consistent with a wave excited by the bounce motion of protons with a peak in the energy distribution at ~100 keV, with the energy source drifting from east to west with the ion gradient B drift.