Dynamo Mechanism in a Rotating Spherical Shell: Competition between Magnetic Field and Convection Vortices

"A strong axial magnetic dipole field having the magnetic energy 15 times larger than the kinetic of thermal convection is realized by a direct numerical simulation of the magneto-hydrodynamic equation of an electrically conducting Boussinesq fluid in a rotating spherical shell which is driven by a temperature difference between the outer and inner boundaries against a gravity force pointed to the system center. Cyclonic and anti-cyclonic convection vortices, which rotate, respectively, in the same and opposite directions of the system rotation, are generated and play a primary role in the magnetic field intensification. The magnetic field is enhanced through the stretching of magnetic lines of force in four particular places of the convection fields, namely, inside anti-cyclones, between cyclones and their western neighbouring anti-cyclones at middle as well as low latitudes, and between anti-cyclones and the outer boundary. A \u27twist-turn\u27 loop of intense magnetic flux density is identified as a fundamental structure which yields the dominant contribution both to the toroidal and poloidal components of the longitudinal averaged magnetic field. Various types of competitive interaction between magnetic field and convection vortices are observed. Among others, a creation-and-annihilation cycle of them in the statistically equilibrium state is particularly important, which is composed of three sequentially recurrent dynamical processes, that is, the generation of convection vortices by the Rayleigh-Benard instability, the growth of anti-cyclones and the intensification of magnetic field by concentrate-and-stretch mechanism, and the break-down of vortices by the Lorentz force followed by diminution of magnetic field. The energy transfer from the velocity to the magnetic fields is taking place predominantly in this dynamical cycle.