自引力气体（M）HD模拟的一个新的Jeans分辨准则：应用于引力驱动的turbulence的磁场放大
A new Jeans resolution criterion for (M)HD simulations of self-gravitating gas: Application to magnetic field amplification by gravity-driven turbulence
摘要：
宇宙结构的形成是以重力、湍流和磁场之间复杂的相互作用为特征的。然而，人们对引力能转化为湍流能和磁能的过程仍然知之甚少。在这里，我们显示与高分辨率，自适应网格模拟，MHD湍流是有效地驱动提取能量的重力势在致密的气体云的崩溃。在收缩期间产生的可压缩运动被转换成螺线管的湍流运动，导致E_sol/E_tot的自然能量比约为2/3。我们发现，引力驱动湍流的能量注入尺度接近于局域的金斯尺度。如果存在小的磁场种子，它们通过小尺度发电机过程以指数方式快速放大。磁场在最小尺度上增长最有效，在这种尺度上，磁力线的拉伸、扭曲和折叠以及湍流涡旋都能得到充分的分辨。我们发现，这个规模对应于约30个网格单元的模拟。因此，我们建议一个新的最小分辨率标准的30个细胞每牛仔长度（磁）流体动力学模拟的自引力气体，以解决湍流的牛仔尺度，并捕捉最小的发电机放大的磁场。然而，由于数值扩散，今天任何现有的模拟最多只能提供物理增长率的下限。我们的结论是，一个小的，初始的磁场可以增长到动态的重要强度的时间尺度上显着短于云的自由落体时间。
英文摘要：
Cosmic structure formation is characterized by the complex interplay between gravity, turbulence, and magnetic fields. The processes by which gravitational energy is converted into turbulent and magnetic energies, however, remain poorly understood. Here, we show with high-resolution, adaptive-mesh simulations that MHD turbulence is efficiently driven by extracting energy from the gravitational potential during the collapse of a dense gas cloud. Compressible motions generated during the contraction are converted into solenoidal, turbulent motions, leading to a natural energy ratio of E_sol/E_tot of approximately 2/3. We find that the energy injection scale of gravity-driven turbulence is close to the local Jeans scale. If small seeds of the magnetic field are present, they are amplified exponentially fast via the small-scale dynamo process. The magnetic field grows most efficiently on the smallest scales, for which the stretching, twisting, and folding of field lines, and the turbulent vortices are sufficiently resolved. We find that this scale corresponds to about 30 grid cells in the simulations. We thus suggest a new minimum resolution criterion of 30 cells per Jeans length in (magneto)hydrodynamical simulations of self-gravitating gas, in order to resolve turbulence on the Jeans scale, and to capture minimum dynamo amplification of the magnetic field. Due to numerical diffusion, however, any existing simulation today can at best provide lower limits on the physical growth rates. We conclude that a small, initial magnetic field can grow to dynamically important strength on time scales significantly shorter than the free-fall time of the cloud.
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