Dynamo Models With a Mercury-Like Magnetic Offset Dipole

Abstract

Mercury has a global magnetic field that is unique in our solar system. It is dominated by a weak axial dipole and a substantial axial quadrupole contribution. The field has a dipole tilt which is smaller than one degree and an offset of the magnetic equator toward north by about 20% of the planetary radius. The only dynamo model that succeeds in continuously reproducing these features without having to rely on an unrealistic heat-flux pattern through the core-mantle boundary is a double-diffusive model by Takahashi et al. (2019), https://doi.org/10.1038/s41467-018-08213-7, where thermal effects cause a thick, stably stratified layer in the outer parts of the core. In this study we further show the sensitivity of this model to the choice of parameters. In the explored parameter set, the model yields solutions which are unrealistic for Mercury when convection becomes vigorous. We present a new single-diffusive model, that captures Mercury's field characteristics during long periods that are interrupted by dipole field reversals causing the magnetic equator to switch between the northern and southern hemisphere. The Mercury-like solutions are stable over a broader range of parameters. An important ingredient for the success is the fact that dynamo action sets in with the equatorially symmetric magnetic field family.