Dipole collapse in rotating stratified dynamos

Abstract

Numerical modelling of convection driven dynamos in the Boussinesq approximation revealed fundamental characteristics of the dynamo-generated magnetic fields, but the relevance of these results remains to be assessed for highly stratified systems, like gas planets and stars. The common approach is then to rely on the anelastic approximation to model the background density stratification. A conclusion from different anelastic studies is that dipolar solutions seem more difficult to obtain in presence of a substantial density contrast. We review some important results obtained by Raynaud et al. (2015), who investigated the influence of the density stratification on the stability of dipolar dynamos. This study indicates that the loss of the dipolar branch does not ensue from a specific modification of the dynamo mechanisms related to the background stratification, but could instead result from a bias as our observations naturally favour a certain domain in the parameter space characterized by moderate values of the Ekman number. In strongly stratified systems, the force balance may vary with depth, and a local increase of inertia close to the outer surface can explain the loss of the dipolar branch, while volume-averaged measures may underestimate the role of inertia on the field topology.