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Distinct Formation and Evolution Characteristics of Outer Radiation Belt Electron Butterfly Pitch Angle Distributions Observed by Van Allen Probes

Using Van Allen Probes pitch angle‐resolved electron flux data, we report intriguing events of electron butterfly pitch angle distributions (PADs) at L > ~5.5 on the nightside with distinct durations, i.e., about 9 hr for the 28–29 April 2013 event and over 2 days for the 2–5 May 2013 event. The formation and evolution of the short‐duration electron butterfly PADs exhibited insignificant dependence on electron energy, probably resulting from a combination of drift shell splitting, magnetopause shadowing, and wave diffusion, but the long‐duration electron butterfly evolution presented strong energy dependence, likely mainly attributed to interactions with MS waves. Both the short‐duration and long‐duration electron butterfly PADs disappeared closely related to enhanced substorm activities, while the responsible waves could be plasmaspheric hiss or chorus waves. Our results demonstrate that radiation belt electron butterfly PADs are naturally complex in both spatial and temporal scales and well connected to solar wind condition, substorm activity, and magnetospheric wave distribution.