Anisotropies in Compressible MHD Turbulence: Probing Magnetic Fields and Measuring Magnetization
Anisotropies in Compressible MHD Turbulence: Probing Magnetic Fields and Measuring Magnetization
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DOI：
10.3847/1538-4357/abea18
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发表时间：
2020-12
期刊：
影响因子：
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通讯作者：
Yue Hu;Siyao Xu;A. Lazarian
中科院分区：
文献类型：
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作者：
Yue Hu;Siyao Xu;A. Lazarian
探测星际介质(ISM)中的磁场是出了名的挑战。在现代磁流体湍流理论和湍流各向异性理论的启发下，我们引入了结构函数分析(SFA)作为一种测量磁场取向和估算磁化强度的新方法。从局部坐标系和全局坐标系的二阶结构函数出发，分析了三维可压缩MHD数值模拟中湍流速度的统计特性。在磁能大于湍流能量的亚Alfvénic湍流中，垂直于磁场方向和平行于磁场方向测量的湍流速度的SFA会有很大的不同。它们的比值与Alfvén马赫数MA成幂函数关系，而后者与磁场强度成反比。我们证明了湍流速度的各向异性结构函数可以用来估计磁场的方向和强度。由于用不同的示踪剂测量湍流速度，我们的方法可以普遍应用于探测多相ISM中的磁场。
Probing magnetic fields in the interstellar medium (ISM) is notoriously challenging. Motivated by the modern theories of magnetohydrodynamic (MHD) turbulence and turbulence anisotropy, we introduce the Structure Function Analysis (SFA) as a new approach to measure the magnetic-field orientation and estimate the magnetization. We analyze the statistics of turbulent velocities in three-dimensional compressible MHD simulations through the second-order structure functions in both local and global reference frames. In the sub-Alfvénic turbulence with the magnetic energy larger than the turbulent energy, the SFA of turbulent velocities measured in the directions perpendicular and parallel to the magnetic field can be significantly different. Their ratio has a power-law dependence on the Alfvén Mach number M A, which is inversely proportional to the magnetic-field strength. We demonstrate that the anisotropic structure functions of turbulent velocities can be used to estimate both the orientation and strength of magnetic fields. With turbulent velocities measured using different tracers, our approach can be generally applied to probing the magnetic fields in the multiphase ISM.