Dynamo activities driven by magneto-rotational instability and Parker
  instability in galactic gaseous disk
  We carried out global three-dimensional magneto-hydrodynamic simulations of
dynamo activities in galactic gaseous disks without assuming equatorial
symmetry. Numerical results indicate the growth of azimuthal magnetic fields
non-symmetric to the equatorial plane. As magneto-rotational instability (MRI)
grows, the mean strength of magnetic fields is amplified until the magnetic
pressure becomes as large as 10% of the gas pressure. When the local plasma
$\beta$ ($ = p_{\rm gas}/p_{\rm mag}$) becomes less than 5 near the disk
surface, magnetic flux escapes from the disk by Parker instability within one
rotation period of the disk. The buoyant escape of coherent magnetic fields
drives dynamo activities by generating disk magnetic fields with opposite
polarity to satisfy the magnetic flux conservation. The flotation of the
azimuthal magnetic flux from the disk and the subsequent amplification of disk
magnetic field by MRI drive quasi-periodic reversal of azimuthal magnetic
fields in timescale of 10 rotation period. Since the rotation speed decreases
with radius, the interval between the reversal of azimuthal magnetic fields
increases with radius. The rotation measure computed from the numerical results
shows symmetry corresponding to a dipole field.
