On the Exchange of Kinetic and Magnetic Energy between Clouds and the Interstellar Medium
On the Exchange of Kinetic and Magnetic Energy between Clouds and the Interstellar Medium
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云与星际介质之间动能和磁能的交换
DOI：
10.1086/307162
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
D. Ryu
中科院分区：
文献类型：
--
作者：
F. Miniati;T. Jones;D. Ryu
我们通过二维MHD数值模拟，研究了倾斜于运动星际云的均匀磁场的相互作用。特别是，我们探索了由于场线伸展而将云动能转化为磁能的问题。以前的一些模拟强调了当磁场垂直于云层运动时，围绕云层形成的“磁屏蔽”可能具有的动力学重要性。然而，目前还不清楚这些发现在多大程度上依赖于假设的野外配置和云层属性。为了扩大我们对这一效应的理解，我们通过改变磁场相对于云运动的方位角(θ)、云-背景密度对比度和云的马赫数来研究几个新的情况。结果表明，在二维空间中，当θ足够大时，磁场张力可以在高密度对比度、低马赫数云的运动动力学中起主导作用。在这种情况下，云团动能的很大一部分可以转化为磁能，云团头部的磁压超过撞击流的冲压。我们推导出了能量“转换”过程的特征时间标度τma。我们还发现，除非云运动与磁场高度一致，否则通过云尾迹中撕裂模不稳定性的重联限制了跟随云的强磁绳特征的形成。最后，我们试图根据我们的结果解释所观察到的磁场的一些性质。
We investigate, through two-dimensional MHD numerical simulations, the interaction of a uniform magnetic field oblique to a moving interstellar cloud. In particular we explore the transformation of cloud kinetic energy into magnetic energy as a result of field line stretching. Some previous simulations have emphasized the possible dynamical importance of a "magnetic shield" formed around clouds when the magnetic field is perpendicular to the cloud motion. It was not clear, however, how dependent those findings were on the assumed field configuration and cloud properties. To expand our understanding of this effect, we examine several new cases by varying the magnetic field orientation angle with respect to the cloud motion (θ), the cloud-background density contrast, and the cloud Mach number. We show that in two dimensions and with θ large enough, the magnetic field tension can become dominant in the dynamics of the motion of high density contrast, low Mach number clouds. In such a case, a significant fraction of the cloud's kinetic energy can be transformed into magnetic energy with the magnetic pressure at the cloud's nose exceeding the ram pressure of the impinging flow. We derive a characteristic timescale, τma, for this process of energy "conversion." We find also that unless the cloud motion is highly aligned with the magnetic field, reconnection through tearing-mode instabilities in the cloud wake limits the formation of a strong flux-rope feature following the cloud. Finally we attempt to interpret some observed properties of the magnetic field in view of our results.