Magnetic Buoyancy Instabilities Of A Shear-generated Magnetic Layer

摘要

We discuss a series of high-resolution fully nonlinear MHD numerical simulations relevant to processes that are expected to operate in the solar tachocline. We study the generation of a strong toroidal magnetic field by the stretching action of velocity shear on a weak background poloidal magnetic field, a process often referred to in solar dynamo theory as the Ω-effect. Here we specifically study the generation of a strong layer of horizontal magnetic field by the action of vertical shear on weak vertical magnetic field in a subadiabatically stratified atmosphere, and we examine the buoyancy properties of the resulting magnetic configurations. We find that magnetic layers can indeed be generated, and that magnetic buoyancy instabilities of this layer can exist and lead to rising magnetic structures that resemble arching tubes. However, the conditions for the instabilities to occur are much more demanding than might be anticipated from previous results where a magnetic layer was initially prescribed, rather than shear-generated. Furthermore, the magnetic flux transport by the buoyancy instabilities in this case is decidedly inefficient. These issues, stemming from the feedback of strong magnetic field on the generating shear, raise serious questions for the efficacy of this process in the current solar dynamo paradigm.