Standard, planar accretion discs operate through a dissipative mechanism, usually thought to be turbulent, and often modelled as a viscosity. This acts to take energy from the radial shear, enabling the flow of mass and angular momentum in the radial direction. In a previous paper we discussed observational evidence for the magnitude of this viscosity, and pointed out discrepancies between these values and those obtained in numerical simulations. In this paper we discuss the observational evidence for the magnitude of the dissipative effects which act in non--planar discs, both to transfer and to eliminate the non--planarity. Estimates based on the model by Ogilvie (1999), which assumes a small--scale, isotropic viscosity, give alignment timescales for fully ionized discs which are apparently too short by a factor of a few compared with observations, although we emphasize that more detailed computations as well as tighter observational constraints are required to verify this conclusion. For discs with low temperature and conductivity, we find that the timescales for disc alignment based on isotropic viscosity are too short by around two orders of magnitude. This large discrepancy suggests that our understanding of viscosity in quiescent discs is currently inadequate.
What Do the Hitomi Observations Tell Us About the Turbulent Velocities in the Perseus Cluster? Probing the Velocity Field with Mock Observations
Hitomi首次直接测量了英仙座星系团的气体运动，这意味着它的核心相当“静止”，速度小于200 km s $\sim$，尽管存在活动星系核和晃动的冷锋。在以前的工作的基础上，我们使用合成Hitomi/SXS观测模拟集群的热等离子体与晃动的气体运动和不同的粘度，以类似的方式分析其速度结构。我们发现，晃动运动可以产生类似于瞳测量的线位移和宽度。我们发现这些测量不受气体粘度值的影响，因为它的影响只在小于SXS $\sim$ 1' PSF的角标度上清楚地表现出来。PSF使靠近核心的区域的谱线偏移偏置多达$\sim 40-50$ km s $^{-1}$，因此仔细模拟这种效应至关重要。我们还推断，如果晃动的运动占主导地位的观察速度梯度，英仙座必须观察到从这些运动的平面有点倾斜的视线，但仍然允许螺旋图案是可见的。最后，我们发现，假设各向同性的运动可以低估的总速度和动能的核心在我们的模拟多达$\sim$ 60%。然而，在我们模拟的集群核心的总动能仍然是不到10%的热能的核心，与瞳观测。
