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

The drift-bounce resonance between ultralow-frequency (ULF) waves 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, we determined that the ULF waves propagate eastward with an azimuthal wave number in the range of 237–277. The ULF waves are excited with free energy related to the enhanced proton phase space density in the magnetic dip.