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arXiv 0909.0270astro-ph.HEastro-ph.CO
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Turbulence与团簇内介质的混合

Turbulence and Mixing in the Intracluster Medium

P. Sharma, B. D. G. Chandran, E. Quataert, I. J. Parrish

备注：
8 pages, 2 figs., submitted to the conference proceedings of "The Monster's Fiery Breath;" a follow up of arXiv:0901.4786 focusing on the general mixing properties of the ICM
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摘要：

团内介质（ICM）在流体动力学意义上是稳定分层的，熵$s$向外增加。然而，沿磁力线的热传导从根本上改变了ICM的稳定性，导致“热通量浮力不稳定性”时，$dT/dr>0$和“磁热不稳定性”时，$dT/dr<0$。因此，不管$dT/dr$和$ds/dr$的迹象如何，ICM都是有害的不稳定的。另一方面，这些温度梯度驱动的不稳定性通过重新定向磁场（当$dT/dr>0$时垂直于$\hat{\bf r}$，当$dT/dr<0$时平行于$\hat{\bf r}$）而饱和，而不会产生持续的对流。我们表明，各向异性导电等离子体达到这种非线性稳定的磁配置后，它经历了浮力回复力，抵抗磁场的进一步扭曲。该回复力类似于稳定分层的绝热等离子体所经受的浮力回复力。我们认为，为了驱动机制（如星系运动或宇宙线浮力），以克服这种回复力，并在ICM中产生湍流，驱动的强度必须超过一个阈值，对应于湍流速度$\gtrsim 10 -100 {km/s}$。对于较弱的驱动，ICM保持在其非线性稳定的磁配置，和湍流混合是有效的缺席。我们讨论了这些发现的影响，在ICM的金属和热量的湍流扩散。

英文摘要：

The intracluster medium (ICM) is stably stratified in the hydrodynamic sense with the entropy $s$ increasing outwards. However, thermal conduction along magnetic field lines fundamentally changes the stability of the ICM, leading to the "heat-flux buoyancy instability" when $dT/dr>0$ and the "magnetothermal instability" when $dT/dr<0$. The ICM is thus buoyantly unstable regardless of the signs of $dT/dr$ and $ds/dr$. On the other hand, these temperature-gradient-driven instabilities saturate by reorienting the magnetic field (perpendicular to $\hat{\bf r}$ when $dT/dr>0$ and parallel to $\hat{\bf r}$ when $dT/dr<0$), without generating sustained convection. We show that after an anisotropically conducting plasma reaches this nonlinearly stable magnetic configuration, it experiences a buoyant restoring force that resists further distortions of the magnetic field. This restoring force is analogous to the buoyant restoring force experienced by a stably stratified adiabatic plasma. We argue that in order for a driving mechanism (e.g, galaxy motions or cosmic-ray buoyancy) to overcome this restoring force and generate turbulence in the ICM, the strength of the driving must exceed a threshold, corresponding to turbulent velocities $\gtrsim 10 -100 {km/s}$. For weaker driving, the ICM remains in its nonlinearly stable magnetic configuration, and turbulent mixing is effectively absent. We discuss the implications of these findings for the turbulent diffusion of metals and heat in the ICM.

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