High-speed standard magneto-rotational instability

Abstract

The large Reynolds number asymptotic approximation of the neutral curve of Taylor-Couette flow subject to axial uniform magnetic field is analysed. The flow has been extensively studied since early 90's as the magneto-rotational instability (MRI) occurring in the flow could possibly explain the origin of the instability observed in certain astrophysical objects. Elsewhere the ideal approximation has been used to study high-speed flows, whilst it sometimes produces paradoxical results. For example, ideal flows must be completely stabilised for strong enough applied magnetic field, but on the other hand the vanishing magnetic Prandtl number limit of the stability should be purely hydrodynamic so instability must occur when Rayleigh's stability condition is violated. The first our discovery is that this apparent contradiction can be resolved by showing the abrupt appearance of the hydrodynamic instability at certain critical value of magnetic Prandtl number, which can be found by asymptotically large Reynolds number limit but with long enough wavelength to retain some diffusive effects. The second our finding concerns so-called Velikhov-Chandrasekhar paradox, namely the mismatch of the zero external magnetic field limit of the Velikhov-Chandrasekhar stability criterion and Rayleigh's stability criterion. We show for fully wide gap cases that the high Reynolds number asymptotic analysis of the MRI naturally yields the simple stability condition that describes smooth transition from Rayleigh to Velikhov-Chandrasekhar stability criteria with increasing Lundquist number.