AbstractVan Allen radiation belt electrons exhibit complex dynamics during geomagnetically active periods. Investigation of electron pitch angle distributions (PADs) can provide important information on the dominant physical mechanisms controlling radiation belt behaviors. Here we report a storm time radiation belt event where energetic electron PADs changed from butterfly distributions to normal or flattop distributions within several hours. Van Allen Probes observations showed that the flattening of butterfly PADs was closely related to the occurrence of whistler‐mode chorus waves. Two‐dimensional quasi‐linear STEERB simulations demonstrate that the observed chorus can resonantly accelerate the near‐equatorially trapped electrons and rapidly flatten the corresponding electron butterfly PADs. These results provide a new insight on how chorus waves affect the dynamic evolution of radiation belt electrons.
