Shear-driven magnetic buoyancy oscillations

TL;DR

This study investigates how uniform horizontal shear influences magnetic buoyancy oscillations in a stratified gas layer, revealing complex magnetic field dynamics and persistent oscillations with implications for magnetic stability.

Contribution

It demonstrates the amplification and evolution of magnetic fields under shear in a stratified medium, highlighting unresolved issues in magnetic field sustainment and stability.

Findings

Magnetic field amplifies to Lundquist numbers of 200-400 then drops to 100-300.

Magnetic field becomes nearly axisymmetric in the final state.

Persistent low amplitude vertical velocity oscillations are observed.

Abstract

The effects of uniform horizontal shear on a stably stratified layer of gas is studied. The system is initially destabilized by a magnetically buoyant flux tube pointing in the cross-stream direction. The shear amplifies the initial field to Lundquist numbers of about 200-400, but then its value drops to about 100-300, depending on the value of the sub-adiabatic gradient. The larger values correspond to cases where the stratification is strongly stable and nearly isothermal. At the end of the runs the magnetic field is nearly axisymmetric, i.e. uniform in the streamwise direction. In view of Cowling's theorem the sustainment of the field remains a puzzle and may be due to subtle numerical effects that have not yet been identified in detail. In the final state the strength of the magnetic field decreases with height in such a way that the field is expected to be unstable. Low amplitude oscillations are seen in the vertical velocity even at late times, suggesting that they might be persistent.