The evolution of a buoyancy disturbance in a stratified incompressible fluid permeated by a uniform vertical magnetic field is investigated. Two regimes are considered in the absence of background rotation – that of strong stratification, where the internal gravity wave frequency $\omega _A$ is much higher in magnitude than the magnetic (Alfvén) wave frequency $\omega _M$ , and that of strong magnetic field, where $\omega _M$ is dominant. For small but finite magnetic diffusion, perturbations that initially lie in the strong-field regime are shown to cross over to the regime of strong stratification, so that small-scale motions may exist as damped internal gravity waves at large times. The induced magnetic field propagates as damped Alfvén waves for a much longer time than the velocity before undergoing the above transition. With strong rotation, the unstably stratified system that satisfies the inequality $|\omega _C| > |\omega _M| \gg |\omega _A| \gg |\omega _\eta |$ , where $\omega _C$ is the inertial wave frequency and $\omega _\eta$ is the diffusion frequency, is of relevance to convection-driven dynamos. Here, a parameter space with $|\omega _M/\omega _C| \sim 0.1$ is found wherein the flow intensity of the slow magnetic-Archimedean-Coriolis (MAC) waves is of the same order of magnitude as that of the fast MAC waves. Slow wave motions at horizontal length scales much smaller than the width of the fluid layer can therefore generate substantial helicity in rapidly rotating dynamos. The excitation of slow MAC waves at scales of $\sim$ 10 km in the Earth's core may play a crucial role in the generation of the axial dipole field.

中文翻译：

---

分层流体中强迫磁流体动力波的演化

研究了由均匀垂直磁场渗透的分层不可压缩流体中浮力扰动的演变。在没有背景旋转的情况下考虑了两种情况——强分层，其中内部重力波频率 $\欧米茄_A$ 幅度远高于磁（Alfvén）波频率 $\欧米茄_M$ , 和强磁场的, 其中 $\欧米茄_M$ 占主导地位。对于小但有限的磁扩散，最初处于强场状态的扰动被证明会跨越到强分层状态，因此小尺度运动可能在很大程度上以阻尼内部重力波的形式存在。感应磁场作为阻尼阿尔文波传播的时间比经历上述转变之前的速度要长得多。具有强旋转，满足不平等的不稳定分层系统 $|\欧米茄_C| > |\欧米茄_M| \gg |\欧米茄_A| \gg |\omega _\eta |$ ， 在哪里 $\欧米茄_C$ 是惯性波频率和 $\欧米茄_\eta$ 是扩散频率，与对流驱动的发电机有关。这里，一个参数空间 $|\欧米茄_M/\欧米茄_C| \sim 0.1$ 发现其中慢磁-阿基米德-科里奥利(MAC)波的流动强度与快速MAC波的流动强度相同的数量级。因此，水平长度的慢波运动比流体层的宽度小得多，因此可以在快速旋转的发电机中产生相当大的螺旋度。慢 MAC 波的激发尺度为 $\sim$ 地核10公里处可能在轴向偶极子场的产生中起关键作用。