Intense bipolar structures from stratified helical dynamos

Abstract

We perform direct numerical simulations of the equations of magnetohydrodynamics with external random forcing and in the presence of gravity. The domain is divided into two parts: a lower layer where the forcing is helical and an upper layer where the helicity of the forcing is zero with a smooth transition in between. At early times, a large-scale helical dynamo develops in the bottom layer. At later times the dynamo saturates, but the vertical magnetic field continues to develop and rises to form dynamic bipolar structures at the top, which later disappear and reappear. Some of the structures look similar to $\delta$ spots observed in the Sun. This is the first example of magnetic flux concentrations, owing to strong density stratification, from self-consistent dynamo simulations that generate bipolar, super-equipartition strength, magnetic structures whose energy density can exceeds the turbulent kinetic energy by even a factor of ten.

Published in Monthly Notices of Royal Astronomical Society

References in corpus (4)

• The case for a distributed solar dynamo shaped by near-surface shear
• 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
• Magnetic fluctuations and formation of large-scale inhomogeneous magnetic structures in a turbulent convection

Cited by in corpus (6)

• Flare-productive active regions
• Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars
• Magnetic concentrations in stratified turbulence: the negative effective magnetic pressure instability
• Explaining the Observed Relation Between Stellar Activity and Rotation
• Bipolar magnetic spots from dynamos in stratified spherical shell turbulence
• Analytic solution of an oscillatory migratory alpha^2 stellar dynamo