Dipolar dynamos in stratified systems
Résumé
Observations of low-mass stars reveal a variety of magnetic topologies ranging from large-scale, axial dipoles to more complex magnetic fields. Spherical simulations of convectivedynamos reproduce a similar diversity, either with Boussinesq or anelastic models taking intoaccount the variation of the density with depth throughout the convection zone. A conclusionfrom anelastic studies is that dipolar solutions are difficult to obtain as soon as substantialstratifications are considered. We rely on a systematic parameter study to investigate inmore detail the impact of the density stratification both on the dynamo onset and thedipole collapse. Our study indicates that the loss of the dipolar branch does not ensue froma modification of the dynamo mechanisms related to the background stratification, butcould instead result from a bias as our observations naturally favour a certain domain in theparameter space characterized by moderate values of the Ekman number, owing to currentcomputational limitations. We also show that the critical magnetic Reynolds number ofthe dipolar branch is scarcely modified, which provides an important insight into the globalunderstanding of the impact of the density stratification on the stability domain of thedipolar dynamo branch.