[论文解读] Stability of the magnetotail current sheet with normal magnetic field and field-aligned plasma flows

Chen Shi, Anton Artemyev+2 more|arXiv (Cornell University)|Oct 16, 2021

Ionosphere and magnetosphere dynamics参考文献 159被引用 4

一句话总结

本研究基于有限法向磁场分量（Bz）和场向离子流的1D多流体磁尾电流片模型，探究撕裂模不稳定性。结果表明，Bz显著稳定电流片，且反向对流离子流（净流量为零）使稳定性达到最大；场向流无法克服Bz引起的稳定效应，暗示亚暴爆发需额外的动能效应。

ABSTRACT

One of the most important problems of magnetotail dynamics is the substorm onset and the related instability of the magneotail current sheet. Since the simplest 2D current sheet configuration with monotonic $B_z$ was proven to be stable to the tearing mode, the focus of the instability investigation moved to more specific configurations, e.g. kinetic current sheets with strong transient ion currents and current sheets with non-monotonic $B_z$ (local $B_z$ minima or/and peaks). Stability of the latter current sheet configuration has been studied both within kinetic and fluid approaches, whereas the investigation of the transient ion effects were limited to kinetic models only. This paper aims to provide detailed analysis of stability of a multi-fluid current sheet configuration that mimics current sheets with transient ions. Using the system with two field-aligned ion flows that mimic the effect of pressure non-gyrotropy, we construct 1D current sheet with a finite $B_z$. This model describes well recent findings of very thin intense magnetotail current sheets. The stability analysis of this two-ion model confirms the stabilizing effect of finite $B_z$ and shows that the most stable current sheet is the one with exactly counter-streaming ion flows and zero net flow. Such field-aligned flows may substitute the contribution of the pressure tensor nongyrotropy to the stress balance, but cannot overtake the stabilizing effect of $B_z$. Obtained results are discussed in the context of magnetotail dynamical models and spacecraft observations.

研究动机与目标

• 采用多流体方法，建立具有有限Bz和场向等离子体流的1D磁尾电流片模型。
• 研究在法向磁场分量存在下，场向离子流如何影响撕裂模不稳定性。
• 确定等离子体流是否能克服薄电流片中Bz的稳定效应。
• 调和理论稳定性与观测到的亚暴爆发之间的矛盾，即尽管配置稳定，仍发生快速磁重连。

提出的方法

• 构建一个1D磁尾电流片模型，包含两个密度相等但场向速度相反的离子流体。
• 施加应力平衡条件，使磁张力（jyBz/c）与场向离子流动量通量平衡。
• 采用线性稳定性分析，计算撕裂模增长率随Lundquist数和流体不对称性的变化。
• 利用SciPy中的数值求解器求解扰动的两点边值问题。
• 考虑净流量趋近于零（反向对流）和净流量最大（单流体情形）的极限，以界定稳定性的极值。
• 分析最大增长率（γm）随Lundquist数（S）的标度关系，识别扩散主导的不稳定性。

实验结果

研究问题

• RQ1 在具有场向离子流的1D电流片中，有限Bz分量是否能稳定撕裂模？
• RQ2 尽管存在Bz的稳定作用，场向等离子体流是否仍能导致电流片不稳定？
• RQ3 净流速在决定电流片稳定性方面起什么作用？
• RQ4 双离子反向对流构型的稳定性与单流体MHD极限相比如何？
• RQ5 该稳定性对亚暴相关磁重连爆发具有何种意义？

主要发现

• 有限Bz分量显著稳定1D电流片中的撕裂模，且在大S极限下最大增长率满足γm ∝ S−1，表明为扩散支持的不稳定性。
• 最稳定构型出现在两个离子流体以相同速度反向对流时，此时净流量为零，撕裂模无增长。
• 任何离子流的不对称性（非零净流量）都会引入自由能，导致电流片不稳定，使最大增长率升高。
• 最不稳定情形对应于两个离子流体均以Alfvén速度同向流动，等价于单流体MHD极限。
• 场向等离子体流无法克服Bz的稳定效应，暗示需额外的动能效应（如压力非回旋对称性）才能触发亚暴爆发。
• 结果表明，现场观测到的极薄且强电流片在无额外自由能源时是稳定的。