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.
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.
The electron radiation belts of the Earth, consisting of relativistic and ultra-relativistic electrons, are composed by an inner belt, a slot region, and a dynamic outer belt where electrons frequently vary because of solar wind or geomagnetic activities. An intense activity (e.g., a geomagnetic storm) would drive various plasma waves, such as whistler mode waves, electromagnetic ion-cyclotron (EMIC) wave, and ultra-low frequency (ULF) wave (e.g.
where ω, m, and ω d are the poloidal wave angular frequency, the azimuthal wave number, and the drift angular velocity, respectively. This equation is derived under an azimuthally symmetric background field, which is obviously applicable for a dipole magnetic field. Considering a dipole magnetic field, the bounce-averaged drift angular velocity for energetic electrons can be expressed as:
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