The large-scale magnetic field advected in the corona of a thin accretion disk

TL;DR

This paper demonstrates that a hot corona above a thin accretion disk can efficiently advect external magnetic fields inward, leading to strong magnetic fields capable of launching outflows, which aligns with numerical simulations and observations.

Contribution

It introduces a model where the corona enhances magnetic field advection in thin disks, explaining field amplification and outflow launching.

Findings

Coronal gas advects magnetic flux more efficiently than the thin disk.

Magnetic field strength is amplified in the inner disk region.

Field lines are inclined to facilitate outflow launching.

Abstract

Large-scale magnetic field is believed to play a key role in launching and collimating jets/outflows. It was found that advection of external field by a geometrically thin disk is rather inefficient, while the external weak field may be dragged inwards by fast radially moving tenuous or/and hot gas above the thin disk. We investigate the field advection in a thin (cold) accretion disk covered with hot corona, in which turbulence is responsible for the angular momentum transfer of the gas in the disk and corona. The radial velocity of the gas in the corona is significantly higher than that in the thin disk. Our calculations show that the external magnetic flux is efficiently transported inwards by the corona, and the field line is strongly inclined towards the disk surface, which help launching outflows. The field configurations are consistent with those observed in the numerical simulations. The strength of the field is substantially enhanced in the inner region of the disk (usually several orders of magnitude higher than the external field strength), which is able to drive a fraction of gas in the corona into outflows. This mechanism may be useful in explaining the observational features in X-ray binaries and active galactic nuclei. Our results may help understanding the physics of the magneto-hydrodynamic (MHD) simulations.