Dynamo Action Driven by Shear and Magnetic Buoyancy

Abstract

We present direct numerical simulations based on the full MHD equations of dynamo action in a nonrotating, convectively stable layer containing a forced, localized velocity shear. The dynamo operates by the interaction of two MHD processes: the production of toroidal magnetic field from poloidal field by the shear, and the regeneration of poloidal loops from toroidal field due to the combined action of magnetic buoyancy and Kelvin-Helmholtz instabilities. The nature of the dynamo process is such that it can occur only if the initial magnetic fields exceed a critical value that typically depends on the magnetic Reynolds number. As such, this dynamo does not operate in the kinematic limit. Several different behaviors are observed, including steady dynamo production and cyclic as well as chaotic activity. In the cyclic regimes, the dynamo process exhibits polarity reversals and periods of reduced activity.