The Excitation of Eastward Second Harmonic Poloidal Waves Through Drift-Bounce Resonance with Protons in the Magnetic Dip

The drift-bounce resonance between ultralow-frequency (ULF) wave and charged particles is an efficient way to transfer energy. In this study, we report the excitation of ULF waves through drift-bounce resonance with protons in the magnetic dip for the first time. On 4 September 2015, Van Allen probe B observed ULF signals with a frequency of ~10 mHz inside the magnetic dip during substorms at the dusk side. The ULF waves are further diagnosed as second harmonic poloidal waves. The 54-67 keV protons in the magnetic dip exhibit oscillations with the same period as ULF waves, providing evidence for drift-bounce resonance. Through finite Larmor radius effects combined with simulations, we determined the ULF waves propagate eastward with an azimuthal wave number of ∼240. The sign of df/dW reveals that the ULF waves are excited and related to the outward radial gradient of proton phase space density.