On the effect of an electrically heterogeneous lower mantle on planetary dynamos

Self-consistent convection-driven dynamos in an electrically conducting fluid spherical shell of constant electric conductivity surrounded by an electrically heterogeneous lower mantle are investigated using an element-by-element finite element method that takes the full advantage of modern massively parallel computers. The primary objective is to explore possible effects of an electrically heterogeneous mantle on the self-consistent convection-driven dynamos in rotating spherical systems. The dynamo solutions obtained with an electrically heterogeneous mantle are compared to that obtained with a uniformly insulating mantle at exactly the same Rayleigh, Ekman and Prandtl numbers. Three important effects are identified: (i) an electrically heterogeneous mantle can induce a vacillating dynamo whose amplitude is determined by the relative phase between the time-dependent dynamo solution and the electrically heterogeneous mantle, (ii) the spatial structure of the radial component of the generated magnetic field in the vicinity of the core-mantle boundary, an observable component of planetary magnetic fields. is strongly influenced by the structure of the electrically heterogeneous lower mantle and (iii) the extra magnetic dissipation taking place in a very thick electrically conducting layer in lower mantle can terminate dynamo action operating in the fluid core. (C) 2008 Elsevier B.V. All rights reserved.