标题： $\textit{Eppur Si Muove}$：多相MHD中的自持流体运动 显示英文标题

标题： $\textit{Eppur Si Muove}$: Self-Sustained Streaming Motions in Multi-Phase MHD

摘要： 辐射冷却可以驱动多相气体的动力学。 一个显著的例子是流体动力学“粉碎”，即冷却云在压力驱动下剧烈的碎片化，这使其与周围环境严重失去压力平衡。 我们进行磁流体动力学（MHD）模拟，以了解磁场如何影响这种粉碎过程。 在MHD中，云不会“随机”粉碎。 相反，在初始碎片化之后，热相和冷相会以长寿命、场对齐、自我维持的高速气体流动方式相干地“流动”（$\sim 100 \, {\rm km \, s^{-1}}$）。 MHD热不稳定性也会产生这样的流动。 这是由于MHD压力支撑的各向异性特性，它仅在磁场垂直方向起作用。 因此，即使当$P_{\rm B} + P_{\rm gas} \approx$为常数时，压力平衡也只在磁场垂直方向成立。 场对齐的气体压力变化没有受到阻碍，导致气体速度$v \sim (2 \Delta P/\rho)^{1/2}$，这是伯努利原理的结果。 引人注目的是，相邻通量管中的气体以相反方向流动（$\textit{counter-stream}$）。 我们证明这是由冷却引起的MHD版本的薄壳不稳定性所导致的。 磁张力在促进皱褶不稳定性以及修改其非线性演化过程中都起着重要作用。 即使在高$\beta$的热气体中，也可以出现流动，因为随着气体冷却和压缩，磁压支撑会增加。 热传导增加了流动云团的大小和速度，但不会定性地改变动力学。 这些结果与太阳日冕雨中观测到的反向流动气体有关，也与星际介质（ISM）、星系际介质（ICM）和星系晕介质（CGM）中的多相气体冷却和凝聚相关。 显示英文摘要

摘要： Radiative cooling can drive dynamics in multi-phase gas. A dramatic example is hydrodynamic `shattering', the violent, pressure-driven fragmentation of a cooling cloud which falls drastically out of pressure balance with its surroundings. We run MHD simulations to understand how shattering is influenced by magnetic fields. In MHD, clouds do not `shatter' chaotically. Instead, after initial fragmentation, both hot and cold phases coherently `stream' in long-lived, field-aligned, self-sustaining gas flows, at high speed ($\sim 100 \, {\rm km \, s^{-1}}$). MHD thermal instability also produces such flows. They are due to the anisotropic nature of MHD pressure support, which only operates perpendicular to B-fields. Thus, even when $P_{\rm B} + P_{\rm gas} \approx$const, pressure balance only holds perpendicular to B-fields. Field-aligned gas pressure variations are unopposed, and results in gas velocities $v \sim (2 \Delta P/\rho)^{1/2}$ from Bernoulli's principle. Strikingly, gas in adjacent flux tubes $\textit{counter-stream}$ in opposite directions. We show this arises from a cooling-induced, MHD version of the thin shell instability. Magnetic tension is important both in enabling corrugational instability and modifying its non-linear evolution. Even in high $\beta$ hot gas, streaming can arise, since magnetic pressure support grows as gas cools and compresses. Thermal conduction increases the sizes and velocities of streaming cloudlets, but does not qualitatively modify dynamics. These results are relevant to the counter-streaming gas flows observed in solar coronal rain, as well as multi-phase gas cooling and condensation in the ISM, CGM and ICM.