Turbulent magnetohydrodynamic dynamo based on alpha and cross-helicity effects, with special reference to geomagnetic fields

Abstract

Magnetohydrodynamic (MHD) state in a wide-gap spherical shell mimicking the earth’s outer core is examined with resort to the mean-field or turbulent-dynamo theory. In the dynamo, the induced mean magnetic field is in a quasi-force-free state under a large alpha effect. The saturation level of the field is determined through the alignment with the mean velocity under a cross-helicity effect. On this basis, the following characteristics of the induced field are elucidated: The energy of the magnetic field becomes much larger than the kinetic one of the fluid motion driven by buoyancy force; the Lorentz force coming from the field remains less dominant than the buoyancy force; the toroidal component of the field is larger than the poloidal one.