基于MMS数据对磁尾等离子体片中磁洞精细结构和带电粒子动力学特征的研究

Magnetic holes are a kind of magnetic dissipative structure, which can occur in some space environments, such as solar wind, planetary magnetosheath and earth magnetosheath. In recent years, magnetic holes have also been observed in the Earth's magnetotail plasma sheet. These structures have excited people great interest since their typical length scale is less than one ion cyclotron radius and their background plasma environment is mirror stable. Several formation mechanisms of the magnetic hole have been proposed. Besides, recent studies show that the area ahead of the dipolarization front could be a possible source region generating these structures. Since the satellite plasma data resolution is not high enough, fine structures and charged particle properties of the magnetic hole are not fully clear, resulting in that the formation mechanism of the magnetic hole is still inconclusive. This project aims to analyze the electromagnetic and charged particle properties of magnetic holes in the plasma sheet, especially near the dipolarization front, then compare these properties of the magnetic holes at different regions, and statistically study the distribution and occurrence rate of the magnetic hole based on multi-satellite joint observation. Our project will mainly use high-resolution data from MMS satellites, combined with 10 years data from THEMS satellites. The purpose of this projection is to understand the fine structure and charged particle properties of the magnetic hole, then to clarify its source region, so as to provide detailed observational materials to reveal its formation mechanisms and evolution.

磁洞是一种磁场耗散结构，可出现在太阳风、行星磁鞘和地球磁鞘等空间等离子体环境里。近年来，在地球磁尾等离子体片里也观测到了磁洞，因其空间尺度小于1个质子回旋半径且周围等离子体是磁镜稳定的而引起人们浓厚的兴趣，提出了多种形成机制。此外，近期的研究表明偶极化锋面前端的区域可能是形成磁洞的一个源区。由于此前卫星的等离子体数据分辨率较低，导致很难深入分析磁洞结构里电磁和带电粒子的相关特征，致使磁洞的形成机制尚无定论。本项目拟采用多卫星联合观测，以具有高分辨率数据的MMS卫星为主，并结合有十年数据的THEMIS卫星，详细分析对等离子体片里特别是偶极化锋面附近磁洞的电磁和带电粒子相关特征，对比不同区域里这些特征的差异，统计磁洞的分布和发生率。本项目目的在于获取磁洞的精细结构和带电粒子动力学特征，明确磁洞形成的源区，从而为揭示磁洞的形成机制和演化过程提供详细的观测资料。

电子尺度磁洞是一个尺度小于局地离子回旋半径的磁场耗散结构，广泛存在于地球磁尾等离子体片、磁鞘和1 AU太阳风中。电子尺度磁洞可以捕获高能电子，为内磁层的电子动力学过程提供种子电子。然而，这些结构中的电子动力学过程和物理形成机制仍不完全清楚。本项目中，我们利用MMS卫星所提供的高精度磁场和等离子体数据，对地球磁尾电流片及1 AU 太阳风中电子尺度磁洞开展了细致研究。我们对电子尺度磁洞中电子涡流的行程开展了详细研究，分析了不同电子漂移过程对电子涡流的贡献。我们利用多卫星联合观测，揭示了电子尺度磁洞的三维结构。此外，我们也在火星附近的太阳风中首次报道了电子尺度磁洞的存在，也首次报道了地球轨道的太阳风中存在电子尺度磁柱；通过统计分析，我们进一步揭示了地球前兆激波是电子尺度磁洞和磁柱的一个重要源区。电子尺度磁洞中存在电流，这些电流在磁尾动力学过程中是否发挥着重要作用尚不清楚。为此，我们也对地球磁尾中的小尺度电流开展了研究。此外，为弄清楚电子尺度磁洞在湍动中发挥的作用，我们也对湍动开展了一系列研究。