A POWERFUL LOCAL SHEAR INSTABILITY IN WEAKLY MAGNETIZED DISKS .1. LINEAR-ANALYSIS
A POWERFUL LOCAL SHEAR INSTABILITY IN WEAKLY MAGNETIZED DISKS .1. LINEAR-ANALYSIS
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DOI：
10.1086/170270
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发表时间：
1991-07-20
影响因子：
4.9
通讯作者：
HAWLEY, JF
中科院分区：
文献类型：
--
作者：
BALBUS, SA;HAWLEY, JF
在本文和一个同伴的工作，我们表明，广泛的一类天体物理吸积盘是动态不稳定的轴对称扰动在弱磁场的存在下。由于磁场的普遍存在，这个结果对气体差动旋转系统的影响相当普遍。这项工作提出了一个线性分析的不稳定性。(The同伴的工作提出了非线性数值模拟的结果。不稳定是局部的，而且非常强大。最大增长率的数量级的角旋转速度和磁场的强度是无关的，只要在场的能量密度小于热能密度。不稳定的轴对称扰动需要极向场分量的存在，并且与环向分量的存在无关。不稳定性也要求角速度向外减小。在没有强耗散过程的情况下，不稳定性就没有其他条件。与不稳定性相关的流体运动直接产生极向和环向场分量。我们详细讨论了不稳定性的物理解释。提出了饱和发生的条件。指出并解释了经典瑞利剪切失稳判据在场强消失极限下的不出现。不稳定性对盘边界条件和本构流体性质都不敏感。它的存在排除了内波（非压缩）在圆盘中传播的可能性。如果存在于天体物理盘中，具有交换性质的不稳定性很可能导致通用和有效的角动量传输，从而解决一个突出的理论难题。
In this paper and a companion work, we show that a broad class of astrophysical accretion disk is dynamically unstable to axisymmetric disturbances in the presence of a weak magnetic field. Because of the ubiquity of magnetic fields, this result bears upon gaseous differentially rotating systems quite generally. This work presents a linear analysis of the instability. (The companion work presents the results of nonlinear numerical simulations.) The instability is local and extremely powerful. The maximal growth rate is of order the angular rotation velocity and is independent of the strength of the magnetic field, provided only that the energy density in the field is less than the thermal energy density. Unstable axisymmetric disturbances require the presence of a poloidal field component, and are indifferent to the presence of a toroidal component. The instability also requires that the angular velocity be decreasing outward. In the absence of a powerful dissipation process, there are no other requirements for instability. Fluid motions associated with the instability directly generate both poloidal and toroidal field components. We discuss the physical interpretation of the instability in detail. Conditions under which saturation occurs are suggested. The nonemergence of the classical Rayleigh criterion for shear instability in the limit of vanishing field strength is noted and explained. The instability is sensitive neither to disk boundary conditions nor to the constituative fluid properties. Its existence precludes the possibility of internal (noncompressive) wave propagation in a disk. If present in astrophysical disks, the instability, which has the character of an interchange, is very likely to lead to generic and efficient angular momentum transport, thereby resolving an outstanding theoretical puzzle.