Dynamo action in a rotating spherical shell at high Rayleigh numbers

We investigate convection-driven dynamos in a rotating spherical shell with the Rayleigh number Ra up to about 53 times the critical value Ra_c, emphasizing Rayleigh number dependence of the thermal convection and the magnetic field generated by dynamo action. The Rayleigh numbers used in calculations are chosen so as to be in a range which allows us to study the sequence of bifurcation. In the low-Ra-dynamo regime, the flow structure is characterized by columnar convection cells, which mainly generate the magnetic field that is predominantly dipolar. Force balance is essentially in a geostrophic state. Both the magnetic energy and the kinetic energy increase with increase in Ra. In the moderate-Ra-dynamo regime, convective motions appear inside the tangent cylinder (TC), where the azimuthal magnetic field is generated through the so-called ω effect. However, the magnetic energy shows saturation due to relatively inefficient magnetic field generation. In the high-Ra-dynamo regime, dominance of convection inside the TC is remarkable. The advection processes play important roles both in force balance and in magnetic field generation. The magnetic field is generated very inefficiently, leading to the reduced magnetic energy in spite of higher kinetic energy. These three dynamo regimes exhibit distinctive differences in the process of generating magnetic field and characteristic dissipation scales.