Transport of Large Scale Poloidal Flux in Black Hole Accretion

TL;DR

This study uses 3D GRMHD simulations to show that magnetic flux in black hole accretion disks is primarily transported via a global coronal mechanism outside the disk, challenging local flux transport models.

Contribution

It reveals the dominance of a coronal magnetic flux transport mechanism and questions previous local models based on effective viscosity and resistivity.

Findings

Coronal mechanism drives magnetic flux evolution outside the disk.

Large-scale magnetic field structures are formed by flux reconnection events.

Flux transport is largely independent of direct accretion within the disk.

Abstract

We report on a global, three-dimensional GRMHD simulation of an accretion torus embedded in a large scale vertical magnetic field orbiting a Schwarzschild black hole. This simulation investigates how a large scale vertical field evolves within a turbulent accretion disk and whether global magnetic field configurations suitable for launching jets and winds can develop. We find that a "coronal mechanism" of magnetic flux motion, which operates largely outside the disk body, dominates global flux evolution. In this mechanism, magnetic stresses driven by orbital shear create large-scale half-loops of magnetic field that stretch radially inward and then reconnect, leading to discontinuous jumps in the location of magnetic flux. In contrast, little or no flux is brought in directly by accretion within the disk itself. The coronal mechanism establishes a dipole magnetic field in the evacuated funnel around the orbital axis with a field intensity regulated by a combination of the magnetic and gas pressures in the inner disk. These results prompt a reevaluation of previous descriptions of magnetic flux motion associated with accretion. Local pictures are undercut by the intrinsically global character of magnetic flux. Formulations in terms of an "effective viscosity" competing with an "effective resistivity" are undermined by the nonlinearity of of the magnetic dynamics and the fact that the same turbulence driving mass motion (traditionally identified as "viscosity") can alter magnetic topology.