Fluid simulations of cosmic ray-modified shocks
Fluid simulations of cosmic ray-modified shocks
复制标题
DOI：
10.1093/mnras/stab1926
复制
发表时间：
2020-08
影响因子：
4.8
通讯作者：
T. Tsung;S. Oh;Yan-Fei Jiang
中科院分区：
文献类型：
--
作者：
T. Tsung;S. Oh;Yan-Fei Jiang
宇宙射线（CR）修改冲击是一个苛刻的测试的数字代码。我们用它们来测试和验证CR流体动力学的两个时刻的方法，以及表征的现实主义的CR冲击加速在双流体模拟，这是不可避免的。在此之前，数值代码无法在这种苛刻的制度中纳入流，并且从未与解析解进行过比较。首先，我们找到了一个新的解析解高度差异的加速效率从标准的解决方案。它是由CRs从次激波中的双向流动引起的，类似于辐射激波中的泽尔多维奇尖峰。由于更少的CR扩散回到上游，这有利于更低的加速效率，通常为${\lesssim}10{{\rm %}}$（即使马赫数> 10），而不是在以前的分析工作中发现的${\gtrsim}50{{\rm %}}$。在马赫数为10时，新的解分为三个分支，分别为有效、中等和无效CR加速。我们的两个时刻的代码准确地恢复这些解决方案在整个参数空间探测，没有特设的封闭关系。对于一般的初始条件，低效的分支被代码鲁棒地选择;中间的分支是不稳定的。优选的分支被CR非常弱地修饰。在高马赫数（1000）时，气体跳跃条件接近于纯流体动力激波的条件，为了获得合理的加速效率，需要对热注入进行亚网格规定。CR-修改的冲击有很长的平衡时间（1000扩散时间）需要开发的前体，这必须解决的1000细胞收敛。非平衡效应、差的分辨率和磁场的不均匀性都降低CR加速效率。在星系尺度模拟中，如果不进行次网格修正，激波对CR加速的贡献一般不大。
Cosmic ray (CR)-modified shocks are a demanding test of numerical codes. We use them to test and validate the two-moment method for CR hydrodynamics, as well as characterize the realism of CR shock acceleration in two-fluid simulations which inevitably arises. Previously, numerical codes were unable to incorporate streaming in this demanding regime, and have never been compared against analytic solutions. First, we find a new analytic solution highly discrepant in acceleration efficiency from the standard solution. It arises from bi-directional streaming of CRs away from the subshock, similar to a Zeldovich spike in radiative shocks. Since fewer CRs diffuse back upstream, this favours a much lower acceleration efficiency, typically ${\lesssim}10{{\ \rm per\ cent}}$ (even for Mach number > 10) as opposed to ${\gtrsim}50{{\ \rm per\ cent}}$ found in previous analytic work. At Mach number ≳10, the new solution bifurcates into three branches, with efficient, intermediate, and inefficient CR acceleration. Our two-moment code accurately recovers these solutions across the entire parameter space probed, with no ad hoc closure relations. For generic initial conditions, the inefficient branch is robustly chosen by the code; the intermediate branch is unstable. The preferred branch is very weakly modified by CRs. At high Mach numbers (≳10), the gas jump conditions approach that of a purely hydrodynamic shock, and a sub-grid prescription for thermal injection is required for reasonable acceleration efficiencies ${\sim}10{{\ \rm per\ cent}}$. CR-modified shocks have very long equilibration times (∼1000 diffusion time) required to develop the precursor, which must be resolved by ≳10 cells for convergence. Non-equilibrium effects, poor resolution, and obliquity of the magnetic field all reduce CR acceleration efficiency. Shocks in galaxy-scale simulations will generally contribute little to CR acceleration without sub-grid modification.