Physics – Plasma Physics
Scientific paper
Oct 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aps..dppum1007s&link_type=abstract
American Physical Society, 43rd Annual Meeting of the APS Division of Plasma Physics October 29 - November 2, 2001 Long Beach, C
Physics
Plasma Physics
Scientific paper
Explaining the origin of the spacially coherent large-scale magnetic fields of galaxies represents one of the main challenges of the astrophysical dynamo theory. A weak seed magnetic field embedded in a turbulent plasma grows while its characteristic scale decays exponentially fast at the eddy-turnover rate of the turbulence. The field distribution is lognormally intermittent. In the interstellar medium and protogalactic plasmas, the magnetic Prandtl number is very large, so the kinematic dynamo produces a broad spectrum of magnetic fluctuations at small (subviscous) scales. These small-scale fields are a major factor and a major difficulty in the galactic dynamo theory. Some form of nonlinear magnetic-energy transfer from small to large scales must take place to generate the observed galactic field. The condition for the onset of nonlinear effects depends on the strength and structure of the field. We show that the magnetic field lines are folded in such a way that the small-scale nature of the field is due to rapid transverse direction reversals, while the variation of the field along itself stays at the velocity scale. Therefore the Lorentz tension becomes important when the magnetic energy approaches that of the smallest turbulent eddies. The nonlinear back reaction leads to saturation of the magnetic energy and a substatial suppression of the intermittency of the field distribution. The exponential decrease of the magnetic-field scale is slowed down and continues at the resistive time scale. The folding pattern persists into the nonlinear stage. No inverse cascade of magnetic energy appears possible in this regime. Our findings derive from a statistical theory of the small-scale magnetic fluctuations in the viscosity-dominated regime and are corroborated by an array of numerical simulations.
Cowley Steven
Malyshkin Leonid
Maron Jason
McWilliams James
Schekochihin Alexander A.
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