Dynamo-driven plasmoid ejections above a spherical surface

TL;DR

This study models turbulent dynamos in spherical geometry to investigate flux emergence and plasmoid ejections, revealing regular ejection intervals and magnetic helicity sign changes akin to solar phenomena.

Contribution

It extends dynamo models to spherical geometry with an exterior, demonstrating flux emergence and plasmoid ejections without magnetic buoyancy influence.

Findings

Plasmoid ejections occur regularly in the model.

Oscillatory dynamo with equatorward migration is observed.

Magnetic helicity changes sign outside the turbulence zone.

Abstract

We extend earlier models of turbulent dynamos with an upper, nearly force-free exterior to spherical geometry, and study how flux emerges from lower layers to the upper ones without being driven by magnetic buoyancy. We also study how this affects the possibility of plasmoid ejection. A spherical wedge is used that includes northern and southern hemispheres up to mid-latitudes and a certain range in longitude of the Sun. In radius, we cover both the region that corresponds to the convection zone in the Sun and the immediate exterior up to twice the radius of the Sun. Turbulence is driven with a helical forcing function in the interior, where the sign changes at the equator between the two hemispheres. An oscillatory large-scale dynamo with equatorward migration is found to operate in the turbulence zone. Plasmoid ejections occur in regular intervals, similar to what is seen in earlier Cartesian models. These plasmoid ejections are tentatively associated with coronal mass ejections. The magnetic helicity is found to change sign outside the turbulence zone, which is in agreement with recent findings for the solar wind.