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A. Schekochihin, S. Cowley, J. Maron (Imperial College), L. Malyshkin (University of Chicago), J. McWilliams (UCLA)
Explaining the origin of the magnetic fields of galaxies represents one of the main tasks of the astrophysical dynamo theory. A weak seed magnetic field embedded in a turbulent plasma grows exponentially while its scale decays. The decay of the field scale is limited by diffusion. In the ISM or protogalaxies, the magnetic Prandtl number is very large, so a broad spectrum of magnetic fluctuations at small (subviscous) scales is produced. These fluctuations are lognormally intermittent and arranged in the form of long folds, so the fine-scale variation of the field is due to transverse spatial oscilations of its direction, while the field lines are approximately straight. It is this field configuration that the nonlinear backreaction acts upon. The growth of the field is saturated, the spectral slope becomes gentler, and an exponential distribution of the field strength is formed, while the folding structure survives. No inverse cascade of the magnetic energy appears possible in this regime. Our findings are based on the statistical theory of viscosity-dominated magnetic turbulence and are corroborated by numerical simulations.
The author(s) of this abstract have provided an email address for comments about the abstract: sure@pppl.gov