The results of observations by the Stare radar of an eastward propagating Pc5 pulsation with short azimuthal wavelength are presented. Previously observed high-m phenomena have been propagating westward. This event occurred in the predawn sector during a time of substorm activity. The azimuthal wave number is m=30¿3. Calculations of the drift and bounce periods of energetic particles show that the most plausible particle resonances are with near-equatorial drifting electrons in the 5 to 20 keV range or with bounce resonant protons in the 0.03-1 keV range. If the drift mirror instability is important it must be with guiding center-drifting electrons or the wave velocity must match the diamagnetic drift velocity which is only possible if the pressure gradient is inwards.