Evolution of generalized two‐dimensional magnetotail equilibria in ideal and resistive MHD
Evolution of generalized two‐dimensional magnetotail equilibria in ideal and resistive MHD
复制标题
理想和电阻 MHD 中广义二维磁尾平衡的演化
DOI：
10.1002/2014ja020651
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
J. Lyon
中科院分区：
文献类型：
--
作者：
V. Merkin;M. Sitnov;J. Lyon
我们给出了地球磁尾的二维(2-D)磁流体动力学(MHD)模拟结果。使用的是Lyon-Fedder-Mobarry全球MHD模型的区域改编。作为初始条件，我们采用了一类在尾端(BZ驼峰)有和没有磁通量积累的渐近磁尾平衡。最近的全粒子模拟表明，前者对具有离子撕裂性质的动力学模式是不稳定的。因此，我们的目标是研究相同平衡在MHD近似下的演化，并协助对动力学模拟的物理解释。这还受到能量原理的考虑，该原理表明，如果系统在运动上不稳定，那么它在理想情况下也可能不稳定。为了寻找类似于在动力学模拟中观察到的动态MHD区域，我们实施了两组边界条件(速度平衡，VB和动量平衡，MB)，一组允许等离子体流通过边界，另一组限制这种流。使用更能反映MB的边界条件会抑制任何显著的动力学，除了初始平衡松弛之外，我们看不到任何实质性的变化。另一方面，VB边界条件允许更有效地松弛初始平衡并吸收随后产生的等离子体流。有了这些边界条件，我们找到了没有磁通积累(即恒定磁场分量垂直于电流片)的平衡，以发展伴随着尾部等离子体流的明显阻性模。同时，具有足够大幅度的BZ驼峰的平衡发展出一种不同的、理想的模式，其特征是自发地产生向地球的等离子体流和相应电场的指数增长。这种增长在性质上类似于相应的完全动力学模拟，尽管在MHD计算中没有明显的向地球移动的BZ峰的爆炸性增长，只是初始通量积累的一部分向地球移动。我们讨论了我们的结果对地球磁尾和全球MHD模拟中这种平衡存在的可能性和影响的影响。
We present results of two‐dimensional (2‐D) magnetohydrodynamic (MHD) simulations of the terrestrial magnetotail. A regional adaptation of the Lyon‐Fedder‐Mobarry global MHD model is used. As initial conditions, we employ a class of asymptotic magnetotail equilibria with and without an accumulation of magnetic flux at the tailward end (a Bz hump). The former have been recently shown by full particle simulations to be unstable to a kinetic mode with formal properties of ion tearing. Thus, our goal here is to investigate the evolution of the same equilibria in the MHD approximation and assist in the physical interpretation of the kinetic simulations. This is additionally motivated by the energy principle considerations which suggest that if the system is unstable kinetically, it may also be unstable ideally. To seek dynamical MHD regimes similar to those observed in kinetic simulations, we implement two sets of boundary conditions (velocity balanced, VB, and momentum balanced, MB), one allowing plasma flows through the boundaries and the other inhibiting such flows. The use of more reflecting MB boundary conditions results in suppression of any significant dynamics, and we see no substantial changes beyond initial equilibrium relaxation. On the other hand, VB boundary conditions allow a more efficient relaxation of initial equilibrium and absorb subsequently generated plasma flows. With these boundary conditions we find the equilibrium without a flux accumulation (i.e., with constant magnetic field component normal to the current sheet) to develop an apparently resistive mode accompanied by tailward plasma flows. At the same time, the equilibria with a Bz hump of sufficiently large amplitude develop a different, ideal, mode characterized by spontaneous generation of earthward plasma flows and an exponential growth of the corresponding electric field. This growth is qualitatively similar to the corresponding fully kinetic simulations although no explosive growth of the earthward moving Bz peak is evident in the MHD calculations, just an earthward shift of a part of the initial flux accumulation. We discuss implications of our results for the possibility of existence and impact of such equilibria in the Earth's magnetotail and in global MHD simulations.