Buoyant Magnetic Loops in a Global Dynamo Simulation of a Young Sun

TL;DR

This paper presents the first 3-D global MHD simulation where turbulent convection and rotation generate buoyant magnetic loops, advancing understanding of magnetic flux emergence in sun-like stars.

Contribution

It demonstrates a self-consistent dynamo model producing buoyant magnetic loops in a global simulation with reduced turbulence diffusion.

Findings

Magnetic wreaths form in the convection zone due to dynamo action.

Buoyant magnetic loops rise through the convection zone via buoyancy and advection.

Simulation achieves reduced diffusion enabling flux emergence phenomena.

Abstract

The current dynamo paradigm for the Sun and sun-like stars places the generation site for strong toroidal magnetic structures deep in the solar interior. Sunspots and star-spots on sun-like stars are believed to arise when sections of these magnetic structures become buoyantly unstable and rise from the deep interior to the photosphere. Here we present the first 3-D global magnetohydrodynamic (MHD) simulation in which turbulent convection, stratification, and rotation combine to yield a dynamo that self-consistently generates buoyant magnetic loops. We simulate stellar convection and dynamo action in a spherical shell with solar stratification, but rotating three times faster than the current solar rate. Strong wreaths of toroidal magnetic field are realized by dynamo action in the convection zone. By turning to a dynamic Smagorinsky model for subgrid-scale turbulence, we here attain considerably reduced diffusion in our simulation. This permits the regions of strongest magnetic field in these wreaths to rise toward the top of the convection zone via a combination of magnetic buoyancy instabilities and advection by convective giant cells. Such a global simulation yielding buoyant loops represents a significant step forward in combining numerical models of dynamo action and flux emergence.