The Wardle Instability in Interstellar Shocks. I. Nonlinear Dynamical Evolution
The Wardle Instability in Interstellar Shocks. I. Nonlinear Dynamical Evolution
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星际冲击中的沃德尔不稳定性。
DOI：
10.1086/304595
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发表时间：
1997
期刊：
影响因子：
--
通讯作者：
J. Stone
中科院分区：
文献类型：
--
作者：
J. Stone
通过依赖时间的磁流体动力学模拟，研究了弱电离等离子体中不稳定 C 型激波的非线性演化。这项研究仅限于磁控等离子体中的冲击（其中中性粒子中的阿尔芬速度大大超过了声速），并且没有遵循电离和复合等微观物理过程。给出了初始平面垂直和倾斜 C 型激波的二维模拟以及垂直激波的全三维模拟。对于这里研究的情况，不稳定性导致形成沿冲击传播方向拉长并垂直于磁场方向的致密气体片。弱 J 型锋面的形成与平衡激波结构的不稳定性增长有关。饱和后，磁场结构由沿激波传播方向向外弯曲的拱形结构组成，并由致密片中增强的离子中性阻力锚定。与磁浮力（帕克）不稳定性类似，当扭曲的磁力线中的磁张力通过片材中的阻力平衡时，就会发生饱和。对于这里研究的磁控冲击，产生饱和的磁场畸变非常小。尽管如此，片材中离子和中性密度的增强非常大，与预震值相比，增强了 2 到 3 个数量级。在这些高密度下，重组过程可能很重要。这些片层随时间缓慢演变，因此在均匀介质中传播的激波可能会留下一个由相交细丝和稠密气体片组成的网络，该网络在激波传播方向上延伸并垂直于平均场。另一篇论文介绍了非线性区域中不稳定 C 型激波的温度结构和发射特性。
The nonlinear evolution of unstable C-type shocks in weakly ionized plasmas is studied by means of time-dependent, magnetohydrodynamic simulations. This study is limited to shocks in magnetically dominated plasmas (in which the Alfvén speed in the neutrals greatly exceeds the sound speed), and microphysical processes such as ionization and recombination are not followed. Both the two-dimensional simulations of initially planar perpendicular and oblique C-type shocks and the fully three-dimensional simulation of a perpendicular shock are presented. For the cases studied here, the instability results in the formation of dense sheets of gas elongated in the direction of shock propagation and oriented perpendicular to the magnetic field. The formation of a weak J-type front is associated with the growth of the instability from an equilibrium shock structure. After saturation, the magnetic field structure consists of arches that bow outward in the direction of shock propagation and are anchored by the enhanced ion-neutral drag in the dense sheets. Analogous to the magnetic buoyancy (Parker) instability, saturation occurs when the magnetic tension in the distorted field lines is balanced by drag in the sheets. For the magnetically dominated shocks studied here, the distortions in the magnetic field that produce saturation are very small. Nonetheless, the enhancements of the ion and neutral densities in the sheets are very large, between 2 and 3 orders of magnitude compared with the preshock values. At these high densities, recombination processes may be important. The sheets evolve slowly in time, so that shocks propagating in a homogeneous medium may leave behind a network of intersecting filaments and sheets of dense gas elongated in the direction of shock propagation and perpendicular to the mean field. The temperature structure and emission properties of unstable C-type shocks in the nonlinear regime are presented in a companion paper.