Bounce Resonance Between ULF Waves and Electrons in the Dayside Outer Magnetosphere

Resonant interactions between ultra‐low‐frequency (ULF) waves and particles are critical for energy transfer in the Earth's magnetosphere, and have been extensively investigated in the inner magnetosphere. However, resonant processes between ULF waves and electrons in the dayside outer magnetosphere remain largely unexplored. Using MMS data, we investigate their bounce resonance in the dayside outer magnetosphere. Quasi‐periodic oscillations of electron fluxes below 1 keV and ULF waves with periods comparable to the electron bounce periods are observed simultaneously. Test‐particle simulations demonstrate that off‐equatorial magnetic field minima can increase the number of resonant pitch angles for electrons at a given energy. Overlap of stripes corresponding to different resonant energies gives rise to observed 360 phase shifts. Furthermore, off‐equatorial minima and temporal evolution of ULF waves broaden the conventional bounce resonance condition , which links the harmonic type of ULF waves to the parity of .