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Microinjection phenomena, characterized by dispersive oscillations of electron fluxes at the Pc5 period and bi‐directional pitch angle anisotropy, are frequently observed by Magnetospheric Multiscale in the dusk to midnight plasma sheet. In this paper, two such events are analyzed and the features of toroidal mode drift resonance measured meanwhile are shown in detail. The prominent observation is that the fluctuations of the electron flux and the electric field have either −90° or +90° phase difference at the resonant energy, and the phase difference rises as the energy increases. We extend the present theory for drift resonance of toroidal mode wave with only the equatorial moving electrons in a dipole field to include bouncing electrons. The predicted phase differences based on the new theory are consistent well with the observations in the microinjection events. It is thus suggested that drift resonance may act as the forming mechanism for the observed microinjections. Plain Language Summary: In the dusk to midnight plasma sheet, the fluxes of counter‐streaming energetic electrons were often observed to fluctuate at the period about 2 ∼ 6 min, with the higher energy fluxes coming to peaks earlier than those at lower energies. These phenomena were considered to be caused by repetitive injections of electrons with smaller time scale than the substorm‐associated injections, so were named "microinjections." In this paper, we analyze two microinjection events and find that they are likely to result from the drift resonance between local electrons and Ultralow‐frequency compressional toroidal waves. To verify this hypothesis, we extend the present theory for toroidal mode drift resonance from only considering electrons with 90° pitch angles to including all bouncing electrons. We find that the predicted phase relationships of electron fluxes and the wave electric field correspond well with observations. The phase difference between the electron fluxes and the electric field equals to −90° or +90° at the resonant energy and increases with energy. Therefore, we propose that the compressional toroidal mode drift resonance may work as the generation mechanism of microinjections in the two events. Key Points: Evidences of drift resonance between compressional Ultralow‐frequency waves and electrons are observed in microinjection eventsCompressional toroidal mode drift resonance theory is extended to off‐equator regionDrift resonance can be a possible mechanism for the generation of microinjection events [ABSTRACT FROM AUTHOR]

In the inner magnetosphere dominated by the dipole magnetic field, energetic electrons of 90° pitch angle drift faster than those of other pitch angles. Electron flux oscillations initiated by injections or drift resonances will be observed earlier at a 90° pitch angle, that is, 90°‐leading boomerang‐shaped stripes on pitch angle distributions(PADs) are expected. In this work, reversed‐boomerang stripes (90° observed last) are first reported from the observations of RBSP‐A at L‐shell ∼5.9 on 11 March 2016. The corresponding solar wind dynamic pressure is over 10 nPa, which suggests that strong compression on the magnetosphere changes the magnetic field and drift frequencies of energetic electrons. Test particles running in an image‐dipole magnetic field (off‐equatorial minima exists at dayside large L‐shell) reproduce the abnormal (90° slower) drift velocities at L‐shell >5.8 but are normal at L‐shell <4.5 dominated by a dipole field. Normal boomerang stripes on PADs are indeed observed by RBSP‐B at L‐shell ∼4.0, indicating the solar wind dynamic pressure effect. Plain Language Summary: The motions of energetically charged particles trapped in the outer radiation belt can be divided into three adiabatic motions: Gyro, bounce, and drift. Usually, the magnetic field within the 3.0 < L‐shell < 6.0 is close to a dipole magnetic field, where charged particles with pitch angles closer to 90° drift faster. Therefore, electron flux oscillations at a 90° pitch angle followed by other pitch angles symmetrically ("boomerang‐shaped" stripes) are often observed in injection/dropout echoes or drift resonance events. In this study, we present an observation where both normal‐(90° observed first) and reversed‐(90° last) boomerang stripes on electron pitch angle distributions are simultaneously observed by two Van Allen Probes on 11 March 2016. Besides, strong compression impacts the magnetosphere (the solar wind dynamic pressure is over 10 nPa). The reversed‐boomerang stripes suggest that the particles with 90° pitch angle drift more slowly, which disagrees with the dipole field. Test particle simulation in an image‐dipole magnetic field can reproduce the observed reversed‐boomerang feature at L‐shell >5.8, which suggests that the reversed‐boomerang stripes result from the strong compression on the magnetosphere. At L‐shell <4.5, normal boomerang stripes are observed since the magnetic field therein is less affected by the compression on the magnetosphere and still dominated by the dipole field. Key Points: Electron normal‐(90° first) and reversed‐(90° last) boomerang stripes on the pitch angle distributions (PADs) are simultaneously observed at different L‐shellsThe strong compression on the magnetosphere results in the dayside off‐equatorial minima, leading to reversed dispersions on electron PADsTest particle simulation in an image dipole model (off‐equatorial minima exists) reproduces the longer drift periods of 90° particles [ABSTRACT FROM AUTHOR]

Ultralow frequency (ULF) wave‐particle interactions play a significant role in the radiation belt dynamic process, during which drift resonance can accelerate and transport energetic electrons in the outer radiation belt. Observations of wave‐electron drift resonance are characterized by quasiperiodic straight or "boomerang‐shaped" stripes in the pitch angle spectrogram. Here we present an ULF wave event on 1 December 2015, during which both kinds stripes were observed by Van Allen Probes A and B, respectively. Using the time‐of‐flight technique based on the pitch angle dependence of electron drift velocities, the "boomerang‐shaped" stripes are inferred to originate from straight stripes at the time and location covered by Probe B. Given that straight stripes were indeed observed by Probe B, our observations strongly support the charged particle interacting with azimuthally localized ULF waves. A new method is provided to identify the location of ULF wave‐particle interaction on the basis of remote observations of electron flux modulations. Plain Language Summary: In the outer radiation belt, energetic electrons drift around the Earth with the periods of tens of minutes up to a few hours, which match the period of ultralow frequency (ULF) waves. Therefore, resonant wave‐particle interactions may occur during the electron drift motion, which are observationally characterized by quasiperiodic stripes, either straight or "boomerang shaped," in the pitch angle spectrograms of electron fluxes. Here we observe, for the first time, the appearance of straight and "boomerang‐shaped" stripes at two Van Allen Probe locations, respectively. These observations support the scenario that the straight stripes are generated by localized wave‐particle drift resonance, and they are gradually distorted by the pitch angle‐dependent electron drift motion, to form "boomerang‐shaped" stripes outside the region of strong wave activity. This study also provides a new approach to identify the region of ULF wave‐particle interactions on the basis of remote observations of electron flux modulations. Key Points: ULF wave‐electron drift resonance can be characterized by quasiperiodic straight or boomerang stripes in the pitch angle spectragramBoomerang stripes evolve from straight ones after electrons drift away from the localized ULF wave‐particle interaction regionWe present a new approach to remotely identify the region of ULF wave activity, which is validated by two‐spacecraft measurements [ABSTRACT FROM AUTHOR]

In this study, we present Van Allen Probe observations showing that seed (hundreds of keV) and core (≳ 1 MeV) electrons can resonate with ultra‐low‐frequency (ULF) wave modes with distinctive m values simultaneously. An unusual electron energy spectrogram with double‐banded resonant structure was recorded by energetic particle, composition, and thermal plasma (ECT)‐magnetic electron ion spectrometer (MagEIS) and, meanwhile, boomerang stripes in pitch angle spectrogram appeared at the lower energy band. A localized drift resonance with m = 10 wave component was responsible for the resonant band peaked at ∼200 keV while a global drift resonance with m = 3 component gave rise to the upper band resonance peaked at ∼1 MeV. Time‐Of‐Flight on boomerang stripes suggested that the localized drift resonance with ∼200 keV electrons was confined within the plasmaspheric plume. Electron flux modulations were reproduced by numerical simulations in good consistency with the observations, supporting the scenario that localized and global drift resonance could coexist in the outer belt electron dynamics simultaneously. Key Points: We, for the first time, report the banded drift resonance between outer belt electrons and ULF wavesULF waves with small and moderate azimuthal wave numbers can resonate with core and seed electrons simultaneouslyAzimuthally localized and global drift resonances are able to coexist [ABSTRACT FROM AUTHOR]

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. Key Points: The flux of ultrarelativistic electrons enhanced by an order of magnitude within 1 hr after SSC on 16 July 2017Drift resonance between 3.4‐MeV electrons and shock‐induced ULF waves is confirmedThe behavior of nonresonant particles in response to the shock indicates the scenario of drift resonance with damping ULF waves [ABSTRACT FROM AUTHOR]

We present an analysis of 'boomerang-shaped' pitch angle evolutions of outer radiation belt relativistic electrons observed by the Van Allen Probes after the passage of an interplanetary shock on 7 June 2014. The flux at different pitch angles is modulated by Pc5 waves, with equatorially mirroring electrons reaching the satellite first. For 90° pitch angle electrons, the phase change of the flux modulations across energy exceeds 180° and increasingly tilts with time. Using estimates of the arrival time of particles of different pitch angles at the spacecraft location, a scenario is investigated in which shock-induced ULF waves interact with electrons through the drift resonance mechanism in a localized region westward of the spacecraft. Numerical calculations on particle energy gain with the modified ULF wavefield reproduce the observed boomerang stripes and modulations in the electron energy spectrogram. The study of boomerang stripes and their relationship to drift resonance taking place at a location different from the observation point adds new understanding of the processes controlling the dynamics of the outer radiation belt. [ABSTRACT FROM AUTHOR]

Electron flux measurements are an important diagnostic for interactions between ultralow‐frequency (ULF) waves and relativistic (∼1 MeV) electrons. Since measurements are collected by particle detectors with finite energy channel width, they are affected by a phase mixing process that can obscure these interactions. We demonstrate that ultrahigh‐resolution electron measurements from the Magnetic Electron Ion Spectrometer on the Van Allen Probes mission—obtained using a data product that improves the energy resolution by roughly an order of magnitude—are crucial for understanding ULF wave‐particle interactions. In particular, the ultrahigh‐resolution measurements reveal a range of complex dynamics that cannot be resolved by standard measurements. Furthermore, the standard measurements provide estimates for the ULF flux modulation amplitude, period, and phase that may not be representative of true flux modulations, potentially leading to ambiguous conclusions concerning electron dynamics. Plain Language Summary: Plasma waves with periods of a few minutes interact with high‐energy electrons in the near‐Earth space environment. Understanding how the waves affect the electrons is important, as these electrons can potentially damage orbiting satellites. However, these interactions are obscured by a process known as phase mixing where electrons with different energies behave differently yet are mixed together in measurements that have finite energy resolution. We show how a unique data set obtained from National Aeronautics and Space Administration's Van Allen Probes mission can dramatically increase the energy resolution, revealing a range of interactions that are obscured by measurements with lower energy resolution. Key Points: Previous electron measurements could not resolve many energy‐dependent ULF wave interactions with >∼1 MeV electrons due to phase mixingMagEIS histogram channels resolve many interactions: dE/E of 2‐3% compared to standard 20‐30% that is already considered high resolutionWithout ultrahigh‐resolution measurements, ambiguous conclusions may be reached concerning >∼1 MeV electron dynamics (e.g., drift resonance) [ABSTRACT FROM AUTHOR]