[论文解读] Auroral signatures of ballooning instability and plasmoid formation processes in the near-Earth magnetotail

Ping Zhu, J. Liang+3 more|arXiv (Cornell University)|Feb 4, 2026

Ionosphere and magnetosphere dynamics被引用 0

一句话总结

该论文对近地磁尾对流失稳的三维MHD模拟及等离子体团形成进行研究，将结果的FAC映射到极光区，并与THEMIS ASI观测结果比较，以揭示磁尾动力学与极光珠串及起始之间的联系。

ABSTRACT

The nonlinear development of ballooning instability and the subsequently induced plasmoid formation in the near-Earth magnetotail demonstrated in MHD simulations has been proposed as a potential trigger mechanism for substorm onset over the past decade, and their connections to the in-situ satellite and ground all-sky auroral optical observations have been a subject of continued research. In this work, a set of THEMIS substorm onset events with good conjunction of auroral observations has been selected for comparative simulation study, whose pre-onset magnetotail configuration and conditions are inferred from in-situ data and compared with the onset conditions of ballooning instability obtained in our MHD simulations. The evolution of the near-Earth magnetotail is followed, where the signatures of ballooning instability and the plasmoid formation are extracted from simulations and compared with the magnetic fields and flow patterns within the magnetotail region from observation data. The field-aligned current (FAC) density is evaluated at the Earth side boundary of the magnetotail domain of simulation, which is further mapped along magnetic field lines to the auroral ionosphere and compared with the auroral pattern and evolution there in terms of growth rate, dominant wavenumber, and absolute auroral intensities. Such validation efforts are also the first step towards the development of a self-consistent coupling model that includes the magnetotail-ionosphere interaction in the substorm onset process.

研究动机与目标

• 研究近地磁尾非线性 ballooning 不稳定性如何触发等离子体团形成。
• 评估尾部动力学如何通过场线辖向电流（FACs）映射到极光电离层信号。
• 在次暴时间观测的基础上将模拟结果与 THEMIS 极光观测结果进行验证，以探究磁尾–电离层耦合。
• 探索朝向自洽的磁尾–电离层耦合模型用于次暴起始的步骤。

提出的方法

• 使用 NIMROD 代码解的电阻性 MHD 方程，模拟近地磁尾的一般化 Harris 脚线。
• 对 x 与 z 采用实心、无滑移边界，y 方向周期性，归一化到平衡尺度长度和阿尔fvén时。
• 在 y 方向引入初始扰动（单色或双模态），演化至非线性 ballooning 与等离子体团形成。
• 在地球侧边界提取尾尺度的 FAC 密度，并沿磁场线映射到极光电离层。
• 使用 TREx-ATM 模型结合 Knight 关系将 FAC 与平行电势、以及就地数据拟合的电子温度，重建极光辐射。
• 从笛卡尔坐标电流片映射过渡到近地磁的 Tsyganenko 89 (T89) 基于偶极场，以更好地匹配观测。

Figure 1: THEMIS-A (a: upper), THEMIS-D (b: lower left) and THEMIS-E (c: lower right) observations of magnetic field (1st row), ion velocity (2nd row), perpendicular ion velocity (3rd row), ion number density (4th row) and ion/magnetic pressure (5th row) as functions of time during the 2009, March 5

实验结果

研究问题

• RQ1 近地磁尾的非线性 ballooning 不稳定性是否能开启影响再连接动力学的等离子体团形成？
• RQ2 建模的 FAC 结构如何映射到极光区图样，是否能再现次暴起始阶段观测到的珠串和弧演化？
• RQ3 多模扰动在形成细结构极光特征和极区极移弧形成中起到何种作用？
• RQ4 磁尾–电离层耦合模型在多大程度上能再现次暴起始信号的时序与时空演化？

主要发现

• 非线性 ballooning 演化驱动近地磁尾中的再连接与等离子体团形成。
• 地球侧边界的 FAC 展现出方位周期性，与电离层中演化的极光结构相映射。
• 双模扰动（n=1 与 n=25）通过引入更短波长的 FAC 组件，提升与观测极光极移弧的吻合度。
• 通过 TREx-ATM 的极光区映射显示从初始珠串向极移弧形成的演进，与观测的起始动力学一致。
• 在单模运行中，大尺度弧结构占主导，随着高次谐波出现后，较小尺度的方位结构逐渐出现，且与观测的吻合度提高。

Figure 2: (a: upper left) The ( $X_{\rm GSM}$ , $Y_{\rm GSM}$ ) coordinates of THEMIS satellite orbit trajectories; (b: upper right) the $B_{z}$ component of magnetic field measured by TH-A (dark solid line) and the fitted value of $B_{z}$ (blue dashed line), (c: lower left) the lobe magnetic field