Computer Science
Scientific paper
Apr 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt.........7c&link_type=abstract
Thesis (PHD). PRINCETON UNIVERSITY , Source DAI-B 57/10, p. 6305, Apr 1997, 137 pages.
Computer Science
4
Magnetohydrodynamics
Scientific paper
This work addresses several questions concerning the origin of the galactic magnetic field. First, the amplification of magnetic energy by turbulent motions in the weak-field limit is examined with the aid of a numerical simulation involving realistically long velocity correlation times. It is found that the growth rate of the magnetic energy is reduced by a factor of about 2 relative to the short-correlation-time approximation of Kulsrud & Anderson (1992). Second, the transition from the weak-field regime to the strong-field regime is also treated. A new form of damping termed 'viscous relaxation' is shown to play an important role in this transition. Viscous relaxation is similar to ambipolar diffusion, in which field lines straighten out under the influence of magnetic forces, except that in viscous relaxation the ions and neutrals move together, while in ambipolar diffusion the ions move more rapidly than the neutrals. Ambipolar diffusion is the dominant damping mechanism when the magnetic field is concentrated at scales smaller than the neutral mean free path, scales at which damping of neutral motions is very effective. Viscous relaxation is the dominant damping mechanism when the field is concentrated at scales larger than the mean free path, scales at which the neutrals motions become larger than the ion-neutral drift. Viscous relaxation places an upper bound on the energy in the magnetic field at scales smaller than the smallest turbulent eddy. This upper bound is R-1/2/ E total, where R is the Reynolds number and E total is the total turbulent kinetic energy. Third, a theory is also presented for the propagation and damping of large-scale elastic waves in the presence of small-scale magnetic fields. This theory supports an argument that reductions of the mean-field-galactic-dynamo growth rate due to small-scale fields cannot depend on the magnetic Reynolds number. Fourth, numerical simulations based on Kraichnan's direct-interaction approximation and Bowman's realizable Markovian closure demonstrate that the RMC and DIA are comparably accurate in simulations of nonlinear dynamos, that the magnetic energy in the protogalaxy grows to six percent of the ambient turbulent kinetic energy, and that the field evolves to scales larger than the smallest eddy size.
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