On the Formation of Magnetic Structures by the Combined Action of Velocity Shear and Magnetic Buoyancy

Using numerical simulations of a compressible, stably stratified, magnetohydrodynamical (MHD) flow, we investigate a mechanism for producing a series of rising tubelike magnetic structures. In this process, a steadily forced shear flow stretches a weak poloidal background magnetic field to create a strong toroidal field that is magnetically buoyant. The subsequent evolution of this system depends on the parameters: At moderate magnetic Reynolds numbers ( R m ), the system reaches a stable nonstatic equilibrium. At larger values of R m , this equilibrium becomes unstable to a shear-buoyant instability, involving a modification of the background velocity shear by the magnetically induced buoyant poloidal flow. The system then produces a series of buoyant magnetic structures at regular intervals that are expelled from the region of strong velocity shear. Even higher R m causes the magnetic intensity of the structures to strengthen and the intervals between expulsion events to become irregular. For large enough kinetic Reynolds numbers ( R e ), the magnetic modification of the background shear can trigger a secondary three-dimensional Kelvin-Helmholtz instability that can twist the magnetic structures into a helical shape.