Multiscale convection in a geodynamo simulation with uniform heat flux along the outer boundary

It is generally expected that Earth's magnetic field, which is generated by convecting liquid metal within its core, will substantially alter that convection through the action of Lorentz forces. In most dynamo simulations, however, Lorentz forces do very little to change convective flow, which is predominantly fine-scaled. An important exception to this observation is in dynamo models that employ uniform heat flux boundary conditions, rather than the usual uniform temperature conditions, in which multiscale convection is observed. We investigate the combined influence of thermal boundary conditions and magnetic fields using four simulations: two dynamos and two nonmagnetic models, with either uniform temperature or heat flux fixed at the outer boundary. Of the four, only the fixed-heat-flux dynamo simulation produces multiscale convective flow patterns. Comparison between the models suggests that the fixed-flux dynamo generates large patches of strong azimuthal magnetic field that suppress small-scale convective motions. By allowing temperature to vary along the outer boundary, the fixed-flux dynamo generates stronger azimuthal flow and, in turn, stronger magnetic field, and the resulting Lorentz forces alter the nature of convective flow. Extrapolation of the analyses presented here suggests that magnetic fields may also suppress small-scale convection in the Earth's core.