团簇磁场对子弹和气泡的覆盖--形态学和动力学效应
Draping of Cluster Magnetic Fields over Bullets and Bubbles -- Morphology and Dynamic Effects
摘要：
高分辨率的X射线观测揭示了星系团内的空洞和“冷锋”，它们在温度、密度和金属丰度方面具有尖锐的边缘。它们的存在造成了一个难题，因为这些功能预计不会是流体动力学稳定的，或在扩散的存在下保持尖锐。然而，即使是在非常弱的磁化等离子体中，一个移动的核心或气泡也必然会扫过足够的磁场，在物体周围建立起一个动态的重要鞘层;该层的强度是由场的“犁起”和场线围绕核心滑动之间的竞争决定的。我们表明，一个二维的方法来解决这个问题是相当普遍的是不可能的。在三维空间中，我们与分析参数和数值实验表明，该磁性层修改的动力学的一个暴跌的核心，极大地修改了流体动力学不稳定性对核心的影响，修改剥离的材料的几何形状，甚至减缓下降的核心通过磁张力。我们推导出的最大磁场强度，层的厚度，和磁尾的张开角的表达式。磁覆盖层的形态意味着抑制穿过该层的热传导，从而在接触表面上保持强的温度梯度。间歇性放大的磁场，以及注入的MHD湍流的核心尾流被确定为是由于涡的产生内的磁披覆层。这些结果对于理解星系团内介质的物理性质和复杂的气体动力学过程具有重要意义，并且非常普遍地适用于通过其他磁化环境的运动，例如，的ISM。
英文摘要：
High-resolution X-ray observations have revealed cavities and `cold fronts' with sharp edges in temperature, density, and metallicity within galaxy clusters. Their presence poses a puzzle since these features are not expected to be hydrodynamically stable, or to remain sharp in the presence of diffusion. However, a moving core or bubble in even a very weakly magnetized plasma necessarily sweeps up enough magnetic field to build up a dynamically important sheath around the object; the layer's strength is set by a competition between `plowing up' of field and field lines slipping around the core. We show that a two-dimensional approach to the problem is quite generally not possible. In three dimensions, we show with analytic arguments and in numerical experiments, that this magnetic layer modifies the dynamics of a plunging core, greatly modifies the effects of hydrodynamic instabilities on the core, modifies the geometry of stripped material, and even slows the fall of the core through magnetic tension. We derive an expression for the maximum magnetic field strength, the thickness of the layer, and the opening angle of the magnetic wake. The morphology of the magnetic draping layer implies the suppression of thermal conduction across the layer, thus conserving strong temperature gradients over the contact surface. The intermittent amplification of the magnetic field as well as the injection of MHD turbulence in the wake of the core is identified to be due to vorticity generation within the magnetic draping layer. These results have important consequences for understanding the physical properties and the complex gasdynamical processes of the intra-cluster medium, and apply quite generally to motions through other magnetized environments, e.g., the ISM.
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