Thermal convection and dynamo action with stable stratification at the top of the Earth's outer core.

The outer core of the Earth, filled with electrically conducting fluid, undergoes thermochemical convection due to super-adiabatic temperature gradients. Near the core-mantle boundary, fluid flow may be restricted due to sub-adiabatic temperature gradients or accumulated light elements forming a layer of stable stratification. The present study investigates the behavior of thermal convection with various buoyancy profiles, using non-uniform radial distribution of heat sources, mimicking the combined presence of convective and stable zones. Role of such modified convection in the evolution and resulting morphology of saturated magnetic fields is the main focus of this study. Apart from the reduction in the threshold for onset, the length scale of the convective instabilities is enhanced with stable stratification, while the frequency is reduced. Despite the confinement of convection to unstable regions, rapid rotation favors penetrative radial convective flows. In presence of a stably stratified layer, the dynamo action is suppressed due to the radial confinement of buoyancy, Coriolis, and Lorentz forces. The suppression of vortex stretching, indicated by the relative asymmetry in axial helicity provides further understanding of the mechanism behind the magnetic field structure. As the stratification becomes stronger, the dynamo action leads to magnetic fields with enhanced axial dipole field strength, although the strength of the dynamo is reduced. The confinement of the toroidal component of the magnetic field to localized concentrated patches in regions of stable stratification near the equatorial plane also inhibits the growth of magnetic fields. Nevertheless, enhanced buoyancy forcing may overcome the suppression of dynamo action and lead to strongly convecting dipolar dominated Earth-like dynamos even with moderate stratification. • Role of a stably stratified layer below the Earth's core-mantle boundary in dynamo action is investigated. • Buoyancy forcing with non-uniform heat sources results in both convective and stable zones. • Threshold and patterns of convective instabilities at onset occur in response to enhanced heat flux due to stable layers. • Topological features of the convective motion characterize the success and failure of dynamo. • Stable layers promote axial dipolarity as well as equatorial anti-symmetry of generated magnetic fields. [ABSTRACT FROM AUTHOR]