In Earth's inner magnetosphere, electromagnetic waves in the ultralow frequency (ULF) range play an important role in accelerating and diffusing charged particles via drift resonance. In conventional drift resonance theory, linearization is applied under the assumption of weak wave-particle energy exchange so particle trajectories are unperturbed. For ULF waves with larger amplitudes and/or durations, however, the conventional theory becomes inaccurate since particle trajectories are strongly perturbed. Here we extend the drift resonance theory into a nonlinear regime, to formulate nonlinear trapping of particles in a wave-carried potential well, and predict the corresponding observable signatures such as rolled-up structures in particle energy spectrum. After considering how this manifests in particle data with finite energy resolution, we compare the predicted signatures with Van Allen Probes observations. Their good agreement provides the first observational evidence for the occurrence of nonlinear drift resonance, highlighting the importance of nonlinear effects in magnetospheric particle dynamics under ULF waves.
Bibliographical noteFunding Information:
This study was supported by NSFC grants 41774168, 414221003, and 41474140. Y. O. acknowledges support from JSPS KAKENHI grant 17H06140, and R. R. acknowledges support from Canadian Space Agency and NSERC. The Van Allen Probes data used in this study can be accessed from NASA’s Space Physics Data Facility at http://spdf.gsfc.nasa.gov/.
- ULF waves
- drift resonance
- nonlinear process
- particle acceleration
- radiation belts
- wave-particle interactions