Second harmonic poloidal waves observed by Van Allen Probes in the dusk‐midnight sector

This paper presents observations of ultralow‐frequency (ULF) waves from Van Allen Probes. The event that generated the ULF waves occurred 2 days after a minor geomagnetic storm during a geomagnetically quiet time. Narrowband pulsations with a frequency of about 7 mHz with moderate amplitudes were registered in the premidnight sector when Probe A was passing through an enhanced density region near geosynchronous orbit. Probe B, which passed through the region earlier, did not detect the narrowband pulsations but only broadband noise. Despite the single‐spacecraft measurements, we were able to determine various wave properties. We find that (1) the observed waves are a second harmonic poloidal mode propagating westward with an azimuthal wave number estimated to be ∼100; (2) the magnetic field fluctuations have a finite compressional component due to small but finite plasma beta (∼0.1); (3) the energetic proton fluxes in the energy ranging from above 10 keV to about 100 keV exhibit pulsations with the same frequency as the poloidal mode and energy‐dependent phase delays relative to the azimuthal component of the electric field, providing evidence for drift‐bounce resonance; and (4) the second harmonic poloidal mode may have been excited via the drift‐bounce resonance mechanism with free energy fed by the inward radial gradient of ∼80 keV protons. We show that the wave active region is where the plume overlaps the outer edge of ring current and suggest that this region can have a wide longitudinal extent near geosynchronous orbit. Key Points Poloidal mode ULF waves near the premidnight sector during a quiet geomagnetic time Second harmonic, high m number poloidal mode at westward propagation likely driven by an inward radial gradient of ring current protons The background plasma density appears to be important for the longitudinal and radial extents of the wave active region