Scaling laws, force balances and dynamo generation mechanisms in numerical dynamo models: influence of boundary conditions

We investigate the influence of different thermal and velocity boundary conditions on numerical geodynamo models. We concentrate on the implications for magnetic field morphology, heat transport scaling laws, force balances and generation mechanisms. The field morphology most strongly depends on the local Rossby number, but there is some variation in the dipolarity of the field with boundary condition. Scaling laws also depend on the boundary conditions, but a diffusivity-free scaling is a good first order approximation for all our dipolar models. Our multipolar models, however, obey different scaling laws from dipolar models implying a different force balance in these models. We find that our dipolar models have a stronger degree of Lorentz-Coriolis balance compared to our multipolar models which have a stronger degree of Lorentz-inertial balance.The models with a stronger Lorentz-Coriolis dominance can be generated by either alpha omega, alpha(2)omega or alpha(2) mechanisms whereas the models with a stronger Lorentz-inertial balance are all alpha(2) dynamos. These results imply that some caution is necessary when extrapolating results from dynamo models to Earth-like parameters since the choice of boundary conditions can have important effects.