Observation of Magnetocoriolis Waves in a Liquid Metal Taylor-Couette Experiment

Abstract

The first observation of fast and slow magnetocoriolis (MC) waves in a laboratory experiment is reported. Rotating nonaxisymmetric modes arising from a magnetized turbulent Taylor-Couette flow of liquid metal are identified as the fast and slow MC waves by the dependence of the rotation frequency on the applied field strength. The observed slow MC wave is damped but the observation provides a means for predicting the onset of the Magnetorotational Instability.

A Unifying Picture of Helical and Azimuthal Magnetorotational Instability, and the Universal Significance of the Liu Limit

“…Besides this relevance to the outer part of accretion disks, to protoplanetary disks (Turner & Sano 2008), and possibly even to planetary cores (Petitdemange et al 2008), the limit of low and vanishing Pm has acquired some additional interest in connection with the recent liquid metal experiments devoted to the study of MRI (Sisan et al 2004;Stefani et al 2006;Nornberg et al 2010).…”

The role of boundaries in the magnetorotational instability

“…This effect has not been observed in the experiment, where the spiral seems to be retrograde even relatively close to the endcaps. This could be due to the effect of the mean shear flow, or the secondary excitation of typical waves of the system, such as magneto-coriolis waves [6].…”

“…In this case, the solution is relatively similar to the one obtained with no-slip boundaries, except that the axial position of the outflowing jet is made arbitrary by the periodic boundaries, and the linear MRI modes are sinusoidal in the z direction. In the linear regime, MRI modes correspond to the unstable branch appearing from the coalescence of two complex conjugate MagnetoCoriolis (MC) waves [6]. If these MC-waves are linearly stable (as in our simulations), the non-linear transition to MRI corresponds, at least close to the threshold S c , to a classical supercritical pitchfork bifurcation: Red curve correspond to periodic boundary conditions in z, whereas the black one corresponds to results obtained with no-slip rotating rings at the endcaps.…”

“…with an angular momentum which increases outward to satisfy Rayleigh stability criterion, but which is MRI unstable (the angular velocity decreases outward). Except where indicated, all simulations reported here use the following rotation rates for the cylinders and rings: (6) where rotation rates have been normalized by Ω 1 . The ratio Ω 2 /Ω 1 ensures a quasi-keplerian regime, whereas Ω 3 and Ω 4 have been carefully chosen to strongly reduce Ekman recirculation and therefore reinforce the hydrodynamical stability of the flow [15].…”

“…The Princeton experiment has been designed to observe this instability in a Taylor-Couette flow of liquid Gallium, with an axial applied magnetic field [5]. So far, MRI has not been identified, but non-axisymmetric modes have been observed when a strong magnetic field is imposed [6]. The PROMISE experiment, in Dresden, is based on a similar set-up, except that the applied field possesses an azimuthal component.…”