Spontaneous Transitions of the Magnetic Field Morphology within Long-Time Evolution Simulations

We present the results of convection-driven spherical dynamo simulations that are evolved up to 30 large-scale magnetic diffusion times. Periods of strongly-stable dipolar magnetic fields lasting up to 6 magnetic diffusion times, and periods of chaotic magnetic polarity reversals lasting up to 9 magnetic diffusion times, are both observed during the same simulation. The transition from stable dipole to chaotic reversals is preceded by an instability at the tangent cylinder, which generally acts as a transport barrier during periods of stable dipolarity. The instability is marked by the radiation of high frequency inertial waves and leads to a net transfer of magnetic energy from the dipole to higher poles; the result of this process is a growth of magnetic energy that eventually exceeds the kinetic energy during the chaotic reversal period. The thermal evolution rate of the system, as characterized by the Nusselt number, increases during the chaotic reversal period when the ohmic dissipation exceeds the viscous dissipation.