P. J. Käpylä, A. Brandenburg, N. Kleeorin, M. J. Mantere, I. Rogachevskii
We investigate the effects of weakly and strongly stratified turbulent
convection on the mean effective Lorentz force, and especially on the mean
effective magnetic pressure. Earlier studies with isotropically forced
non-stratified and stratified turbulence have shown that the contribution of
the turbulence to the mean magnetic pressure is negative for mean horizontal
magnetic fields that are smaller than the equipartition strength, so that the
effective mean magnetic pressure that takes into account the turbulence
effects, can be negative. Compared with earlier cases of forced turbulence with
an isothermal equation of state, we find that the turbulence effect is similar
to or even stronger in the present case of turbulent convection. This is argued
to be due to the anisotropy of turbulence in the vertical direction. Another
important difference compared with earlier studies is the presence of an
evolution equation for the specific entropy. Mean-field modelling with entropy
evolution indicates that the negative effective magnetic pressure can still
lead to a large-scale instability which forms local flux concentrations, even
though the specific entropy evolution tends to have a stabilizing effect when
applied to a stably stratified (e.g., isothermal) layer. It is argued that this
large-scale instability could be important for the formation of solar
large-scale magnetic structures such as active regions.
View original:
http://arxiv.org/abs/1104.4541
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