AAS 198th Meeting, June 2001
Session 90. Turbulence
Oral, Thursday, June 7, 2001, 2:00-3:30pm, C212-214

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[90.04] Nonlinear Saturation of Dynamo with Large Magnetic Prandtl Number

S. A. Boldyrev (Institute for Theoretical Physics, Santa Barbara), S. C. Cowley (University of California, Los Angeles)

The theory of magnetic dynamo is one of the leading candidates for explaining the galactic magnetic fields. However, for magnetic fields generated in galaxy or protogalaxy, there is a huge gap between the velocity field viscous scale and the resistive scale, which spans from 8 to 12 orders of magnitude. This presents the main difficulty for the theory -- at the initial stage of the generation, the magnetic field energy "leaks" into this gap and is transferred exponentially fast to the smallest (resistive) scales. This seems not to explain the observed large-scale-structured galactic magnetic fields. We argue that this leak can be stopped by the nonlinear reaction of the growing magnetic field on the velocity field well before the magnetic energy hits the resistive scale. When the velocity field inertial interval is very large (about 4 orders of magnitude for the Galaxy), it is this non-resistive nonlinear stage that is responsible for the formation of the inverse cascade of the magnetic energy towards the large scales. To illustrate this, we introduce the so-called restricted dynamical model describing motion and deformation of a fluid element with magnetic field frozen into it and preventing it from the deformation. This consideration is novel and may be effective in explaining the generation of large-scale magnetic fields observed in galaxies.

The author(s) of this abstract have provided an email address for comments about the abstract: boldyrev@itp.ucsb.edu

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