Reversals in nature and the nature of reversals

One of the most interesting features of Earth's magnetic field reversals is their asymmetric shape including a rather slow decay of a given axial dipole and a very fast recreation of the dipole with opposite polarity. With focus on this asymmetry, we consider a simple mean-field dynamo model with a spherically symmetric isotropic helical turbulence parameter alpha that is quenched by the magnetic field energy and subjected to some noise. With an appropriate radial dependence of alpha(r) (including at least one sign change), this dynamo model exhibits typical features of reversals. The asymmetric shape and the very fast recreation are attributed to the dynamical behaviour in the vicinity of a branching point of square root type (exceptional point) of the spectrum of the non-selfadjoint dynamo operator. Other features, like the possible correlation of magnetic field amplitude and reversal rate, the bimodal field distribution, and the inhibition time are also addressed within our simple model. We discuss a tendency of highly supercritical dynamos to self-tune into reversal prone states, and hypothesize that reversing dynamos might be much more common in nature than what could be expected from a purely kinematic perspective.