Effects of boundary layers on magnetic field behavior in an MHD dynamo model

Abstract

Numerical simulations of three-dimensional self-consistent MHD dynamos in a rotating spherical shell are performed to examine the structures of the velocity and the magnetic fields and the mechanism of magnetic field generation. Emphasis is put on an important role of the boundary layer which arises for the no-slip boundary condition. The most important is precise computation in the boundary layers, in which the number of grid points must be large enough to ensure spatial resolution there. The result of computation shows that the dipole field is dominant and that the magnetic field is concentrated in the convection columns. Such results are the same as those derived from our previous study for the stress-free boundary condition. A marked difference is the strong toroidal magnetic field generated by strong shear flow inside the boundary layers. Also the effect of magnetic diffusion is strong and more significant than that of magnetic induction near the spherical surfaces. This suggests that the so-called frozen-flux hypothesis, which has usually been used to estimate core surface flows, does not necessarily hold for the cases in which significant boundary layers appear.