Earth’s magnetic field is generated by the turbulent dynamics of the electrically conducting liquid core in a process known as geodynamo. The turbulent motion is driven by convective currents between the inner solid core and the core-mantle boundary and is under the influence of a strong rotation. The geomagnetic field has a large-scale dipolar structure roughly aligned with the rotation axis which can reverse its polarity. That is, Earth’s magnetic north and south poles can switch places in a process occurs at irregular intervals, with a typical time between reversals of ~300.000 years and takes around ~10.000 years to fully reverse.

Recent seismic, mineral physics, and geomagnetic investigations have indicated the existence of a stably-stratified layer (SSL) beneath the core-mantle boundary of Earth. This layer, potentially present in other celestial bodies such as Mercury and Saturn, may originate from thermal or compositional influences. In the case of Earth, its depth ranges from 0 to 300 km, exhibiting a stratification strength denoted as N/Ω ∈ (0, 50), where N is the buoyancy frequency and Ω the rotation rate. The presence of the SSL on Earth remains a topic of debate, evidenced by the diverse range of parameters yielded by various models. An important aspect of the SSL is its influence on the overall configuration of the magnetic field and on the occurrence of magnetic polarity reversals on a large scale.

In this work, we conduct an extensive study performing numerical simulations of the MHD equations under the Boussinesq approximation in a spherical shell. The SSL is introduced by assuming a background temperature gradient that undergoes a sign change, being negative near the inner-core boundary and positive in proximity to the core-mantle boundary. We show that the introduction of the SSL generates a large-scale magnetic field with a strong dipolar structure that can undergo through magnetic reversals. This mechanism can be understood by a low-dimensional model given by the non-linear interaction between the dipolar and quadrupolar modes of the magnetic field, where the control parameter is the equatorial symmetry of the flow controlled by imposing an heterogeneous heat flux at the outer core.