An investigation of reversing numerical dynamos driven by either differential or volumetric heating

Abstract We present a study of reversing Boussinesq dynamos driven thermally in a rotating spherical shell. For differentially heated dynamos, we investigate the Ekman number dependence of the strong/weak field transition. It is demonstrated that several non-asymptotic features emerge at high Ekman number E > 1 × 1 0 − 3 . At lower E 1 × 1 0 − 3 , it is shown that reversing dynamos are in weak field states with little influence of the Lorentz force on the flow. A non-magnetic investigation of the onset of convection inside the tangent cylinder (TC) highlights this transition to the lower E regime. Based on these results we choose to study the detailed reversal mechanisms of a dynamo at E = 3.16 × 1 0 − 4 as a compromise between low-E features and numerical expense. For this case comparison with known visual interpretation has been made. We have developed a filtering method where the Lorentz force is being damped consistently in selected regions of the shell. It is demonstrated that upwelling plumes at the inner core boundary (ICB) inside the TC destabilise the magnetic dipole to some extent. However, the induction processes in the main part of the TC interior, in particular at the core-mantle boundary (CMB), stabilise the dipole. The net effect of the TC interior is dipole stabilisation. The polar TC-surface and the equatorial CMB region also contribute to dipole stabilisation. Equatorial upwelling plumes at the ICB outside the TC are necessary to the strong/weak field transition at this Ekman number and act as pronounced reversal builders/triggers. The filtering method highlights the ICB plumes as promoters of dipole instability. For comparison we have therefore also investigated volumetrically heated dynamos where buoyancy is stronger at the CMB. In contrast to the above solutions, these dynamos may be in strong field reversing states. The latter require sufficiently high magnetic Prandtl number Pm. For lower Pm we find solutions with a weak, erratically reversing dipole. The dependence on the inner core size has been determined showing that the erratic dipole dynamos extend to higher Pm for smaller inner cores. A non-magnetic investigation of the onset of convection suggests that these results should be close to the case without an inner core. The filtering method shows that the dipole position of the erratic dipole dynamos is destabilised by flow fluctuations anywhere in the shell. Like for differentially heated dynamos, the reversing strong field dynamos are stabilised by the flow inside the TC where the CMB region reinforces the stabilisation. In sharp contrast, however, the ICB region plays no role to reversals. In fact, the equatorial CMB region contains the processes that build/trigger reversals.