Axel Brandenburg, Koen Kemel, Nathan Kleeorin, Igor Rogachevskii
To understand the basic mechanism of the formation of magnetic flux
concentrations, we determine by direct numerical simulations the turbulence
contributions to the mean magnetic pressure in a strongly stratified isothermal
layer with large plasma beta, where a weak uniform horizontal mean magnetic
field is applied. The negative contribution of turbulence to the effective mean
magnetic pressure is determined for strongly stratified forced turbulence over
a range values of magnetic Reynolds and Prandtl numbers. Small-scale dynamo
action is shown to reduce the negative effect of turbulence on the effective
mean magnetic pressure. However, the turbulence coefficients describing the
negative effective magnetic pressure phenomenon are found to be converged for
magnetic Reynolds numbers between 60 and 600, which is the largest value
considered here. In all these models the turbulent intensity is nearly constant
in height, so the kinetic energy density decreases with height due to the
decrease in density. In a second series of numerical experiments, the turbulent
intensity increases with height such that the turbulent kinetic energy density
is nearly independent of height. Turbulent magnetic diffusivity and turbulent
pumping velocity are determined with the test-field method for both cases. The
vertical profile of the turbulent magnetic diffusivity is found to agree with
what is expected based on simple mixing length expressions. Turbulent pumping
is shown to be down the gradient of turbulent magnetic diffusivity, but it is
twice as large as expected. Corresponding numerical mean-field models are used
to show that a large-scale instability is to be expected in both cases.
View original:
http://arxiv.org/abs/1005.5700
No comments:
Post a Comment