Convective, absolute and global azimuthal magnetorotational instabilities

We study the convective and absolute forms of azimuthal magnetorotational instability (AMRI) in a cylindrical Taylor-Couette (TC) flow with an imposed azimuthal magnetic field. We show that the domain of the convective AMRI is wider than that of the absolute AMRI. Actually, it is the absolute instability which is the most relevant and important for magnetic TC flow experiments. The absolute AMRI, unlike the convective one, stays in the device, displaying a sustained growth that can be experimentally detected. We also study the global AMRI in a TC flow of finite height using direct numerical simulation and find that its emerging butterfly-type structure - a spatio-temporal variation in the form of axially upward and downward travelling waves - is in a very good agreement with the linear analysis, which indicates the presence of two dominant absolute AMRI modes in the flow giving rise to this global butterfly pattern.

Automated summary

We investigate the convective and absolute forms of azimuthal magnetorotational instability in a cylindrical Taylor-Couette flow with an imposed azimuthal magnetic field. Our results show that the convective AMRI domain is wider than the absolute AMRI domain, but the absolute instability is more relevant and important for magnetic TC flow experiments. The global AMRI in a TC flow of finite height exhibits a butterfly-type structure, with two dominant absolute AMRI modes giving rise to this pattern in a good agreement with linear analysis.