Quenching and anisotropy of hydromagnetic turbulent transport

Nordita Astrophysics seminars

Wednesday 06 August 2014
from 13:30
to 14:15 at
122:026

Speaker :

Bidya Binay Karak (Nordita)

Abstract :

Hydromagnetic turbulence affects the evolution of the large-scale magnetic field through turbulent diffusion and the effect, for example. For stronger fields, these effects are usually suppressed or quenched, and additional anisotropies are introduced. Using different variants of the so-called test-field method, we determine these effects for different flows: forced Roberts flow, forced turbulence, and thermal convection. For turbulent diffusion, we confirm earlier results with quenching proportional to the first power of the mean field. However, turbulent diffusion proportional to the second derivative along the mean magnetic field is quenched much less, especially for larger values of the magnetic Reynolds number. The magnetic field is expressed in terms of the equipartition value based either on the original or the quenched level of turbulence. In the former case, the quenching exponent is generally larger, for example 4 instead of 1.3 for the Roberts flow, 1.5 instead of 1.1 for turbulent diffusion in forced turbulence, and 3 instead of 2 for the alpha effect in rotating convection. Quenching of turbulent pumping follows the same exponents as turbulent diffusion, while the coefficient describing the Omega x J effect follows nearly the same exponent as alpha. We find that the quenched diffusion coefficients in axisymmetric mean field dynamos with dominant toroidal field are the same for poloidal and toroidal fields.