Effect of the inner core on the numerical solution of the magnetohydrodynamic dynamo

We report two simulation results of the magnetohydrodynamic dynamo applied to rapidly rotating spherical systems using fully nonlinear equations under Boussinesq approximation. Calculations were carried out under the same par ameter conditions but for a spherical shell and a sphere. We assume that a uniform internal heat source distributed in the whole sphere drives the con vection and dynamo and that the physical properties of the inner core are i dentical to those of the fluid outer core except for its rigidity. This tre atment enables us to compare two cases under the same condition, except the existence of the inner core. Magnetic field is effectively generated by st rong velocity shear and helicity of the fluid near the top (and bottom) bou ndaries. A stable axial dipole field develops in the case of the spherical shell because of the steady field generation at both the outer and inner bo undaries, while the magnetic field in the sphere fluctuates with time from lack of the bottom boundary before it reaches the dipole dominant state at last. This result suggests that the Earth's magnetic field may be stabilize d as the inner core grows, even though the total energy input is the same. This study provides a first step to interpret the paleointensity data from the Archaean when there was a transition due to the growth of the inner cor e. (C) 1999 Elsevier Science B.V. All rights reserved.