Dynamo activities driven by magneto-rotational instability and Parker instability in galactic gaseous disk

TL;DR

This study uses 3D MHD simulations to explore how magneto-rotational and Parker instabilities drive magnetic field amplification and reversals in galactic disks, revealing quasi-periodic magnetic cycles and flux escape mechanisms.

Contribution

It demonstrates the combined roles of MRI and Parker instability in galactic dynamo processes without assuming equatorial symmetry, highlighting magnetic field reversals and flux escape.

Findings

Magnetic fields grow to 10% of gas pressure via MRI.

Magnetic flux escapes through Parker instability when plasma β < 5.

Magnetic field reversals occur every 10 rotation periods.

Abstract

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.