LAUSR

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.