发表机构
Helmholtz-Zentrum Dresden-Rossendorf; Abastumani Astrophysical Observatory; Faculty of Exact and Natural Sciences, Tbilisi State University; LyRIDS, ECE-Paris Engineering School; LUTH, UMR 8102 CNRS, Observatoire de Paris-Meudon（德累斯顿-罗斯多夫亥姆霍兹中心; 阿巴斯图马尼天文台; 第比利斯国立大学理学院; 巴黎ECE工程学院LyRIDS; 巴黎-默东天文台CNRS联合研究实验室）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
研究开普勒盘中弱垂直磁场下的扰动，发现MRI驱动的不可压缩磁模式可在径向波数过零时激发螺旋密度波，为可压缩运动提供线性机制。
AI 中文摘要
我们在剪切盒近似下，研究受弱均匀垂直磁场作用的开普勒盘中非轴对称扰动的线性动力学。扰动被分解为剪切波，通过数值积分线性化理想磁流体力学（MHD）方程演化。该盘流支持三种基本扰动模式：两种不可压缩模式——发生磁旋转不稳定性（MRI）的磁模式、惯性-磁波，以及可压缩螺旋密度波。磁模式和惯性-磁波的频率分别为阿尔文频率和轨道频率量级，而密度波为高频。我们引入模式本征函数和控制模态方程分析各模式动力学。对于非轴对称模式，因开普勒旋转的剪切作用，模态方程耦合，产生新的剪切诱导线性模式耦合过程，该过程源于剪切流的非自伴性质。我们重点研究主导的MRI不稳定磁模式产生密度波的过程，结果显示，初始施加的磁模式经MRI增长，当径向波数过零时会突然激发密度波；当扰动的方位向和垂直波长与盘的标高相当，密度波与MRI的耦合最有效。由于密度波可压缩、MRI不可压缩，该波激发过程也可视为通过MRI驱动的不可压缩运动产生可压缩运动的线性机制，本文还讨论了其对具有非零净垂直磁场的MRI湍流的可压缩性影响。
英文摘要
We investigate the linear dynamics of non-axisymmetric perturbations in Keplerian discs subject to a weak uniform vertical magnetic field in the shearing box approximation. Perturbations are decomposed into shearing waves and evolved by numerically integrating the linearized ideal MHD equations. The disc flow supports three basic perturbation modes: two incompressible modes -- a magnetic mode that exhibits magnetorotational instability (MRI) and inertia-magnetic wave -- and a compressible spiral density wave. The magnetic mode and inertia-magnetic wave have a low frequency of the order of Alfvén and orbital frequencies, respectively, while density wave has a high frequency. We introduce mode eigenfunctions and governing modal equations to analyse the dynamics of individual modes and their interaction. For non-axisymmetric modes, the modal equations are coupled due to the shear of the disc's Keplerian rotation, giving rise to a new shear-induced linear mode coupling process, which is rooted in the non-self-adjoint nature of shear flows. We focus on the generation of density waves by the dominant MRI-unstable magnetic mode. We show that initially imposed magnetic mode undergoes MRI growth and abruptly excites density waves when its radial wavenumber crosses zero. This density wave--MRI coupling is most efficient when the azimuthal and vertical wavelengths of perturbations are comparable to the disc scale height. Since density waves are compressible, whereas MRI is incompressible, this wave excitation process can also be regarded as a linear mechanism generating compressible motions via MRI-driven incompressible ones. Its implications for compressible non-zero net vertical field MRI turbulence are also discussed.
Comments14 pages, 6 figures, published in MNRAS
