Three dimensional non-linear evolution of a magnetic flux tube in a spherical shell: Influence of turbulent convection and associated mean flows

TL;DR

This study uses 3D MHD simulations in spherical geometry to explore how magnetic flux tubes evolve non-linearly within turbulent convection zones, highlighting the effects of initial field strength, twist, and large-scale flows on flux emergence.

Contribution

First 3D MHD simulation of magnetic flux tube evolution in spherical geometry including self-consistent mean flows and turbulent convection, revealing new insights into flux emergence dynamics.

Findings

Strong flux tubes rise radially regardless of latitude.

Weaker flux tubes are influenced by convective flows, favoring Omega-loop emergence.

Differential rotation hampers low-latitude flux tube ascent.

Abstract

We present the first 3D MHD study in spherical geometry of the non-linear dynamical evolution of magnetic flux tubes in a turbulent rotating convection zone. We study numerically the rise of magnetic toroidal flux ropes from the base of a modelled convection zone up to the top of our computational domain where bipolar patches are formed. We compare the dynamical behaviour of flux tubes in a fully convective shell possessing self-consistently generated mean flows such as meridional circulation and differential rotation, with reference calculations done in a quiet isentropic zone. We find that two parameters influence the tubes during their rise through the convection zone: the initial field strength and amount of twist, thus confirming previous findings in Cartesian geometry. Further, when the tube is sufficiently strong with respect to the equipartition field, it rises almost radially independently of the initial latitude (either low or high). By contrast, weaker field cases indicate that downflows and upflows control the rising velocity of particular regions of the rope and could in principle favour the emergence of flux through Omega-loop structures. For these latter cases, we focus on the orientation of bipolar patches and find that sufficiently arched structures are able to create bipolar regions with a predominantly East-West orientation. Meridional flow seems to determine the trajectory of the magnetic rope when the field strength has been significantly reduced near the top of the domain. Finally differential rotation makes it more difficult for tubes introduced at low latitudes to reach the top of the domain.