Dynamo action and its temporal variation inside the tangent cylinder in MHD dynamo simulations

Results of numerical simulations of three-dimensional, self-consistent, convection driven magnetohydrodynamic dynamos in a rotating spherical shell are reported. The electrical conductivity of the inner core and its solid body rotation relative to the reference frame are taken into account. Emphasis is put on the magnetic field generation inside the so-called tangent cylinder (TC) and its importance on the overall dynamo process. Convection inside the TC consisting of some upward and downward plumes takes place at a relatively high supercritical Rayleigh number. These plumes shear the toroidal field to the north-south direction, generating a new poloidal field, which does not grow significantly due to spatial and temporal variations in the plumes. The toroidal field often exhibits quadrupolar structure inside the TC and reverses its direction according to the direction of differential rotation. Such a quadrupolar symmetric toroidal field is destroyed by strong intermittent fluctuation in the meridional circulation inside the TC and the dipolar symmetric field recovers. It turns out, however, that the magnetic field generation inside the TC is at most several times weaker than the outside.