Magnetospheric Multiscale Observations of Foreshock Transients at Their Very Early Stage
Magnetospheric Multiscale Observations of Foreshock Transients at Their Very Early Stage
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DOI：
10.3847/1538-4357/abb249
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发表时间：
2020-08
期刊：
影响因子：
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通讯作者：
T. Liu;X. An;Hui Zhang;D. Turner
中科院分区：
文献类型：
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作者：
T. Liu;X. An;Hui Zhang;D. Turner
前震瞬变是离子前震中的离子动力学结构。由于它们的动态压力扰动，它们可以扰乱弓激波和磁层-电离层系统。它们还可以加速导致冲击加速的粒子。然而，目前还不清楚它们到底是如何形成的。最近的细胞内粒子模拟指出了电场和霍尔电流在形成过程中的重要作用。为了进一步研究这一点，我们使用磁层多尺度（MMS）任务的数据对刚刚开始形成的两个小型（1000-2000公里）前震瞬态事件进行案例研究。在事件 1 中，MMS 呈四面体结构，我们表明决定磁场分布的电流密度配置主要由消磁前震离子产生的霍尔电流驱动。由此产生的磁场时间变化感应出一个电场，驱动冷等离子体随着磁场线向外移动。在事件 2 中，MMS 呈珍珠串状，我们分析了场和等离子体参数的演变。我们表明，磁通量和质量通量从核心向外传输，导致边界变陡。陡峭的边界捕获了更多的前震离子并引起更强的前震离子退磁，非线性地进一步增强了霍尔电流。根据我们的观察，我们提出了一种物理形成过程，其中前震离子对由前震离子霍尔电流引起的变化磁场的正反馈使得结构“不稳定”并生长。
Foreshock transients are ion kinetic structures in the ion foreshock. Due to their dynamic pressure perturbations, they can disturb the bow shock and magnetosphere–ionosphere system. They can also accelerate particles contributing to shock acceleration. However, it is still unclear how exactly they form. Recent particle-in-cell simulations point out the important role of the electric field and Hall current in the formation process. To further examine this, we use data from the Magnetospheric Multiscale (MMS) mission to apply case studies on two small (1000–2000 km) foreshock transient events that just started to form. In event 1 where MMS was in a tetrahedral formation, we show that the current density configuration, which determined the magnetic field profile, was mainly driven by Hall currents generated by demagnetized foreshock ions. The resulting time variation of the magnetic field induced an electric field that drove cold plasma moving outward with magnetic field lines. In event 2 where MMS was in a string-of-pearls formation, we analyze the evolution of field and plasma parameters. We show that the magnetic flux and mass flux were transported outward from the core, resulting in the steepening of the boundary. The steepened boundary, which trapped more foreshock ions and caused stronger demagnetization of foreshock ions, nonlinearly further enhanced the Hall current. Based on our observations, we propose a physical formation process wherein the positive feedback of foreshock ions on the varying magnetic field caused by the foreshock ion Hall current enables an “instability” and the growth of the structure.