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Bypassing Cowling's theorem in axisymmetric fluid dynamos
Christophe Gissinger1, Emmanuel Dormy, Stephan Fauve
1Laboratoire de Physique Statistique de l'Ecole Normale Supérieure, CNRS UMR 8550, 24 Rue Lhomond, 75231 Paris Cedex 05, France.
Physical Review Letters
|October 15, 2008
Summary
This study shows how a magnetic field can form in a spherical domain, even without complex flow variations. An axisymmetric dipolar magnetic field is generated from an equatorial dipole through a secondary bifurcation.
Area of Science:
- Geophysics
- Astrophysics
- Fluid Dynamics
Background:
- Understanding the generation of magnetic fields in celestial bodies is crucial.
- Dynamo theory explains how fluid motion can sustain magnetic fields.
- Axisymmetric flows are fundamental in many astrophysical contexts.
Purpose of the Study:
- To numerically investigate magnetic field generation in a spherical domain with axisymmetric forcing.
- To explore the possibility of generating a mean magnetic field without nonaxisymmetric velocity fluctuations.
- To analyze the dynamics of interacting dipolar modes.
Main Methods:
- Numerical simulation of magnetohydrodynamics in a spherical shell.
- Analysis of bifurcations in a forced flow system.
- Derivation of simplified amplitude equations from symmetry arguments.
Main Results:
- A mean magnetic field with a dominant axisymmetric dipolar component is generated.
- This generation occurs via a secondary bifurcation from an equatorial dipole.
- The absence of nonaxisymmetric velocity fluctuations does not preclude field generation.
Conclusions:
- Axisymmetric flows alone can sustain a significant magnetic field.
- The interaction between equatorial and axial dipolar modes governs the field dynamics.
- Symmetry arguments provide insights into the amplitude evolution of these modes.
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