Multiphase Circumnuclear Gas in a Low-$\beta$ Disk: Turbulence and Magnetic Field Reversals

TL;DR

This study uses 3D MHD simulations to explore the magnetic field structures and turbulence in multiphase gas disks around supermassive black holes, revealing magnetic reversals driven by MRI and Parker instability.

Contribution

It provides new insights into the magnetic field dynamics and turbulence in multiphase AGN disks, including magnetic reversals and the role of magnetoconvective instability.

Findings

Magnetic energy is comparable to thermal and kinetic energies in turbulence.

Magnetorotational instability develops a dominant toroidal magnetic field.

Magnetic field reversals occur periodically due to flux escape and instabilities.

Abstract

We studied the magnetic field structures and dynamics of magnetized multiphase gas on parsec scales around supermassive black holes by using global 3D magnetohydrodynamics (MHD) simulations. We considered the effect of radiative cooling and X-ray heating due to active galactic nuclei (AGNs). The gas disk consists of a multiphase gas with (1) cold ($\leq 10^3$ K) and thin, and (2) warm ($\sim 10^4$ K) and thick components with a wide range of number densities. The turbulent magnetic energy at maximum is comparable to the thermal and turbulent kinetic energies in the turbulent motion. We confirmed that the turbulent velocity of the warm gas in the ambient cold gas is caused by magnetoconvective instability. The turbulent magnetic field due to magnetorotational instability (MRI) is developed in the disk, but the mean toroidal magnetic field dominates and supports in a quasi-steady state, where the plasma-$\beta$, the ratio between gas pressure and magnetic pressure, is low ($\beta < 1$). As often seen in adiabatic MHD simulations of rotating disks, the direction of the mean toroidal field periodically reverses with time even in multiphase gas structures. The direction reversal is caused by magnetic flux vertically escaping from the disk and by the combination of the MRI and the Parker instability.