Abstract
The paper deals with the dynamo action of the Roberts flow, that is, a flow depending periodically on two cartesian coordinates, X and Y , but being independent of the third one, Z . In particular the case is considered in which the magnetic fields, which are periodic in X, Y and Z , have period lengths in the XY -plane being integer multiples of that of the flow. Two approaches are used. Firstly, the equations governing the magnetic field are reduced to a matrix eigenvalue problem, which is solved numerically. Secondly, a mean magnetic field is defined by averaging over proper areas in the XY -plane, corresponding equations are derived, in which the induction effect of the flow occurs as an anisotropic f -effect, and analytic solutions are given. The results are of particular interest for the Karlsruhe dynamo experiment, which works with a Roberts type flow consisting of 52 cells inside a cylindrical volume. In order to check the reliability of predictions concerning self-excitation based on the mean-field approach, analogous predictions are derived for a rectangular box containing 50 cells, and are compared with results obtained with the help of direct solutions of the eigenvalue problem mentioned. It turns out that the simple mean-field approach in general underestimates the requirements for self-excitation. The corresponding results agree with those obtained in the subharmonic approach only if the side length L of the box, its height H and the edge length l of a spin generator satisfy $ L \\gg H \\gg l $ . In Appendix B, some comments on previous results concerning $\\cal {ABC}$ dynamos are made in the light of the subharmonic formalism used in the paper.
Highlights
It is widely believed that the magnetic fields of cosmic bodies are due to dynamo action
We gave analytic solutions of an equation for the mean magnetic field, which can be derived under certain assumptions from the induction equation
Both kinds of solutions have been used for estimates of the self-excitation condition of the Karlsruhe dynamo experiment
Summary
It is widely believed that the magnetic fields of cosmic bodies are due to dynamo action. The fluid motions in the cosmic objects are rather complex it is useful to consider simple steady flow patterns which are periodic with respect to two or three cartesian coordinates In this way we may find some understanding of the basic dynamo mechanisms. Roberts (1972) demonstrated that flows of this kind are capable of dynamo action In his numerical investigations, he considered magnetic fields with the same periodicity in X and Y as the flow pattern. We further consider a mean-field approach to the Roberts dynamo problem, apply it for an estimate of the self-excitation condition in the Karlsruhe experiment and compare this with estimates gained with the help of subharmonic solutions (Section 3).
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