Periodic and aperiodic dynamo waves |
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Authors: | N O Weiss F Cattaneo C A Jones |
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Institution: | 1. Department of Applied Mathematics and Theoretical Physics , University of Cambridge , England , CB3 9EW;2. School of Mathematics, University of Newcastle-upon-Tyne , England , NE1 7RU |
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Abstract: | Abstract In order to show that aperiodic magnetic cycles, with Maunder minima, can occur naturally in nonlinear hydromagnetic dynamos, we have investigated a simple nonlinear model of an oscillatory stellar dynamo. The parametrized mean field equations in plane geometry have a Hopf bifurcation when the dynamo number D=1, leading to Parker's dynamo waves. Including the nonlinear interaction between the magnetic field and the velocity shear results in a system of seven coupled nonlinear differential equations. For D>1 there is an exact nonlinear solution, corresponding to periodic dynamo waves. In the regime described by a fifth order system of equations this solution remains stable for all D and the velocity shear is progressively reduced by the Lorentz force. In a regime described by a sixth order system, the solution becomes unstable and successive transitions lead to chaotic behaviour. Oscillations are aperiodic and modulated to give episodes of reduced activity. |
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