Superionic effect and anisotropic texture in Earth’s inner core driven by geomagnetic field

Abstract

Abstract Seismological observations suggest that Earth’s inner core (IC) is heterogeneous and anisotropic, presenting faster seismic velocity in the polar direction than in the equatorial direction1-4, significant anisotropy changes with depth5-10, and hemispherical dichotomy10-15. Increasing seismological observations make the understanding of the mineralogy and mechanism for the complex IC texture extremely challenging, and the driving force for the anisotropic texture remains unclear16-18. Under IC conditions, hydrogen becomes highly diffusive like liquid in the hexagonal-close-packed (hcp) solid Fe lattice, which is known as the superionic state.19,20. Here, we reveal that H-ion diffusion in superionic Fe-H alloy is anisotropic with the lowest barrier energy along the c-axis. In the presence of external electric field, the alignment of the Fe-H lattice with the c-axis pointing to the field direction is energetically favourable. Due to this effect, Fe-H alloys are aligned with the c-axis parallel to the equatorial plane by the diffusion of the north–south dipole geomagnetic field into the inner core. The aligned texture driven by the geomagnetic field presents significant seismic anisotropy, which explains the anisotropic seismic velocities in the IC, suggesting a strong coupling between the IC structure and geomagnetic field.