A study is presented of a storm time Pc 5 pulsation, observed by GEOS 2 particle detectors and magnetometers at geocentric orbit and by the STARE auroral radar in the ionosphere. These measurements allow the comparison of phase relationships between the magnetic field components, energetic particle flux, and electric field in the ionosphere over a substantial region, They also permit measurements of the wavelength of the disturbance in the ionosphere. A theory of the coupling between a drift mirror wave and a standing Alfv¿n wave is developed. The results are compared with this theory and are found to support it. In particular, it is found that the disturbance is of wavelike nature, propagating westward in the equatorial plane at 6.6 RE with a wavelength of about 1 RE. Its period is initially 213 s, lengthening to 300 s later in the event. In the equatorial plane the disturbance in the particle flux and the compressional magnetic field are out of phase as is consistent with a drift mirror wave. There is a large transverse magnetic field oscillation in the meridian plane as is consistent with a guided poloidal oscillation. In the ionosphere the oscillation is consistent with the existence of a guided poloidal Alfv¿n wave and is entirely different from toroidal oscillations that have been observed at Pc 5 frequencies on other occasions. The observations are consistent with a particle driven drift mirror wave, coupled to a standing Alfv¿n wave. The azimuthal wavelength is consistent with a drift mirror wave whose Doppler shifted frequency matches the natural frequency of the standing Alfv¿n wave.