Although foreshock transients can generate strong magnetospheric Pc5 ultralow‐frequency (ULF) waves, whether they can modulate the energy of magnetospheric ions is still poorly understood. In this study, we analyze the strong magnetospheric ion energy modulations in a foreshock transient event on October 30, 2008, based on the magnetospheric observations by the time history of events and macroscale interactions during substorms A and D in the prenoon sector. ULF wave‐induced drift accelerated the cold ions up to ∼10 keV and the enhanced ion fluxes have wave‐like patterns. There is another portion of enhanced ion fluxes from ∼0.8 to ∼10 keV but with strong energy dispersions in this event. By comparing the observations and the theoretical prediction, we for the first time found that the drift‐bounce resonances played a major role in modulating the energy of those ions with energy dispersions, during the interactions between the ions and the foreshock transient‐driven Pc5 ULF wave with growing and damping effects.

The ultra‐low frequency wave‐particle drift‐bounce resonance in the inner magnetosphere has been studied in detail, due to its important role in particle energization. However, it remains an open question how drift‐bounce resonance manifests in the dayside outer magnetosphere, where particles' orbits show bifurcations because of off‐equatorial magnetic field minima. In this study, we investigate this question, by analyzing Magnetospheric Multiscale observations of the January 20, 2017 event. A test‐particle simulation is conducted to help us understand the observations. The observed pitch angle‐time spectrograms show “pawtrack‐like” structures. We find there are more than two resonant pitch angles at fixed energy, since off‐equatorial minima change the relationship between the bounce (drift) frequency and pitch angle from unimodal function to trimodal function. These results reveal a new drift‐bounce acceleration mechanism in the dayside outer magnetosphere, which potentially affects the efficiency of particle energization during geomagnetic activities like geomagnetic storms.

Pulsating auroras are usually observed with ultralow-frequency (ULF) waves in the Pc 3–5 band (period 10–600 s). These auroras are thought to result from interactions between energetic electrons and chorus waves, but their relationship with ULF waves remains an open question. In this study, we investigated this question by conducting a comparative study on two ULF wave events with pulsating auroras observed near the magnetic footprints. Conjugate observations from the Magnetospheric Multiscale mission and the Chinese Yellow River Station were used. In both events, lower-band chorus waves were observed, which were suggested to be connected with the auroral pulsations by wavelet analysis. The intensity of these waves oscillates at the period of the ULF waves, but the physics laid behind them differs by events. During the event of 22 January 2019, compressional ULF waves changed the threshold for the whistler anisotropy instability periodically, affecting the emission of chorus waves. In the event on 10 January 2016, poloidal ULF waves modulated the chorus wave generation by regulating electron temperature anisotropy through drift resonance. ULF waves in these events may originate from perturbations in the solar wind. We highlight the role of ULF waves in the solar wind–magnetosphere–ionosphere coupling, which requires further study.

Drift‐bounce resonance between ultra‐low‐frequency (ULF) waves and ring current ions has been widely studied, because of its important role in ring current acceleration and relevant geomagnetic activities. To identify drift‐bounce resonance in observations, 180° phase shifts across resonant pitch angle have been proposed as diagnostic signatures. This study, however, presents observations that suggest this criterion may be invalid when phase space density (PSD) distributions vary nonmonochromatically with energy. We identified 14 ULF wave‐ion interaction cases from 2‐years Van Allen Probes data. In these cases, 180° phase shifts across pitch angle are observed at particular energies. Near these energies, pitch angle‐dependent PSD bump‐on‐tail distributions were also observed. As a result, at fixed energies, the sign of ion PSD energy gradient changes across pitch angle, which then can result in the observed 180° phase shift. Based on the observations, we suggest 180° phase shifts across pitch angle can also result from pitch angle‐dependent bump‐on‐tail distributions, which should be taken into account in future ULF wave‐ion interaction studies.