Steady states of the Parker instability: the effects of rotation

TL;DR

This study uses 3D MHD simulations to explore how rotation influences the nonlinear behavior of the Parker instability, revealing effects on magnetic fields, cosmic rays, and dynamo processes in galactic environments.

Contribution

It demonstrates that rotation significantly alters the nonlinear state of the Parker instability, leading to oscillatory magnetic fields and large-scale structures similar to observed galactic halos.

Findings

Stronger steady magnetic fields with rotation.

Persistent gas outflows lasting around 2 Gyr.

Reversal and buoyant spreading of magnetic fields.

Abstract

We model the Parker instability in vertically stratified isothermal gas using non-ideal MHD three-dimensional simulations. Rotation, especially differential, more strongly and diversely affects the nonlinear state than the linear stage (where we confirm the most important conclusions of analytical models), and stronger than any linear analyses predict. Steady state magnetic fields are stronger and cosmic ray energy density higher than in comparable nonrotating systems. Transient gas outflows induced by the nonlinear instability persist longer, of order 2 Gyr, with rotation. Stratification combined with (differential) rotation drives helical flows, leading to mean-field dynamo. Consequently, the nonlinear state becomes oscillatory (while both the linear instability and the dynamo are non-oscillatory). The horizontal magnetic field near the midplane reverses its direction propagating to higher altitudes as the reversed field spreads buoyantly. The spatial pattern of the large-scale magnetic field may explain the alternating magnetic field directions in the halo of the edge-on galaxy NGC 4631. Our model is unique in producing a large-scale magnetic structure similar to such observation. Furthermore, our simulations show that the mean kinetic helicity of the magnetically driven flows has the sign opposite to that in the conventional non-magnetic flows. This has profound consequences for the nature of the dynamo action and large-scale magnetic field structure in the coronae of spiral galaxies which remain to be systematically explored and understood. We show that the energy density of cosmic rays and magnetic field strength are not correlated at scales of order a kiloparsec.