Off-equatorial Minima Effects on ULF Wave-ion Interaction in the Dayside Outer Magnetosphere

The ultra-low frequency (ULF) wave-particle drift-bounce resonance in the inner magnetosphere has been studied in detail, due to its important role in particle energization. However, it remains an open question how drift-bounce resonance manifests in the dayside outer magnetosphere, where particles' orbits show bifurcations because of off-equatorial magnetic field minima. In this study, we investigate this question, by analyzing Magnetospheric Multiscale (MMS) observations of the January 20, 2017 event. A test-particle simulation is conducted to help us understand the observations. The observed pitch angle-time spectrograms show "pawtrack-like" structures. We find there are more than two resonant pitch angles at fixed energy, since the presence of off-equatorial minima changes the relationship between the bounce (drift) frequency and pitch angle from unimodal function to trimodal function. These results indicate a new drift-bounce acceleration mechanism in the dayside outer magnetosphere, which provides new perspectives on the origination of magnetospheric activities connected with particle acceleration, such as the magnetic storm.