Equatorially asymmetric convection inducing a hemispherical magnetic _eld in rotating spheres and implications for the past martian dynamo
Résumé
The convective instability in a rapidly rotating, self-graviting sphere sets up in the form of equatorially symmetric, non-axisymmetric columnar vortices aligned with the rotation axis, carrying heat away in the cylindrical radial direction. In this study, we present numerical simulations of thermal convection and dynamo action driven by internal heating (intended to model a planetary core subject to uniform secular cooling) in a rotating sphere where, from the classical columnar convection regime, we _nd a spontaneous transition towards an unexpected and previously unobserved ow regime in which an equatorially antisymmetric, ax- isymmetric (EAA) mode strongly inuences the ow. This EAA mode carries heat away along the rotation axis and is the nonlinear manifestation of the _rst linearly unstable axisymmetric mode. When the amplitude of the EAA mode reaches high enough values, we obtain hemispherical dynamos with one single hemisphere bearing more than 75 percent of the total magnetic energy at the surface of the rotating sphere. We per- form the linear analysis of the involved convective modes and the nonlinear study of this hydrodynamic transition, with and without dynamo action, to obtain scaling laws for the regime boundaries. As secular cooling in a full sphere (i.e. without inner core) is a con_guration which has probably been widespread in the early solar system in planetary cores, including the core of Mars, we discuss the possible implications of our results for the past martian dynamo.
Domaines
Sciences de la Terre
Origine : Fichiers produits par l'(les) auteur(s)
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