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