Early-Earth dynamos in a full-sphere

While the Earth's magnetic field has existed for 4Gyr or more, its inner core is much younger - recent estimates for the age of the inner core go back no further than 1.5Gyr. Consequently, Earth’s dynamo has been running in a full sphere for much of its life, in contrast to the present day dynamo operating in a spherical shell. However, despite their geophysical relevance, full shpere dynamos remain rare in literature.

Here, we present results from a first parameter study on rapidly rotating dynamos in a full sphere geometry, representative of the Earth's dynamo before the nucleation of the inner core. Since we cannot rely on the buoyancy release of the inner core, our dynamos are driven by internal heat sources and fixed flux boundary conditions take account of the secular cooling of the planet. We show that - depending on the input paramters (Ekman, Rayleigh and magnetic Prandtl number) - such dynamos can produce a variety of different solutions, including dipolar and multipolar dynamos as well as stable and chaotically-reversing dynamos.