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A. Schekochihin, J. Maron, M. Opher, S. Cowley (UCLA)
A weak magnetic field passively advected by a turbulent velocity field grows exponentially while its characteristic scale decays. In the interstellar medium and protogalactic plasmas, the magnetic Prandtl number is very large. and the kinematic stage of magnetic dynamo therefore produces a broad spectrum of magnetic fluctuations on small (subviscous) scales. The distribution of the field stregth in the kinematic regime is lognormal (highly intermittent). A study of statistical correlations that are set up in the field pattern shows that the magnetic field lines possess a folding structure, where most of the characteristic-scale decrease is due to the field variation across itself (rapid transverse direction reversals), while the scale of the field variation along itself stays approximately constant. The field structure determines the conditions under which the nonlinear effects set in. We find that the advent of Lorentz back reaction leads to saturation of the magnetic energy and a substatial suppression of the intermittency of the field distribution. The folding pattern persists into the nonlinear stage, but the decrease of the magnetic-field scales is arrested. Our findings derive from the statistical theory of the small-scale magnetic fluctuations in the viscosity-dominated regime and are corroborated by an array of numerical simulations.
This work was supported by the NSF~Grant~No.~AST~97-13241 and the DOE~Grant~No.~DE-FG03-93ER54~224.
The author(s) of this abstract have provided an email address for comments about the abstract: sure@physics.ucla.edu