In this paper we present details of an unusual Pc5 ULF wave with a large azimuthal wavenumber (m) which is observed within the plasmasphere by ground-based magnetometers. In a previous statistical study, 129 Pc5 events were identified at midlatitudes by the U.K. Sub-Auroral Magnetometer Network (SAMNET), but only three displayed large azimuthal wavenumbers (m>10). Analysis of the variation of wave parameters during one of these events using a novel complex demodulation technique is presented. The technique reveals the temporal variation of wave characteristics, including the azimuthal wavenumber and wave frequency, and allows us to study the azimuthal dispersion characteristics of the wave and estimate the azimuthal phase and group speeds. The method used to estimate the azimuthal wavenumbers resolves the ambiguity resulting from aliasing, which can occur for large-m waves observed at stations more than one azimuthal wavelength apart. The drift-bounce resonance mechanism is discussed as a possible wave excitation mechanism for this event, and resonance with H+ and/or O+ ions at the inner edge of the ring current is presented as a possible excitation scenario. ¿ 1999 American Geophysical Union