Disintegration and reformation of intermediate‐shock segments in three‐dimensional MHD bow shock flows
Disintegration and reformation of intermediate‐shock segments in three‐dimensional MHD bow shock flows
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三维MHD弓激流中中间激波段的分解与重组
DOI：
10.1029/2000ja000205
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发表时间：
2001
影响因子：
--
通讯作者：
S. Poedts
中科院分区：
文献类型：
--
作者：
H. Sterck;S. Poedts
最近，它已被证明，对于强的上游磁场，固定的三维磁流体动力学（MHD）的弓激波流表现出复杂的双前锋激波拓扑结构与特定段的中间MHD激波类型的激波阵面。研究了这种新的弓形激波拓扑的大尺度稳定性。描述了两种类型的数值实验，其中上游流以时间依赖的方式扰动。研究发现，大振幅非周期局部扰动可能导致中间激波的解体，而中间激波在积分振幅超过临界值的扰动下确实是不稳定的，但在驱动弓形激波问题中，总是存在中间激波动态重组的激波前缘段，从而导致在扰动过去之后具有中间激波段的新的双波前拓扑的再现。这些MHD的结果表明，在不稳定的空间物理弓形激波流的中间类型的激波段的可能间歇性形成的理论机制时，上游磁场是强的，例如，在地球弓形激波期间的强行星际磁场，这是更常见的在太阳极大的条件下，或在领先的冲击前引起的快速日冕物质抛射在太阳日冕。当考虑到真实的空间等离子体中的动力学效应和实际耗散时，是否会形成中间激波段还有待证实。在弓形激波流中，真实的、波浪状的循环扰动与中间激波段的详细相互作用可能导致由（随时间变化的）中间激波、旋转间断和非线性波列组成的非定常结构，如Markovskii和Skorokhodov [2000]提出的方案。讨论了新的弓形激波拓扑结构与空间天气现象的中间激波的可能相关性。
Recently, it has been shown that for strong upstream magnetic field, stationary three-dimensional magnetohydrodynamic (MHD) bow shock flows exhibit a complex double-front shock topology with particular segments of the shock fronts being of the intermediate MHD shock type. The large-scale stability of this new bow shock topology is investigated. Two types of numerical experiments are described in which the upstream flow is perturbed in a time-dependent manner. It is found that large-amplitude noncyclic localized perturbations may cause the disintegration of the intermediate shocks, which are indeed known to be unstable against perturbations with integrated amplitudes above critical values, but that in the driven bow shock problem there are always shock front segments where intermediate shocks are reformed dynamically, resulting in the reappearance of the new double-front topology with intermediate-shock segments after the perturbation has passed. These MHD results indicate a theoretical mechanism for the possible intermittent formation of shock segments of intermediate type in unsteady space physics bow shock flows when upstream magnetic fields are strong, for example, in the terrestrial bow shock during periods of strong interplanetary magnetic field, which are more common under solar maximum conditions, or in leading shock fronts induced by fast coronal mass ejections in the solar corona. It remains to be confirmed if intermediate-shock segments would be formed when kinetic effects and realistic dissipation in real space plasmas are taken into account. The detailed interaction of realistic, wave-like cyclic perturbations with the intermediate-shock segments in bow shock flows may lead to unsteady structures composed of (time-dependent) intermediate shocks, rotational discontinuities, and nonlinear wave trains, as in the scenarios proposed by Markovskii and Skorokhodov [2000]. The possible relevance of the new bow shock topology with intermediate shocks for space weather phenomena is discussed.