Accessing the dipole-multipole transition in rapidly rotating spherical shell dynamos

The Earth’s magnetic field is generated by turbulent convective motions of molten iron in the outer core. The field is strongly dipolar, yet the paleomagnetic record shows many polarity reversals, where the North and South poles of the magnetic field have switched position. The processes causing polarity transitions are poorly understood: numerical simulations to date have struggled to obtain both a strongly dipolar field and polarity reversals. Control parameters for simulations are necessarily far from those in Earth’s liquid inner core, due to the highly turbulent nature of the fluid flow and the extreme influence of rotation through the Coriolis force, represented by the Ekman number (Ek). State of the art DNS simulations reach Ek~10-7, while the Earth has Ek~10-15. Much work has been done to find scaling laws and path theories to infer knowledge about the Geodynamo from these simulations. We investigate two path theories built on assuming a constant magnetic Reynolds number, and a balance between the Inertial, Magnetic, Archimedian and Coriolis (IMAC) forces on the fluid flow, and a third that removes inertia from the balance (MAC). We report results from around 20 numerical simulations each ran on ARCHER2 using Leeds Dynamo Code, written in modern fortran, using FFTW, netcdf and MPI. Along all paths we find that dipolarity increases as more Earth-like conditions are reached, but by moderately increasing convective driving the multipolar/reversing regime can be reached, demonstrating that our approach is an efficient method to seek the transition in rapidly rotating simulations.