Tuesday, December 18, 2012

1212.4077 (I. R. Losada et al.)

Competition of rotation and stratification in flux concentrations    [PDF]

I. R. Losada, A. Brandenburg, N. Kleeorin, I. Rogachevskii
In a strongly stratified turbulent layer, a uniform horizontal magnetic field can become unstable to form spontaneously local flux concentrations due to a negative contribution of turbulence to the large-scale (mean-field) magnetic pressure. This mechanism is of interest in connection with dynamo scenarios in which most of the magnetic field resides in the bulk of the convection zone, and not at the bottom, as is usually assumed. Recent work using the mean-field hydromagnetic equations has shown that this negative effective magnetic pressure instability (NEMPI) becomes suppressed at rather low rotation rates with Coriolis numbers as low as 0.1. Here we extend these earlier investigations by studying the effects of rotation both on the development of NEMPI and on the effective magnetic pressure (turbulent and non-turbulent contributions). We quantify the kinetic helicity resulting from rotation and stratification and compare with earlier work at smaller scale-separation ratios. We also determine the sensitivity of surface diagnostics of magnetic helicity. We use direct numerical simulations (DNS) and mean-field calculations of the three-dimensional hydromagnetic equations in a Cartesian domain and analytical studies using the $\tau$ approach. We find that the growth rates of NEMPI in earlier mean-field calculations are well reproduced with DNS and that the rotational effect on the effective magnetic pressure is negligible as long as the production of flux concentrations is not inhibited by rotation. In that case, kinetic and magnetic helicity are also found to be weak. Production of magnetic flux concentrations through the suppression of turbulent pressure appears to be possible only in the upper-most layers of the Sun, where the convective turnover time is less than 2 hours.
View original: http://arxiv.org/abs/1212.4077

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