Longitudinal structure of Earth’s magnetic field controlled by lower mantle heat flow

Thermal interactions between Earth’s core and mantle provide the power that maintains the geomagnetic field. However, the effect of these interactions and, in particular, the thermochemical piles at the base of the mantle on magnetic field behaviour remains uncertain. Here we present numerical dynamo simulations with strong lateral variations in heat flow imposed at the core–mantle boundary to reproduce conditions within Earth and indicate how the mantle controls core dynamics. Comparing these simulations to recent global magnetic field models, based on observational data spanning tens of thousands of years, they successfully reproduce the morphology and secular variation of Earth’s modern field and the inferred large-scale flow structure at the top of the core. These simulations reveal that the long-term geomagnetic signatures of thermal core–mantle interactions are evident in the longitudinal structure of the geomagnetic field as equatorial patches of reverse flux, rather than the high-latitude patches suggested by less Earth-like simulations. Comparison of these simulations with the field models also suggests that the amplitude of the present-day longitudinal hemispheric imbalance in secular variation is anomalously large, indicating our present-day geomagnetic field may be unusual.