Bipolar region formation in stratified two-layer turbulence

Abstract

This work presents an extensive study of the previously discovered formation of bipolar flux concentrations in a two-layer model. We interpret the formation process in terms of negative effective magnetic pressure instability (NEMPI), which is a possible mechanism to explain the origin of sunspots. In our simulations, we use a Cartesian domain of isothermal stratified gas that is divided into two layers. In the lower layer, turbulence is forced with transverse nonhelical random waves, whereas in the upper layer no flow is induced. A weak uniform magnetic field is imposed in the entire domain at all times. In this study we vary the stratification by changing the gravitational acceleration, magnetic Reynolds number, strength of the imposed magnetic field, and size of the domain to investigate their influence on the formation process. Bipolar magnetic structure formation takes place over a large range of parameters. The magnetic structures become more intense for higher stratification until the density contrast becomes around $100$ across the turbulent layer. For the Reynolds numbers considered, magnetic flux concentrations are generated at magnetic Prandtl number between 0.1 and 1. The magnetic field in bipolar regions increases with higher imposed field strength until the field becomes comparable to the equipartition field strength of the turbulence. A larger horizontal extent enables the flux concentrations to become stronger and more coherent. The size of the bipolar structures turns out to be independent of the domain size. In the case of bipolar region formation, we find an exponential growth of the large-scale magnetic field, which is indicative of a hydromagnetic instability. Additionally, the flux concentrations are correlated with strong large-scale downward and converging flows. These findings imply that NEMPI is responsible for magnetic flux concentrations.

15 pages, 13 figures, Astronomy and Astrophysics, published

References in corpus (16)

• The case for a distributed solar dynamo shaped by near-surface shear
• Numerical simulations of quiet Sun magnetism: On the contribution from a small-scale dynamo
• A simulation of convective dynamo in the solar convective envelope: maintenance of the solar-like differential rotation and emerging flux
• On the Formation of Active Regions
• Multiple dynamo modes as a mechanism for long-term solar activity variations
• Magnetic fluctuations and formation of large-scale inhomogeneous magnetic structures in a turbulent convection
• On the cause of solar-like equatorward migration in global convective dynamo simulations
• Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars
• Helioseismology of Pre-Emerging Active Regions II: Average Emergence Properties
• The radial gradient of the near-surface shear layer of the Sun
• Suppression of small scale dynamo action by an imposed magnetic field
• Intense bipolar structures from stratified helical dynamos
• Magnetic flux concentrations from turbulent stratified convection
• Surface appearance of dynamo-generated large-scale fields
• Magnetic twist: a source and property of space weather
• Bipolar magnetic spots from dynamos in stratified spherical shell turbulence

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• Magnetic concentrations in stratified turbulence: the negative effective magnetic pressure instability
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• The dynamics of Wolf numbers based on nonlinear dynamo with magnetic helicity: comparisons with observations
• The origin and early evolution of a bipolar magnetic region in the solar photosphere
• Development of active regions: flows, magnetic-field patterns and bordering effect
• The mean tilt of sunspot bipolar regions: theory, simulations and comparison with observations
• Current systems of coronal loops in 3D MHD simulations
• Spontaneous Formation of Surface Magnetic Structure from Large-scale Dynamo in Strongly-stratified Convection
• Nonlinear mean-field dynamo and prediction of solar activity
• Large Scale Properties of Tilt of Sunspot Groups and Joy's Law Near The Solar Equator
• Magnetic fields of low-mass main sequences stars: Nonlinear dynamo theory and mean-field numerical simulations
• Magnetic bipoles in rotating turbulence with coronal envelope