Surface appearance of dynamo-generated large-scale fields

TL;DR

This study investigates how large-scale magnetic fields generated by a turbulent dynamo appear above the surface without magnetic buoyancy, revealing recurrent plasmoid ejections and arcade-like structures.

Contribution

It demonstrates the emergence of twisted magnetic structures and plasmoid ejections in a dynamo-driven magnetic field without buoyancy effects.

Findings

Twisted arcade-like magnetic structures emerge above the turbulence zone.

Recurrent plasmoid ejections are observed in the exterior.

The exterior magnetic field is highly force-free, reaching up to 95% force-free ratio.

Abstract

Twisted magnetic fields are frequently seen to emerge above the visible surface of the Sun. This emergence is usually associated with the rise of buoyant magnetic flux structures. Here we ask how magnetic fields from a turbulent large-scale dynamo appear above the surface if there is no magnetic buoyancy. The computational domain is split into two parts. In the lower part, which we refer to as the turbulence zone, the flow is driven by an assumed helical forcing function leading to dynamo action. Above this region, which we refer to as the exterior, a nearly force-free magnetic field is computed at each time step using the stress-and-relax method. Twisted arcade-like field structures are found to emerge in the exterior above the turbulence zone. Strong current sheets tend to form above the neutral line, where the vertical field component vanishes. Time series of the magnetic field structure show recurrent plasmoid ejections. The degree to which the exterior field is force free is estimated as the ratio of the dot product of current density and magnetic field strength to their respective rms values. This ratio reaches values of up to 95% in the exterior. A weak outward flow is driven by the residual Lorentz force.