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Alpha-effect dynamos with zero kinetic helicity.

Karl-Heinz Rädler1, Axel Brandenburg

  • 1Astrophysical Institute Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany.

Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|March 21, 2008
PubMed
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This study presents a Roberts-type dynamo example demonstrating that the alpha effect can exist without kinetic helicity. This finding challenges the necessity of kinetic helicity for alpha-effect dynamos, impacting astrophysical dynamo theory.

Area of Science:

  • * Astrophysics
  • * Fluid Dynamics
  • * Plasma Physics

Background:

  • * Mean-field electrodynamics is crucial for understanding cosmic magnetic fields.
  • * The alpha effect is a key mechanism in dynamo theory, often linked to fluid helicity.
  • * Previous models suggested kinetic helicity is essential for the alpha effect.

Purpose of the Study:

  • * To provide an explicit example of a Roberts-type dynamo.
  • * To demonstrate that the alpha effect can occur without kinetic helicity.
  • * To establish a mean-field theory for Roberts-type dynamos and verify findings numerically.

Main Methods:

  • * Construction of a simple, explicit Roberts-type dynamo model.
  • * Development of a mean-field theory using the second-order correlation approximation.

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  • * Numerical solutions of the original dynamo equations without approximations.

    • Main Results:

    • * An explicit example shows the alpha effect existing despite pointwise vanishing kinetic helicity.
    • * Theoretical and numerical results confirm dynamo action is possible without kinetic helicity.
    • * The study validates that nonzero kinetic helicity is not a prerequisite for alpha-effect dynamos.

    Conclusions:

    • * The alpha effect in mean-field electrodynamics does not strictly require nonzero kinetic helicity.
    • * Dynamo action is possible even when fluid flow has zero kinetic helicity.
    • * This work broadens the understanding of magnetic field generation in astrophysical bodies.