具有各向异性动量和热量输运的热分层团内介质的动力学稳定性
Dynamical stability of a thermally stratified intracluster medium with anisotropic momentum and heat transport
摘要：
在弱碰撞等离子体中，如团内介质（ICM），热量和动量输运相对于局部磁场方向变得各向异性。各向异性热传导导致慢磁声波在重力方向温度升高时发生磁热不稳定性（MTI），在重力方向温度降低时发生热流驱动浮力不稳定性（HBI）。伴随这些不稳定性的磁场强度的局部变化引起压力各向异性，其对平行于磁场的运动进行粘性阻尼。在本文中，我们采用了线性稳定性分析，以阐明各向异性粘度（即布拉金斯基压力各向异性）的MTI和HBI的影响。通过抑制磁场线的会聚/发散，压力各向异性显著影响ICM如何与温度梯度相互作用。依赖于磁场线的会聚/发散产生不稳定浮力运动（HBI）的不稳定性在大部分波数空间上被抑制，而那些以其他方式被场线会聚/发散（MTI）阻碍的不稳定性被加强。因此，HBI在ICM中基本上未被抑制的波数具有太小的平行分量，不能严格地被认为是局部的。当磁场变得越来越与温度梯度正交时，这一点尤其正确。相反，增长最快的MTI模式不受各向异性粘度的影响。然而，我们发现，各向异性的粘度夫妇慢，阿尔芬波在这样一种方式，以不稳定的阿尔芬波动时，温度在重力的方向上增加。因此，许多以前被认为是MTI稳定或缓慢增长的波数实际上是最不稳定的。（节略）
英文摘要：
In weakly-collisional plasmas such as the intracluster medium (ICM), heat and momentum transport become anisotropic with respect to the local magnetic field direction. Anisotropic heat conduction causes the slow magnetosonic wave to become buoyantly unstable to the magnetothermal instability (MTI) when the temperature increases in the direction of gravity and to the heat-flux--driven buoyancy instability (HBI) when the temperature decreases in the direction of gravity. The local changes in magnetic field strength that attend these instabilities cause pressure anisotropies that viscously damp motions parallel to the magnetic field. In this paper we employ a linear stability analysis to elucidate the effects of anisotropic viscosity (i.e. Braginskii pressure anisotropy) on the MTI and HBI. By stifling the convergence/divergence of magnetic field lines, pressure anisotropy significantly affects how the ICM interacts with the temperature gradient. Instabilities which depend upon the convergence/divergence of magnetic field lines to generate unstable buoyant motions (the HBI) are suppressed over much of the wavenumber space, whereas those which are otherwise impeded by field-line convergence/divergence (the MTI) are strengthened. As a result, the wavenumbers at which the HBI survives largely unsuppressed in the ICM have parallel components too small to rigorously be considered local. This is particularly true as the magnetic field becomes more and more orthogonal to the temperature gradient. In contrast, the fastest-growing MTI modes are unaffected by anisotropic viscosity. However, we find that anisotropic viscosity couples slow and Alfven waves in such a way as to buoyantly destabilise Alfvenic fluctuations when the temperature increases in the direction of gravity. Consequently, many wavenumbers previously considered MTI-stable or slow-growing are in fact maximally unstable. (abridged)
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