Using data from the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) satellite near local noon, and the AMPTE Ion Release Module (IRM) and United Kingdom Subsatellite (UKS) on the morning flank, we investigate the tailward propagation of a compressional Pc5 wave (380 s period) on October 28, 1984, and suggest that the observation represents a magnetospheric waveguide mode. The observed wave is a transient oscillation lasting about five wave cycles with an observed time of flight from CCE to IRM/UKS of 180 s. The first three or four cycles at the leading edge of the event display a remarkable wave packet coherence between CCE and IRM/UKS despite their large separation in the magnetosphere (~6.2 RE). A time of flight analysis suggests a waveguide mode group speed ~220 km s-1, being much less than the local Alfv¿n speed and comparable to the expected sub-Alfv¿nic propagation speed of a waveguide mode. The ground-based signature of the event is also observed near local noon by the European Incoherent Scatter magnetometer cross, displaying the same period and lasting for the same number of cycles. The ground-based data show no evidence of a field line resonance; the event is monochromatic, and there is little variation of amplitude and polarization with latitude. An increase in solar wind ram pressure of ~30% is observed by IMP 8 upstream in the solar wind just prior to the event, and this may have provided an impulsive energy source for the waveguide mode. This is the first time the signature of a particular waveguide mode harmonic has been observed by satellites which are widely spaced in the magnetosphere. Moreover, we present the first unambiguous observation of the downtail propagation of a waveguide mode. The observation also provides possible evidence of dispersion in the Earth's outer magnetospheric waveguide. ¿ 1998 American Geophysical Union