We study electron behavior in the outer radiation belts during the 16 July 2017 storm sudden commencement (SSC), in which prompt intensification of ultrarelativistic electron fluxes was observed at around L = 4.8 by Van Allen Probe B immediately after an interplanetary shock. The electron fluxes in multiple energy channels show clear oscillations in the Pc5 frequency range, although the oscillation characteristics are quite different in different energy channels. At energies above ∼1 MeV, the oscillation periods were very close to the electron drift period, which resembles an energy spectrogram evolution expected for an energetic particle injection event and its drift echoes. At lower energies, however, the oscillation periods hardly depended on the energy: They were very close to the ultralow frequency (ULF) wave period derived from electric field measurements (about 250 s according to wavelet analysis). These complex signatures are consistent with the picture of drift resonance between electrons and short‐lived ULF waves with low azimuthal wave numbers. Good agreement between the observations and numerical simulations confirms that shock‐induced global‐scale ULF waves can efficiently accelerate outer belt ultrarelativistic electrons up to 3.4 MeV over a time scale shorter than 1 hr.

“Zebra stripes” are newly found energetic electron energy‐spatial (L shell) distributed structure with an energy between tens to a few hundreds keV in the inner radiation belt. Using high‐quality measurements of electron fluxes from Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on board the twin Van Allen Probes, we carry out case and statistical studies from April 2013 to April 2014 to study the structural and evolutionary characteristics of zebra stripes below L = 3. It is revealed that the zebra stripes can be transformed into evenly spaced patterns in the electron drift frequency coordinate: the detrended logarithmic fluxes in each L shell region can be well described by sinusoidal functions of drift frequency. The “wave number” of this sinusoidal function, which corresponds to the reciprocal of the gap between two adjacent peaks in the drift frequency coordinate, increases in proportion to real time. Further, these structural and evolutionary characteristics of zebra stripes can be reproduced by an analytic model of the evolution of the particle distribution under a single monochromatic or static azimuthal electric field. It is shown that the essential ingredient for the formation of multiple zebra stripes is the periodic drift of particles. The amplitude of the zebra stripes shows a good positive correlation with Kp index, which indicates that the generation mechanism of zebra stripes should be related to geomagnetic activities.

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.

We examine the drift‐resonant particle dynamics for toroidal ultralow frequency (ULF) waves in a pure dipole background geomagnetic field. We confirm that the resonant condition originally believed to apply only for poloidal ULF waves, , also applies for compressional toroidal waves. The predicted phase relationships have been confirmed from Van Allen Probes observations. Their good agreement provides the first observational evidence for the drift resonance condition controlled by the compressional toroidal ULF wave. Moreover, we extend the drift resonance theory into a nonlinear regime. The resulting particle motion can be described by a modified pendulum equation with solutions depending on the wave number . For high‐ toroidal waves, the resonant islands become asymmetric to perturb the particle trajectories within each potential well and consequently increase the trapping widths in both energy and L‐shell. We further carry out test‐particle simulations to show the evolution of electron distribution functions when they interact with either toroidal or poloidal waves. These findings demonstrate that toroidal ULF waves, like their poloidal counterparts, play an important role in magnetospheric particle dynamics.

The existence of planetary radiation belts with relativistic electron components means that powerful acceleration mechanisms are operating within their volume. Mechanisms that bring charged particles planetward toward stronger magnetic fields can cause their heating. On the basis that electron fluxes in Saturn’s radiation belts are enhanced over discrete energy intervals, previous studies have suggested that rapid inward plasma flows may be controlling the production of their most energetic electrons. However, rapid plasma inflows languish in the planet’s inner magnetosphere, and they are not spatially appealing as a mechanism to form the belts. Here we show that slow, global-scale flows resulting from transient noon-to-midnight electric fields successfully explain the discretized flux spectra at quasi- and fully relativistic energies, and that they are ultimately responsible for the bulk of the highest energy electrons trapped at Saturn. This finding is surprising, given that plasma flows at Saturn are dominated by the planetary rotation; these weak electric field perturbations were previously considered impactful only over a very narrow electron energy range where the magnetic drifts of electrons cancel out with corotation. We also find quantitative evidence that ultrarelativistic electrons in Jupiter's radiation belts are accelerated by the same mechanism. Given that similar processes at Earth drive a less efficient electron transport compared to Saturn and Jupiter, the conclusion is emerging that global-scale electric fields can provide powerful relativistic electron acceleration, especially at strongly magnetized and fast-rotating astrophysical objects.

Interplanetary (IP) shocks have great impacts on Earth's magnetosphere, especially in causing global dynamic changes of energetic particles. In order to study the response of energetic electrons (50keV–1.5MeV) at geosynchronous orbit to IP shocks, we have systematically analyzed 215 IP shock events based on ACE, GOES, and LANL observations during 1998–2007. Our study shows that after the shock arrival low‐energy electron fluxes increase at geosynchronous orbit. However, in higher energy channels fluxes show smaller increases and eventually become unchanged or even decrease. The oscillations of electron fluxes following the shock arrival have also been studied in this paper. Statistical analysis revealed a frequency preference for 2.2 mHz and 3.3 mHz oscillations of energetic electron fluxes. The amplitude of these oscillations is larger under southward interplanetary magnetic field (IMF) than under northward IMF. Furthermore, oscillations from high‐energy and low‐energy electron fluxes show different phase characteristics and power distributions. The phase angles of the oscillations are the same in the dawn, dusk, and noon sectors for low‐energy channels (50–500keV), while they have a difference between two adjacent local time sectors for high‐energy channels (0.5–1.5MeV). The wave power distribution of electron fluxes shows different dawn‐dusk asymmetries for low‐energy channels and high‐energy channels. The results presented in this paper provide an energetic particle point of view of the magnetospheric response to the interplanetary shock impact.