Simulations of the Poynting‐Robertson Cosmic Battery in Resistive Accretion Disks

Abstract

We describe the results of numerical ``2.5-dimensional MHD simulations of an initially unmagnetized disk orbiting a central point mass and responding to the continual generation of poloidal magnetic field by a secular source emulating the Poynting-Robertson (PR) drag on electrons in the vicinity of a luminous stellar or compact accreting object. The secular PR term has dual purpose both as the magnetic field source and the trigger of magnetorotational instability (MRI). The disk and surrounding hotter atmosphere fluids have finite resistivity, allowing the magnetic field to diffuse out of its generation sites, while at the same time the disk differential rotation twists the poloidal field and induces a substantial toroidal-field component. For moderate disk resistivity (diffusion timescales up to ~16 local dynamical times) and after ~100 orbits, the MRI allows the fluid of the disk inner edge along with its magnetic flux to flow toward the central point mass where a new, magnetized, nuclear disk forms. The toroidal field in this nuclear disk is amplified by differential rotation and, when near equipartition, unwinds vertically, producing episodic jetlike outflows. For diffusion times longer than the flow time, the poloidal field in the inner region cannot diffuse out; it grows linearly in time, as the outer sections of new poloidal loops are drawn outward by the MRI while their inner sections continue to accumulate onto the compact inner disk. However, for low resistivity (diffusion timescales larger than ~16 local dynamical times), the inflowing matter does not form a nuclear disk or jets and the linear growth of the poloidal magnetic field is interrupted after ~20 orbits because of magnetic reconnection and asymmetric outflows.