We present fully three-dimensional local MRI simulations with the object of studying the excitation of non-axisymmetric spiral density waves that are observed to always be present in such simulations. They are potentially important for affecting protoplanetary migration through the action of associated stochastic gravitational forces and producing residual transport in MHD inactive regions. The simulations we perform are with zero net flux and produce mean activity levels of alpha ~ 0.005. We reveal the nature of the mechanism responsible for the excitation of these waves by determining the time dependent evolution of the Fourier transforms of the participating state variables. The dominant waves are found to have no vertical structure and to be excited during periodically repeating swings in which they change from leading to trailing. The initial phase of the evolution of such a swing is found to be in excellent agreement with that expected from the WKBJ theory developed in a preceding paper by Heinemann & Papaloizou.
We demonstrate that the important source terms causing excitation of the waves are related to a quantity that reduces to the potential vorticity for small perturbations from the background state with no vertical dependence. We find that the root mean square density fluctuations associated with the waves are positively correlated with both this quantity and the general level of hydromagnetic turbulence. The mean angular momentum transport associated with spiral density waves generated in our simulations is estimated to be a significant fraction of that associated with the turbulent Reynolds stress.
Cold HI in Turbulent Eddies and Galactic Spiral Shocks
HI 21厘米线自吸收（HISA）揭示了银盘中冷原子云的形状和分布。许多这样的云缺乏相应的CO排放，尽管在平衡模型中比纯原子气体更冷。HISA需要以相同速度的背景线发射，因此可以产生这样的背景的机制。弱的，小规模的，和广泛的吸收可能会产生湍流涡旋，而强大的，大规模的吸收出现在云复合体沿螺旋臂冲击组织。在后者中，气体可能在恒星形成之前从原子状态演化为分子状态，这可以解释HISA-CO协议的不完整性。
Thermal and Magnetorotational Instability in the ISM: Two-Dimensional Numerical Simulations
银河系和其他盘状星系中弥漫气体的结构和动力学可能会受到大约1-100 pc尺度上的热不稳定性和磁旋转不稳定性（TI和MRI）的强烈影响。我们开始研究这些过程，使用二维数值流体动力学和磁流体动力学（MHD）模拟与原子星际介质（ISM）的条件。我们证明，与以前的工作相一致，“纯TI”的非线性发展产生了一个网络的灯丝冷凝成冷云在其交叉点，产生一个独特的两相热/冷介质约20万年内。对于均匀的初始P/k=2000 K cm^-3，TI驱动的云湍流运动在亚音速振幅下达到饱和。MRI以前已经在近均匀介质中进行了研究;我们的模拟包括TI+MRI模型，它从均匀密度条件开始，云+MRI模型，它从两相多云介质开始。TI+MRI和云+MRI模型都表明，MRI在几个银河系轨道时间内发展，就像均匀介质一样。云之间的平均间隔可以影响哪个MRI模式主导演变。提供云间的分离不超过一半的MRI波长，我们发现MRI的增长率是类似于相应的均匀介质。这打开了一种可能性，如果低云体积填充因子增加MRI耗散时间相比，在一个均匀的介质中，MRI驱动的运动在ISM中可以达到的幅度与观察到的HI湍流线宽。
