Competition of rotation and stratification in flux concentrations

Abstract

In a strongly stratified turbulent layer, a uniform horizontal magnetic field can become unstable to spontaneously form local flux concentrations due to a negative contribution of turbulence to the large-scale (mean-field) magnetic pressure. This mechanism, called the negative effective magnetic pressure instability (NEMPI), is of interest in connection with dynamo scenarios where most of the magnetic field resides in the bulk of the convection zone, and not at the bottom. Recent work using the mean-field hydromagnetic equations has shown that 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. We also quantify the kinetic helicity from direct numerical simulations (DNS) and compare with earlier work.}{To calculate the rotational effect on the effective magnetic pressure we consider both DNS and analytical studies using the $\tau$ approach. To study the effects of rotation on the development of NEMPI we use both DNS and mean-field calculations of the 3D hydromagnetic equations in a Cartesian domain.}{We find that the growth rates of NEMPI from earlier mean-field calculations are well reproduced with DNS, provided the Coriolis number is below about 0.06. In that case, kinetic and magnetic helicities are found to be weak. For faster rotation, dynamo action becomes possible. However, there is an intermediate range of rotation rates where dynamo action on its own is not yet possible, but the rotational suppression of NEMPI is being alleviated.}{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.}

13 pages, 13 figures submitted to A&A

References in corpus (6)

• Minimal tau approximation and simulations of the alpha effect
• Magnetic fluctuations and formation of large-scale inhomogeneous magnetic structures in a turbulent convection
• New scaling for the alpha effect in slowly rotating turbulence
• Magnetic twist: a source and property of space weather
• Simulations of the anisotropic kinetic and magnetic alpha effects
• Rotational effects on the negative magnetic pressure instability

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• The dynamics of Wolf numbers based on nonlinear dynamo with magnetic helicity: comparisons with observations
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• The turbulent pressure of magnetoconvection for slow and rapid rotation
• Spontaneous flux concentrations from the negative effective magnetic pressure instability beneath a radiative stellar surface
• Magnetic bipoles in rotating turbulence with coronal envelope