Although Pc3 magnetic pulsations are commonly observed in the dayside magnetosphere and are believed to propagate into the inner magnetosphere and to the ground, how and where they establish themselves as a regular oscillation are not completely understood. In particular, it is not clear whether the cavity mode plays a significant role in determining the spectral properties of Pc3 pulsations. The mode, in principle, can be identified by multipoint observations of the spatial variation of the amplitude and phase of magnetic field perturbations, with at least one satellite providing evidence of compressional magnetic field oscillation in the magnetosphere. Motivated by these requirements for detection of the cavity mode, we surveyed combined magnetic field data from the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite and the Kakioka ground station. We identified an interval on November 4--5, 1984, during which both the satellite (dipole L=3--6) and ground station (L=1.25) were on the morningside (near 8-hour local time) with a small local time separation (within 0.9 hour) and observed Pc3 magnetic pulsations exhibiting a compressional component in space. Unlike previous studies, which used satellites located at L>6, we observed high coherence in the Pc3 band between the compressional component (bz) in space and the horizontal component (H) on the ground. A cross-phase analysis of the merged bz and H time series indicates that these components oscillated in antiphase over a range of satellite radial distance. We discuss whether the observations can be taken as evidence of a cavity mode oscillation in the magnetosphere. ¿ 1998 American Geophysical Union