Measurements of ULF waves in the Pc5 frequency range are presented and discussed. The waves were observed during the magnetic cloud event of January 1997 by instruments on the Polar satellite and ground instrumentation. These large-amplitude waves are best interpreted as traveling shear Alfv¿n waves rather than in the usual standing-wave scenario. Characterization of the associated complex particle environment shows that the waves were largely confined to the plasma trough. Energetic protons modulated by the wave are shown to have caused modulated proton aurora. It is argued that the waves were caused by the interaction of a magnetic hole with the magnetosphere. ¿ 2000 American Geophysical Union