A nonlinear vacillating dynamo induced by an electrically heterogeneous mantle

Abstract

This paper reports the first spherical numerical dynamo based on a three-dimensional finite element method. We investigate a nonlinear dynamo in a turbulent electrically conducting fluid spherical shell of constant electric conductivity surrounded by an electrically heterogeneous mantle. Magnetic fields in the form of a three-dimensional azimuthally traveling dynamo wave are generated by a prescribed time-dependent α in the fluid shell. In the inner sphere, we assume that there is a solid electrical conductor with the same conductivity as that of the fluid shell. Equilibration of the generated magnetic fields is achieved by the nonlinear process of α-quenching. We show for the first time that finite element methods can be effectively and efficiently employed to simulate three-dimensional dynamos in spherical systems. We also show that an electrically heterogeneous mantle can modulate the core dynamo, leading to a vacillating dynamo whose amplitude depends upon the relative phases between the generated magnetic field and the heterogeneous mantle.