A compressional Pc5 wave was observed on an ISEE 1 and 2 outbound path on September 28, 1981 at L=5.6-7.3 near the magnetic equator at ~10 hr local time during the recovery phase of a geomagnetic storm. The wave propagated westward with a large azimuthal wave number of ~30 and exhibited in-phase oscillations of plasma density and magnetic field magnitude. During this event, component-dependent variations in phase and amplitude were observed for the magnetic field oscillations. The radial and compressional components had a constant phase and their amplitude was finite. In contrast, the azimuthal component changed its phase by 180¿ and its amplitude became zero during the middle of the wave event. We interpret this observation as indicating the spatial structure of the Pc5 wave. The polarization reversal is likely to be caused by a crossing of a node of a standing wave located 4¿ above the geomagnetic equator.