Multi-Parametric Study of Rising 3D Buoyant Flux Tubes in an Adiabatic Stratification Using AMR

TL;DR

This study uses high-resolution 3D MHD simulations with AMR to analyze how magnetic flux tubes rise in an adiabatic environment, focusing on the effects of twist, curvature, and diffusion regimes on their evolution.

Contribution

It demonstrates the importance of low diffusivity and magnetic twist in preventing flux tube splitting and highlights the role of vorticity and vortex shedding in flux tube dynamics.

Findings

Higher flux retention with increased curvature in high-diffusivity regimes.

Magnetic twist prevents vortex formation and flux splitting in low-diffusivity regimes.

Flux tubes exhibit oscillatory trajectories with vortex shedding patterns.

Abstract

We study the buoyant rise of magnetic flux tubes embedded in an adiabatic stratification using two-and three-dimensional, MHD simulations. We analyze the dependence of the tube evolution on the field line twist and on the curvature of the tube axis in different diffusion regimes. To be able to achieve a comparatively high spatial resolution we use the FLASH code, which has a built-in Adaptive Mesh Refinement (AMR) capability. Our 3D experiments reach Reynolds numbers that permit a reasonable comparison of the results with those of previous 2D simulations. When the experiments are run without AMR, hence with a comparatively large diffusivity, the amount of longitudinal magnetic flux retained inside the tube increases with the curvature of the tube axis. However, when a low-diffusion regime is reached by using the AMR algorithms, the magnetic twist is able to prevent the splitting of the magnetic loop into vortex tubes and the loop curvature does not play any significant role. We detect the generation of vorticity in the main body of the tube of opposite sign on the opposite sides of the apex. This is a consequence of the inhomogeneity of the azimuthal component of the field on the flux surfaces. The lift force associated with this global vorticity makes the flanks of the tube move away from their initial vertical plane in an antisymmetric fashion. The trajectories have an oscillatory motion superimposed, due to the shedding of vortex rolls to the wake, which creates a Von Karman street.