The cosmic-ray staircase: the outcome of the cosmic-ray acoustic instability
The cosmic-ray staircase: the outcome of the cosmic-ray acoustic instability
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宇宙射线阶梯：宇宙射线声学不稳定性的结果
DOI：
10.1093/mnras/stac1123
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发表时间：
2022
影响因子：
4.8
通讯作者：
Jiang, Yan-Fei
中科院分区：
文献类型：
--
作者：
Tsung, Tsun Hin Navin;Oh, S. Peng;Jiang, Yan-Fei
最近，宇宙射线(CRS)已经成为驱动银河系风的主要候选者。小范围的过程会极大地影响全球风的特性。我们对CR流进行了两个时刻的模拟，以研究相移CR力和CR加热如何驱动声波不稳定。我们验证了线性理论增长率。当声波变得非线性时，它们会陡峭地形成一系列传播的准周期激波；激波密度的跳跃造成了CR瓶颈。传播瓶颈的深度既取决于密度跳跃，也取决于其速度；对于快速移动的瓶颈，ΔPC较小。一系列瓶颈形成了CR楼梯结构，这可以从凸壳结构中理解。系统在新扰动的增长和阶梯合并之间达到稳定状态。CRS在高原时是分离的，但在楼梯跳跃时施加强烈的力和加热。高原没有CR加热会导致冷却、强烈的气体压力梯度和进一步的激波。如果瓶颈是静止的，它们可以极大地改变全球流动；如果它们的传播时间与动态时间相当，它们对全球动量和能量转移的影响是温和的。CR声学不稳定性可能与冷气体和热气体之间的热界面以及银河系风有关。与辐射流动中不透明度的增加类似，由于瓶颈导致的CR压力的积累可以显著增加质量流出速率，最高可达一个数量级。它孕育了不同寻常的热不稳定形式，在∼kpc尺度上，这些激波可能具有明显的观测特征。
Recently, cosmic rays (CRs) have emerged as a leading candidate for driving galactic winds. Small-scale processes can dramatically affect global wind properties. We run two-moment simulations of CR streaming to study how sound waves are driven unstable by phase-shifted CR forces and CR heating. We verify linear theory growth rates. As the sound waves grow non-linear, they steepen into a quasi-periodic series of propagating shocks; the density jumps at shocks create CR bottlenecks. The depth of a propagating bottleneck depends on both the density jump and its velocity; ΔPcis smaller for rapidly moving bottlenecks. A series of bottlenecks creates a CR staircase structure, which can be understood from a convex hull construction. The system reaches a steady state between growth of new perturbations, and stair mergers. CRs are decoupled at plateaus, but exert intense forces and heating at stair jumps. The absence of CR heating at plateaus leads to cooling, strong gas pressure gradients and further shocks. If bottlenecks are stationary, they can drastically modify global flows; if their propagation times are comparable to dynamical times, their effects on global momentum and energy transfer are modest. The CR acoustic instability is likely relevant in thermal interfaces between cold and hot gas, as well as galactic winds. Similar to increased opacity in radiative flows, the build-up of CR pressure due to bottlenecks can significantly increase mass outflow rates, by up to an order of magnitude. It seeds unusual forms of thermal instability, and the shocks could have distinct observational signatures, on ∼kpc scales.