Magnetic fluctuations and formation of large-scale inhomogeneous magnetic structures in a turbulent convection

TL;DR

This paper investigates how magnetic fluctuations in turbulent convection influence large-scale magnetic structure formation, revealing a threshold magnetic field strength and a mechanism for inhomogeneous magnetic field development relevant to solar phenomena.

Contribution

It demonstrates the dependence of magnetic fluctuation energy on magnetic Reynolds number and identifies a large-scale instability driven by turbulent convection that forms inhomogeneous magnetic structures.

Findings

Magnetic fluctuation energy depends on magnetic Reynolds number below a threshold.

Large-scale inhomogeneous magnetic structures can form even in initially uniform fields.

The instability energy source is small-scale turbulent convection.

Abstract

Magnetic fluctuations generated by a tangling of the mean magnetic field by velocity fluctuations are studied in a developed turbulent convection with large magnetic Reynolds numbers. We show that the energy of magnetic fluctuations depends on magnetic Reynolds number only when the mean magnetic field is smaller than $B_{eq} / 4 Rm^{1/4}$, where $B_{eq}$ is the equipartition mean magnetic field determined by the turbulent kinetic energy and Rm is magnetic Reynolds number. Generation of magnetic fluctuations in a turbulent convection with a nonzero mean magnetic field results in a decrease of the total turbulent pressure and may cause formation of the large-scale inhomogeneous magnetic structures even in an originally uniform mean magnetic field. This effect is caused by a negative contribution of the turbulent convection to the effective mean Lorentz force. The inhomogeneous large-scale magnetic fields are formed due to the excitation of the large-scale instability. The energy for this instability is supplied by the small-scale turbulent convection. The discussed effects might be useful for understanding the origin of the solar nonuniform magnetic fields, e.g., sunspots.