Double diffusive convection in the Earth's core and the morphology of the geomagnetic field

The convection in the Earth's outer core is driven by thermal and compositional buoyancy. Here we examine the effects of the co-existence of these two buoyancy sources on the core dynamics and morphology of the geomagnetic field using numerical dynamo models with double diffusive convection at the thermal Prandtl number, _{Pr T}=0.1 and compositional Prandtl number, _{Pr C}=1. We find that the morphology of the magnetic field is determined by the dynamic ratio of the two driving mechanisms. A dipolar magnetic field is maintained as long as the power injected by thermal buoyancy comprises less than 60% of the total. Otherwise, non-dipolar fields prevail due to helicity reduction. The dominantly dipolar structure of the present geomagnetic field suggests that the fraction of power injection by thermal convection in the present geodynamo is below the threshold.