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.

中文翻译：

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偶极锋后面磁通堆积区内电子节距角分布的局部转变


偶极锋（DF），即具有强磁场的向地球传播的磁瞬变，是有利于磁尾中高能电子加速的重要区域。 DF驱动的电子加速通常会在通量堆积区域（FPR）内产生相干的俯仰角分布（PAD），即DF后面的强磁场区域，例如煎饼分布、蝴蝶分布和雪茄分布，这些分布在不同的尾部区域和经常单独发生。在这里，我们展示了 FPR 内部电子 PAD 演化的独特观察结果，表明电子 PAD 经历了从雪茄分布到蝴蝶分布，再到煎饼分布的局部转变，形成 U 形分布。在局部跃迁期间，电子垂直通量（相对于局部磁场）与磁场强度反相关，这与传统预期相反。电子U形分布的意外特征与不同尺度的多个物理过程相关，包括通量管的局部膨胀和中性片附近的俯仰角变化。这些非典型观测可以增进我们目前对磁层中电子加速和传输的理解。