The recent study of a storm time Pc 5 plasma wave event by Lanzerotti, Fukunishi, Lin, and Cahill is extended by the analysis of recently available high-latitude ground-based magnetometer data and hot plasma particle (~1-40 keV) distribution data from a satellite in the magnetosphere. These additional data suggest that an odd mode shear Alfv¿n wave (period ~7 min) was excited by a high &bgr; drift instability at a high-altitude gradient region of the hot plasma distribution. The region of occurrence of the instability was characterized by a cold plasma density of ~100 cm-3.