In recent years the STARE system has been used to analyze Pc5 pulsations of several different types. In this paper a detailed description of a new type of pulsation is given. The event described occurred during local magnetic afternoon; it had small amplitude (‖E‖≲5mV m-1 in the ionosphere), a period which varied from 220 to 385 s during the event, and a large azimuthal wave number (m~35) which varied such as to keep the azimuthal phase velocity approximately constant for a given geomagnetic latitude; propagation was geomagnetic westward. When maped into the equatorial plane, the properties of the wave strongly suggest that the wave phase velocity was determined by hot proton drift motions near the equatorial plane. The observational results are compared with theoretical properties of the drift mirror instability. While the results do not contradict the predicted properties, it is felt that further theoretical development is required.