Selective Acceleration of O⁺ by Drift‐Bounce Resonance in the Earth's Magnetosphere: MMS Observations

Abstract

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 ~10–30 keV occurred at MLT ~ 5 hr and L ~ 8–9 during a storm recovery phase. These flux oscillations were accompanied by a toroidal Pc5 wave and a high‐speed solar wind (~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 ~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 ~ 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 (Ea). This statistical result supports the selective acceleration of O+ due to the N = 2 drift‐bounce resonance.

Off‐Equatorial Minima Effects on ULF Wave‐Ion Interaction in the Dayside Outer Magnetosphere

“…One candidate for this interaction is ultra-low frequency (ULF) wave-particle interaction. Previous studies have proved that drift-bounce resonance, a primary type of ULF wave-particle interaction, can efficiently realize the energy exchange between waves and particles (e.g., Oimatsu et al, 2018Oimatsu et al, , 2020Ren et al, 2015Ren et al, , 2016Ren et al, , 2019Southwood & Kivelson, 1981Yamamoto et al, 2019;Yang et al, 2010;Zong et al, 2007Zong et al, , 2009Zong, Wang, et al, 2017). According to Southwood and Kivelson (1981), drift-bounce resonance occurs when the following condition is satisfied:  is particle bounce-averaged drift frequency, and b E  is particle bounce frequency.…”

Nonlinear Wave Growth Analysis of Chorus Emissions Modulated by ULF Waves

“…depending on the initial particle distributions over energy and/or pitch angle (Kivelson & Southwood, 1985). When drift or drift-bounce resonance occurs, there are clear phase differences between particle flux oscillations and wave signals (Li, Zhou, Omura, et al, 2021;Oimatsu et al, 2020;Ren et al, 2019;Yang et al, 2010;Zhu et al, 2020).…”

Pitch Angle Phase Shift in Ring Current Ions Interacting With Ultra‐Low‐Frequency Waves: Van Allen Probes Observations

“…The interaction between ultra-low-frequency (ULF) waves and charged particles plays an important role in energy transport in the Earth's magnetosphere. According to previous studies (e.g., Liu et al, 2020;Oimatsu et al, 2018Oimatsu et al, , 2020Ren et al, 2015Ren et al, , 2016Ren et al, , 2018Ren et al, , 2019Ren, Zong, Miyoshi et al, 2017;Southwood & Kivelson, 1981Yamamoto et al, 2019;Yang et al, 2010;Yang, Zong, Fu, Takahashi, et al, 2011;Zhu et al, 2020;Zong et al, 2007Zong et al, , 2009Zong et al, , 2012Zong, Wang, et al, 2017), a special type of resonance-drift-bounce resonance-can occur in this interaction, and cause efficient energy exchange between ULF waves and particles. When the time evolution of ULF waves can be neglected, drift-bounce resonance would cause 180° phase shift across resonant pitch angle, manifesting as basically vertical stripes beyond resonance but strong twist near resonance on particle pitch angle spectrograms observed near the equator (Southwood & Kivelson, 1981;Zhu et al, 2020).…”