Relativistic Outflows from a GRMHD Mean-field Disk Dynamo

Abstract

In this work, we present simulations of thin accretion disks around black holes, in order to investigate a mean-field disk dynamo, using our resistive GRMHD code, which is able to produce a large-scale magnetic flux. We consider a weak seed field in an initially thin disk, a background (turbulent) magnetic diffusivity, and the dynamo action itself. A standard quenching mechanism is applied to mitigate an otherwise exponential increase in the magnetic field. Comparison simulations of an initial Fishbone–Moncrief torus suggest that reconnection may provide another quenching mechanism. The dynamo-generated magnetic flux expands from the disk interior into the disk corona, becomes advected by disk accretion, and fills the axial region of the domain. The dynamo leads to an initially rapid increase in magnetic energy and flux, while for later evolutionary stages the growth stabilizes. Accretion toward the black hole depends strongly on the type of magnetic-field structure that develops. The radial field component supports extraction of angular momentum, and thus accretion. It also sets the conditions for launching a disk wind, initially from the inner disk area. When a strong field engulfs the disk, strong winds are launched, predominantly driven by the pressure gradient of the toroidal field. For rotating black holes, we identify a Poynting flux-dominated jet, driven by the Blandford–Znajek mechanism. This axial Poynting flux is advected from the disk, and therefore accumulates at the expense of the flux carried by the disk wind, which is itself regenerated by the disk dynamo.