A Pc5 ULF wave with large azimuthal wavenumber observed within the morningsector plasmasphere by Sub-Auroral Magnetometer Network

In this paper we present details of an unusual Pc5 ULF wave with a large az imuthal wavenumber (m) which is observed within the plasmasphere by ground- based magnetometers. In a previous statistical study, 129 Pc5 events were i dentified at midlatitudes by the U.K. Sub-Auroral Magnetometer Network (SAM NET), but only three displayed large azimuthal wavenumbers (m > 10). Analys is of the variation of wave parameters during one of these events using a n ovel complex demodulation technique is presented. The technique reveals the temporal variation of wave characteristics, including the azimuthal wavenu mber and wave frequency, and allows us to study the azimuthal dispersion ch aracteristics of the wave and estimate the azimuthal phase and group speeds . The method used to estimate the azimuthal wavenumbers resolves the ambigu ity resulting from aliasing, which can occur for large-m waves observed at stations more than one azimuthal wavelength apart. The drift-bounce resonan ce 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 c urrent is presented as a possible excitation scenario.