Data from three instruments, the magnetometer, the charge‐energy‐mass spectrometer, and the medium‐energy particle, analyzer onboard the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (CCE) spacecraft have been used to study a compressional Pc 5 wave observed at 1925–2200 UT on day 202 (July 21) of 1986 at a radial distance of ≃8 RE in the postmidnight sector near the beginning of minor geomagnetic activity. The wave exhibited harmonically related transverse and compressional magnetic oscillations, modulation of the flux of medium energy protons (E ≳ 10 keV), and a large azimuthal wave number (m ∼ 65). These properties are similar to those of compressional Pc5 waves observed previously at geostationary orbit. The unique observations associated with the CCE event are the occurrence in the postmidnight sector, the eastward (or sunward) propagation with respect to the spacecraft, and the left‐handed polarization of the perturbed magnetic field. These are opposite to previous geostationary observations. We propose that the unique propagation and polarization are caused by a dawn‐to‐dusk dc electric field which convects the plasma sunward. The wave, probably propagating westward in the plasma rest frame, appears to propagate eastward to the observer because the electric field drift velocity is larger than the wave phase velocity.
