Flux emergence in a magnetized convection zone

TL;DR

This study uses 3D MHD simulations to explore how a dynamo magnetic field affects the buoyant rise and emergence of twisted flux-ropes in a convection zone, revealing their impact on global magnetic topology and polarity.

Contribution

It provides new insights into the global effects of dynamo fields on flux emergence, including influences on flux-ropes' dynamics, surface signatures, and system polarity.

Findings

Flux-ropes' magnetic field strength scales with density as rho^a, with a less than or approximately 1.

Flux emergence can cause polarity reversals in the global magnetic field.

Emerged flux is advected poleward and interacts with the background dynamo field.

Abstract

We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux-ropes, and their influence on the global external magnetic field. We ran 3D MHD numerical simulations using the ASH code and analysed the dynamical evolution of such buoyant flux-ropes from the bottom of the convection zone until the post-emergence phases. The global nature of this model represents very crudely and inaccurately the local dynamics of the buoyant rise, but allows to study the influence of global effects such as self-consistently generated differential rotation, meridional circulation and Coriolis forces. Although motivated by the solar context, this model cannot be thought of as a realistic model of the rise of magnetic structures and their emergence in the Sun where the local dynamics are completely different. The properties of initial phases of the buoyant rise in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increase as the flux-ropes evolve. During the buoyant rise across the CZ, the flux-rope's magnetic field strength and scales as $rho^a$, with $a\lesssim 1$. An increase of velocity, density and current precedes flux emergence at all longitudes. The geometry, latitude and relative orientation of the flux-ropes with respect to the background magnetic field influences the rise speeds, zonal flow amplitudes (which develop within the flux-ropes) and the corresponding surface signatures. This influences the morphology, duration and amplitude of the associated surface shearing and Poynting flux. The emerged flux influences the system's global polarity, leading in some cases to a polarity reversal while inhibiting background dynamo from doing so in some others. The emerged magnetic flux is slowly advected poleward, while being diffused and assimilated by the background dynamo field.