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Large- and small-scale interactions and quenching in an alpha2-dynamo
Peter Frick1, Rodion Stepanov, Dmitry Sokoloff
1Institute of Continuous Media Mechanics, 1, Korolev, Perm, 614013, Russia.
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|February 7, 2007
Summary
This study shows that low magnetic Prandtl numbers (Pm) can enhance large-scale magnetic field generation in turbulent dynamos, even suppressing small-scale dynamos. Efficient dynamo action occurs when Pm is low, potentially leading to superequipartition of magnetic energy.
Area of Science:
- * Astrophysics and plasma physics
- * Magnetohydrodynamics (MHD) and turbulence
Background:
- * Investigates the generation and evolution of large-scale magnetic fields in turbulent conducting fluids.
- * Utilizes a multiscale alpha2-dynamo model combining mean-field theory and shell formalism for turbulence.
Purpose of the Study:
- * To analyze the alpha quenching and the influence of the magnetic Prandtl number (Pm) on dynamo action.
- * To understand the dynamic saturation mechanisms of large-scale magnetic field generation.
- * To explore the relationship between alpha effect, magnetic field energy, and time lags.
Main Methods:
- * Developed a multiscale alpha2-dynamo model incorporating poloidal and toroidal magnetic field components.
- * Employed a shell model for small-scale magnetohydrodynamical turbulence.
- * Ensured strict adherence to conservation laws in model conjugation.
Main Results:
- * Demonstrated a significant magnetic contribution to the alpha effect, capable of generating large-scale fields via current helicity alone.
- * Found negligible simultaneous cross-correlation between alpha and large-scale magnetic field energy (EB), but substantial coupling at moderate time lags.
- * Revealed that decreasing Pm does not hinder magnetic field generation; low Pm values favor efficient large-scale dynamos, suppressing small-scale ones and potentially leading to EB > Eu (superequipartition).
- * Observed that increasing Pm does not enhance large-scale dynamo action, as the magnetic alpha effect increasingly counteracts it, reducing saturated mean magnetic energy.
Conclusions:
- * The saturation mechanism of dynamo action is dynamic and influenced by time lags.
- * Optimal large-scale dynamo operation occurs at low magnetic Prandtl numbers, contrary to common assumptions.
- * Superequipartition of large-scale magnetic fields is achievable at low Pm, highlighting a complex interplay between scales and parameters.
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