Large- and small-scale interactions and quenching in an α2 -dynamo

Details Large- and small-scale interactions and quenching in an α2 -dynamo Large- and small-scale interactions and quenching in an α2 -dynamo

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006phrve..74f6310f&link_type=abstract

Physical Review E, vol. 74, Issue 6, id. 066310

Physics

9

Turbulence Simulation And Modeling, Magnetohydrodynamics, Magnetohydrodynamics And Plasmas, Origins And Models Of The Magnetic Field, Dynamo Theories

Scientific paper

The evolution of the large-scale magnetic field in a turbulent flow of conducting fluid is considered in the framework of a multiscale α2 -dynamo model, which includes the poloidal and the toroidal components for the large-scale magnetic field and a shell model for the small-scale magnetohydrodynamical turbulence. The conjugation of the mean-field description for the large-scale field and the shell formalism for the small-scale turbulence is based on strict conformity to the conservation laws. The model displays a substantial magnetic contribution to the α effect. It was shown that a large-scale magnetic field can be generated by current helicity even solely. The α quenching and the role of the magnetic Prandtl number (Pm) are studied. We have determined the dynamic nature of the saturation mechanism of dynamo action. Any simultaneous cross correlation of α and large-scale magnetic field energy EB is negligible, whereas coupling between α and EB becomes substantial for moderate time lags. An unexpected result is the behavior of the large-scale magnetic energy with variation of the magnetic Prandtl number. Diminishing of Pm does not have an inevitable ill effect on the magnetic field generation. The most efficient large-scale dynamo operates under relatively low Prandtl numbers—then the small-scale dynamo is suppressed and the decrease of Pm can lead even to superequipartition of the large-scale magnetic field (i.e., EB>Eu ). In contrast, the growth of Pm does not promote the large-scale magnetic field generation. A growing counteraction of the magnetic α effect reduces the level of mean large-scale magnetic energy at the saturated state.

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