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

Abstract

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 (Rm), the system reaches a stable nonstatic equilibrium. At larger values of Rm, 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 Rm causes the magnetic intensity of the structures to strengthen and the intervals between expulsion events to become irregular. For large enough kinetic Reynolds numbers (Re), 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. Subject headings: MHD — Sun: interior — Sun: magnetic fields — sunspots