Electromagnetic induction in a conducting D″ layer --- I: Effects of boundary charge accumulation and the boundary conditions

Abstract

In response to the primary electric field originated by the fluid-core-dynamo, electric charges accumulate at the boundaries of a solid conducting D″ layer due to the poor circuitry conditions until the irrotational primary electric field is neutralized. The neutralization undoes the internal induction excited by the primary electric field and, as a result, eliminates the secondary toroidal magnetic field inside the D″. The primary magnetic field generated by the fluid-core-dynamo gives rise to a sustainable secondary poloidal magnetic field inside the conducting D″ layer. We introduce the general kinematic boundary conditions that are valid with different induction processes and parameter jumps across the core mantle boundary. We find by dimensional analysis that the strength of the secondary poloidal magnetic field is much weaker, by a factor of ∼10−2, than the irrotational primary. These findings indicate that the D″ is in effect an insulator for the geodynamo regardless of its material composition. Any mechanism for the core-mantle magnetic coupling should take this fact into consideration. Keywords: D′′ layer, Magnetic induction, Boundary conditions