Magnetic flux concentrations in a polytropic atmosphere

Abstract

Strongly stratified hydromagnetic turbulence has recently been identified as a candidate for explaining the spontaneous formation of magnetic flux concentrations by the negative effective magnetic pressure instability (NEMPI). Much of this work has been done for isothermal layers for which the density scale height is constant throughout. We now study the validity of earlier conclusions about the size and growth rate of magnetic structures in the case of polytropic layers, which scale height decreases sharply towards the surface. To allow for a continuous transition from isothermal to polytropic layers, we employ a generalization of the exponential function known as the q-exponential. Now, the top of the polytropic layer shifts with the polytropic index such that the scale height at some reference height is always the same. We use both mean-field and direct numerical simulations of forced stratified turbulence to determine the resulting flux concentrations. Magnetic structures begin to form at a depth where magnetic field strength is about 3-4% the local equipartition field strength with respect to the turbulent kinetic energy. Unlike the isothermal case where stronger fields can give rise to magnetic flux concentrations at larger depths, in the polytropic one the growth rates decreases for structures deeper down. For vertical fields, magnetic structures of super-equipartition strengths are formed because such fields survive downward advection, unlike NEMPI under horizontal magnetic fields. The horizontal cross-section of such structures is approximately circular. Results based on isothermal models can be applied locally to polytropic layers. For vertical fields, magnetic flux concentrations of super-equipartition strengths form, which supports suggestions that sunspot formation might be a shallow phenomenon.

12 pages, 17 figures, acepted A&A

References in corpus (8)

• The case for a distributed solar dynamo shaped by near-surface shear
• Kinematic alpha effect in isotropic turbulence simulations
• On the Formation of Active Regions
• Magnetic fluctuations and formation of large-scale inhomogeneous magnetic structures in a turbulent convection
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• Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field
• Magnetic flux concentrations in a polytropic atmosphere

Cited by in corpus (9)

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• Magnetic flux concentrations from turbulent stratified convection
• Magnetic concentrations in stratified turbulence: the negative effective magnetic pressure instability
• Bipolar region formation in stratified two-layer turbulence
• The dynamics of Wolf numbers based on nonlinear dynamo with magnetic helicity: comparisons with observations
• Bipolar magnetic spots from dynamos in stratified spherical shell turbulence
• Magnetic flux concentrations in a polytropic atmosphere
• Spontaneous flux concentrations from the negative effective magnetic pressure instability beneath a radiative stellar surface
• Magnetic bipoles in rotating turbulence with coronal envelope