Dynamo action in a stably stratified core

Abstract

Summary. The main part of the geomagnetic field is regenerated by dynamo action in the fluid outer core. It is commonly assumed that convective motion drives the dynamo, although there is no proof that the core is convecting. The density profile through the core is not sufficiently resolved to discriminate between a homogeneous (convecting) core and one that is stably stratified. The core may be stably stratified, at least in limited regions. We have formulated a theory for dynamo action in a stably stratified fluid. We have demonstrated that a field of inertial gravity waves propagating through a rotating, electrically conducting fluid can act as a dynamo. The effect of the Coriolis acceleration is to give the waves an elliptical polarization and a net helicity. This helicity can give rise to an α-effect and therefore a field of waves can generate a dynamo of the α2-type. We have developed a kinematic dynamo model that is driven only by inertial gravity waves. The model consists of a field of small-amplitude waves propagating horizontally in a plane fluid layer that has a uniform stratification and is rotating about an arbitrary axis. The steady state mean-field induction equation has been solved for the case of a monofrequency wavefield. It reveals that dynamo action can occur if the wavefield has a preferred propagation direction relative to the rotation axis. The efficiency of the dynamo depends on the dimensionless parameter S which is the ratio of the Brunt–Vaisala frequency to the rotation rate. Dynamo efficiency is inversely proportional to S, so that increased stratification requires increased wave energy to reach dynamo conditions. Stable stratification does not prevent dynamo action; indeed, a stably stratified core can under proper conditions support an efficient dynamo.