Nonlinear magnetic buoyancy instability and turbulent dynamo

TL;DR

This paper investigates how magnetic buoyancy instability interacts with dynamo processes in galactic disks, revealing that buoyancy can both accelerate and quench magnetic field growth, leading to oscillations and affecting disk structure.

Contribution

It introduces a coupled model of dynamo action and magnetic buoyancy instability, demonstrating their nonlinear interplay and effects on magnetic field evolution in galactic disks.

Findings

Magnetic buoyancy can accelerate initial magnetic field growth.

Nonlinear effects cause oscillations in the magnetic field.

Magnetic buoyancy can quench the dynamo and influence disk structure.

Abstract

Stratified disks with strong horizontal magnetic fields are susceptible to magnetic buoyancy instability (MBI). Modifying the magnetic field and gas distributions, this can play an important role in galactic evolution. The MBI and the Parker instability, in which cosmic rays exacerbate MBI, are often studied using an imposed magnetic field. However, in galaxies and accretion discs, the magnetic field is continuously replenished by a large-scale dynamo action. Using non-ideal MHD equations, we model a section of the galactic disc (we neglect rotation and cosmic rays considered elsewhere), in which the large-scale field is generated by an imposed $\alpha$-effect of variable intensity to explore the interplay between dynamo instability and MBI. The system evolves through three distinct phases: the linear (kinematic) dynamo stage, the onset of linear MBI when the magnetic field becomes sufficiently strong and the nonlinear, statistically steady state. Nonlinear effects associated with the MBI introduce oscillations which do not occur when the field is produced by the dynamo alone. The MBI initially accelerates the magnetic field amplification but the growth is quenched by the vertical motions produced by MBI. We construct a 1D model, which replicates all significant features of 3D simulations to confirm that magnetic buoyancy alone can quench the dynamo and is responsible for the magnetic field oscillations. Unlike with an imposed magnetic field (arXiv:2305.03318,arXiv:2212.03215), the nonlinear interactions do not reduce the gas scale height, so the consequences of the magnetic buoyancy depend on how the magnetic field is maintained.