The role of magnetic boundaries in kinematic and self-consistent magnetohydrodynamic simulations of precession-driven dynamo action in a closed cylinder

We numerically examine dynamo action generated by a flow of an electrically conducting fluid in a precessing cylindrical cavity.

We compare a simplified kinematic approach based on the solution of the magnetic induction equation with a prescribed velocity field with the results from a self-consistent three-dimensional simulation of the complete set of magnetohydrodynamic equations.

The different kinematic models show a strong influence of the electromagnetic properties of outer layers, which manifests itself in the existence of two different branches with dynamo action.

In all cases, we observe a minimum for the onset of dynamo action in a transitional regime, within which the hydrodynamic flow undergoes a change from a large-scale to a more small-scale, turbulent behaviour.

However, significant differences in the absolute values for the critical magnetic Reynolds number occur, and they can be explained by the low rotation rate in the MHD simulations achievable for technical reasons.

In these models the character of the dynamo solution is small-scale and the magnetic energy remains significantly smaller than the kinetic energy of the flow.

In irregular intervals, we observe dynamo bursts with a local concentration of the magnetic field, resulting in a global increase of the magnetic energy by a factor of 3 to 5.

However, diffusion of the local patches caused by strong local shear is too rapid, causing these features to exist for only a short period (i.e., shorter than a rotation period) so that their dynamical impact on the dynamo remains small.

As the magnetic field is small-scale and weak, the nonlinear feedback on the flow through the Lorentz force remains small and arises essentially in terms of a slight damping of the fast timescales, whereas there is no noticeable change in flow amplitude compared to the hydrodynamic case.