Local Transition of Electron Pitch Angle Distribution within Flux Pileup Region behind Dipolarization Front

Dipolarization fronts (DFs), earthward-propagating magnetic transients with a strong magnetic field, are important regions favorable for energetic electron acceleration in the magnetotail. The DF-driven electron acceleration usually generates coherent pitch angle distributions (PADs) inside flux pileup regions (FPRs), i.e., strong magnetic field regions behind the DFs, such as pancake, butterfly, and cigar distributions, which dominate at different tail regions and often occur separately. Here we present unique observations of electron PAD evolution inside the FPR, showing that electron PAD underwent local transition from cigar distribution, to butterfly distribution, then toward pancake distribution, forming a U-shaped distribution. During the local transition, electron perpendicular flux (relative to the local magnetic field) is anticorrelated with magnetic field strength, contrary to traditional expectation. The unexpected feature of the electron U-shaped distribution is associated with multiple physical processes at different scales, including local expansion of flux tubes and pitch angle variation near the neutral sheet. These atypical observations can advance our current understanding of electron acceleration and transport in the magnetosphere.