8. On Thermal Driving of the Geodynamo

It is widely believed that the main geomagnetic field is created by the dynamo action of motions in the Earth’s fluid core that are driven by thermal and compositional buoyancy. Early numerical simulations of the geodynamo that succeeded in generating strong, Earth-like dipole magnetic fields had to assume, for computational reasons, an unrealistically high viscosity for the core fluid. Some recent high-resolution models have used more realistic, smaller viscosities, but have unexpectedly produced only non-dipolar or dipolar but comparatively weak magnetic fields, which are less Earth-like. We recently advanced a possible explanation for this paradoxical behavior: we argued that these models had used the geophysically unrealistic outer boundary condition of uniform temperature on the core-mantle interface. In support of this opinion, we integrated two otherwise identical models, in one of which we applied the uniform temperature condition and in the other the more realistic condition of horizontally uniform heat flux. In the latter model, we obtained large-scale convective flows and a comparatively strong dipole-type magnetic field; for the former, we found solutions resembling those obtained by other models that had assumed uniform temperature on the core-mantle boundary. Further explanations for the very different character of the solutions are given here.