Magnetic field evolution in high and low $\beta$ disks with initially-toroidal fields

TL;DR

This study uses high-resolution 3D MHD simulations to explore how initial toroidal magnetic field strength influences the evolution and poloidal field generation in accretion disks, revealing persistent effects of initial conditions.

Contribution

It demonstrates that initial magnetic field strength significantly impacts disk evolution and poloidal field formation, with long-lasting effects observed in high-resolution simulations.

Findings

Simulations diverge early and remain distinct over time.

Both simulations produce poloidal fields from toroidal initial fields.

Weak field simulation shows potential artifacts from numerical effects.

Abstract

We present results from a pair of high resolution, long timescale ($\sim10^5 GM/c^3$), global, three dimensional magnetohydrodynamical accretion disk simulations with differing initial magnetic plasma $\beta$ in order to study the effects of initial toroidal field strength on production of large-scale poloidal field. We initialize our disks in approximate equilibrium with purely toroidal magnetic fields of strength $\beta_0=5$ and $\beta_0=200$. We also perform a limited resolution study. We find that simulations of differing field strength diverge early in their evolution and remain distinct over the time studied, indicating that initial magnetic conditions leave a persistent imprint in our simulations. Neither simulation enters the Magnetically Arrested Disk (MAD) regime. Both simulations are able to produce poloidal fields from initially-toroidal fields, with the $\beta_0=5$ simulation evolving clear signs of a large-scale poloidal field. We make a cautionary note that computational artifacts in the form of large-scale vortices may be introduced in the combination of initially-weak field and disk-internal mesh refinement boundaries, as evidenced by the production of an $m=1$ mode overdensity in the weak field simulation. Our results demonstrate that the initial toroidal field strength plays a vital role in simulated disk evolution for the models studied.