GROUND-SATELLITE COHERENCE ANALYSIS OF PC3 PULSATIONS

Although Pc3 magnetic pulsations are commonly observed in the dayside magnetosphere and are believed to propagate into the inner magnetosphe re and to the ground, how and where they establish themselves as a reg ular oscillation are not completely understood, In particular, it is n ot clear whether the cavity mode plays a significant role in determini ng 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 le ast one satellite providing evidence of compressional magnetic field o scillation in the magnetosphere. Motivated by these requirements for d etection of the cavity mode, we surveyed combined magnetic field data from the Active Magnetospheric Particle Tracer Explorers Charge Compos ition Explorer (AMPTE CCE) satellite and the Kakioka ground station. W e 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 m orningside (near 8-hour local time) with a small local time separation (within 0.9 hour) and observed Pc3 magnetic pulsations exhibiting a c ompressional component in space, Unlike previous studies, which used s atellites located at L > 6, we observed high coherence in the Pc3 band between the compressional component (b(z)) in space and the horizonta l component (H) on the ground, A cross-phase analysis of the merged b, and H time series indicates that these components oscillated in antip hase 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.