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We studied O+ drift-bounce resonance using Magnetospheric Multiscale (MMS) data. A case study of an event on 17 February 2016 shows that O+ flux oscillations at similar to 10-30 keV occurred at MLT similar to 5 hr and L similar to 8-9 during a storm recovery phase. These flux oscillations were accompanied by a toroidal Pc5 wave and a high-speed solar wind (similar to 550 km/s). The azimuthal wave number (m-number) of this Pc5 wave was found to be approximately -2. The O+/H+ flux ratio was enhanced at similar to 10-30 keV corresponding to the O+ flux oscillations without any clear variations of H+ fluxes, indicating the selective acceleration of O+ ions by the drift-bounce resonance. A search for similar events in the time period from September 2015 to March 2017 yielded 12 events. These events were mainly observed in the dawn to the afternoon region at L similar to 7-12 when the solar wind speed is high, and all of them were simultaneously identified on the ground, indicating low m-number. Correlation analysis revealed that the O+/H+ energy density ratio has the highest correlation coefficient with peak power of the electric field in the azimuthal component (E-a). This statistical result supports the selective acceleration of O+ due to the N = 2 drift-bounce resonance., QC 20200622

A Pc5 wave is observed by the Exploration of energization and Radiation in Geospace Arase satellite in the inner magnetosphere (L similar to 5.4-6.1) near postmidnight (L-magnetic local time similar to 1.8-2.5 hr) during the storm recovery phase on 27 March 2017. Its azimuthal wave number (m-number) is estimated using two independent methods with satellites and ground observations to be -8 to -15. The direct measurement of the m-number enables us to calculate the resonance energy. The flux oscillations of H+ and O+ ions at >= 56.3 keV are caused by drift resonance and those of O+ ions at <= 18.6 keV by bounce resonance. Resonances of O+ ions at multiple energies are simultaneously observed for the first time. The enhancement of the O+/H+ flux ratio at <= 18.6 keV indicates selective acceleration of O+ ions through bounce resonance. Plain Language Summary Geomagnetic pulsations are magnetic fluctuations excited by solar wind or plasma instabilities in the magnetosphere. Pc5 waves are continuous geomagnetic pulsations with a period of 150-600 s. A Pc5 wave was observed in the inner magnetosphere during a magnetic storm on 27 March 2017. It propagated westward with a wave number of 8 to 15 and resonated with charged particles, resulting in oscillations of the H+ and O+ ion fluxes at >= 56.3 keV and the O+ ion fluxes at <= 18.6 keV. Resonances of O+ ions at multiple energies are simultaneously observed for the first time. At the same time, the O+/H+ flux ratio at <= 18.6 keV enhanced corresponding to the O+ ion flux oscillations, which indicates selective acceleration of O+ ions through resonances., QC 20180919