Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows

Details Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows

2011-09-08

arxiv.org/abs/1109.1811v1

Astronomy and Astrophysics

Astrophysics

Solar and Stellar Astrophysics

10 pages, 6 figures, accepted for publication in Physical Review E

Scientific paper

The nature of dynamo action in shear flows prone to magnetohydrodynamic 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 the 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 non-axisymmetric 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 understand the transitional and statistical properties of subcritical magnetorotational turbulence.

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