Magnetic field generation on Earth has probably persisted for at least 3.5 Gyr (refs. 1,2), initially sustained by secular cooling of the Earth’s core and, more recently, by the growth of the solid inner core3. Numerical models of the present-day geodynamo have proved to be successful in producing Earth-like magnetic fields4–7 and approaching realistic dynamic regimes8–11. However, thermal evolution12,13 and palaeomagnetic records14,15 suggest that the geodynamo operated for most of geomagnetic history without a solid inner core. Dynamo action in a whole fluid core remains poorly understood. Here we show dynamo actions that are independent of fluid viscosity in the correct geometry of the Earth’s core in the deep past at extremely low viscosity, demonstrating the negligible role of fluid viscosity in our dynamo simulations. Our early-Earth geometry models produce magnetic field intensity and morphologies that are compatible with the palaeomagnetic data in the deep past while showing remarkable similarity to the present-day magnetic field. This raises questions about the role of the solid inner core in producing the spatial-temporal variations of the observed Earth’s magnetic field7,16–18.
