ULF waves can accelerate/decelerate the charged particles including the ring current ions via drift‐bounce resonance, which play an important role in the dynamics of ring current during storm times. This study compares the different behaviors of oxygen ions (10.5–35.1 keV) and protons (0.3–12.3 keV) which simultaneously interact with Pc5 ULF waves observed by Cluster on 3 June 2003. The ULF waves are identified as the fundamental mode oscillations. Both oxygen ions and protons show periodic energy dispersion and pitch angle dispersion signatures, which satisfy the drift‐bounce resonance condition of N=2. The different behaviors of oxygen ions and protons include (1) the resonant energy of oxygen ions is higher than that of protons due to mass difference; (2) the phase space density (PSD) of oxygen ions show relative variations (3.6–6.3) that are much larger than that of protons (<0.4), which indicates a more efficient energy exchange between oxygen ions and ULF waves; (3) the PSD spectra show that oxygen ions are accelerated, while protons are decelerated, which depend on the radial gradient of their PSD; (4) the pitch angle distributions (PADs) of the oxygen ions and protons show negative slope and bidirectional field‐aligned features, respectively, which is related to the preexisting state of ion PADs before the interaction with the ULF waves. In addition, the resonant ions with peak fluxes tracing back to the magnetic equator are always collocated with the accelerating (westward) electric field, which indicate that the ions are mainly accelerated near the magnetic equator and the electric field intensity of ULF waves peaks there.

We report observational evidence of cold plasmaspheric electron (<200 eV) acceleration by ultralow frequency (ULF) waves in the plasmaspheric boundary layer on 10 September 2015. Strongly enhanced cold electron fluxes in the energy spectrogram were observed along with second harmonic mode waves with a period of about 1 min which lasted several hours during two consecutive Van Allen Probe B orbits. Cold electron (<200 eV) and energetic proton (10–20 keV) bidirectional pitch angle signatures observed during the event are suggestive of the drift‐bounce resonance mechanism. The correlation between enhanced energy fluxes and ULF waves leads to the conclusions that plasmaspheric dynamics is strongly affected by ULF waves. Van Allen Probe A and B, GOES 13, GOES 15, and MMS 1 observations suggest that ULF waves in the event were strongest on the duskside magnetosphere. Measurements from MMS 1 contain no evidence of an external wave source during the period when ULF waves and injected energetic protons with a bump‐on‐tail distribution were detected by Van Allen Probe B. This suggests that the observed ULF waves were probably excited by a localized drift‐bounce resonant instability, with the free energy supplied by substorm‐injected energetic protons. The observations by Van Allen Probe B suggest that energy transfer between particle species in different energy ranges can take place through the action of ULF waves, demonstrating the important role of these waves in the dynamical processes of the inner magnetosphere.

Ultralow frequency (ULF) waves have long been known to resonate with magnetospheric charged particles through their drift and bounce motions. Most research interest has focused on the resonance with drift motion, which can accelerate charged particles at very high energies. The role of the bounce motion, especially for particles with lower energies, has attracted less attention so far. Here we start from the general theory of wave-particle interactions to predict the characteristic, observable signatures of drift-bounce resonance. Such signatures can be described in the particle pitch angle spectrum as a series of inclined stripes, with the inclination angle depending on the latitude of the observing spacecraft. Each stripe is also twisted at two conjugated pitch angles, suggesting significant phase shifts across resonant pitch angles. These predicted signatures are found consistent with observations from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft, and therefore provide an identification of drift-bounce resonance together with a validated picture over the importance of particle's bounce motion in the ULF wave-particle interactions.

We explore the phase relationship between the poloidal mode ULF wave electric field oscillations and drift‐bounce resonant oxygen ions under the resonant condition of N = 2 at the magnetic equator. Using Cluster data from 2001 to 2004, we identify 55 fundamental poloidal mode wave events, among which 42 show “negative slope” pitch angle dispersion signatures in the southern hemisphere, 11 show “positive slope” dispersion in the northern hemisphere, and two near‐equatorial events are associated with in‐phase field‐aligned signatures. For each event, the off‐equatorial resonant ions are traced along their bouncing trajectories to determine the last time they moved across the equator. The resulting time series of the resonant oxygen ion fluxes at the equator are found to be statistically in antiphase with the wave electric fields. The resonant ion flux variation depends on both ion energy change and radial transportation. This antiphase relationship in statistics suggests two possibilities: (1) the fundamental poloidal mode wave electric fields are generally characterized by electric field intensity peaking near the magnetic equator if the flux variation is mainly caused by energy change and (2) the radial gradient of phase space density is positive if the flux variation caused by radial transportation is dominant.