title: Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic-field generation in shear flows.
abstract: The nature of dynamo action in shear flows prone to magnetohydrodynamc instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic magnetorotational dynamo solution to three-dimensional dissipative incompressible magnetohydrodynamic equations with rotation and shear. We discuss the physical mechanism behind the cycle and show that it results from a combination of linear and nonlinear interactions between a large-scale axisymmetric toroidal magnetic field and nonaxisymmetric perturbations amplified by the magnetorotational instability. We demonstrate that this large-scale dynamo mechanism is overall intrinsically nonlinear and not reducible to the standard mean-field dynamo formalism. Our results therefore provide clear evidence for a generic nonlinear generation mechanism of time-dependent coherent large-scale magnetic fields in shear flows and call for new theoretical dynamo models. These findings may offer important clues to understanding the transitional and statistical properties of subcritical magnetorotational turbulence.
authors: Université de Toulouse, UPS-OMP, IRAP, F-31400 Toulouse, France.
authors: J
authors: J
authors: Herault
authors: F
authors: F
authors: Rincon
authors: C
authors: C
authors: Cossu
authors: G
authors: G
authors: Lesur
authors: G I
authors: GI
authors: Ogilvie
authors: P-Y
authors: PY
authors: Longaretti