Texturing of the Earth's inner core by Maxwell stresses

Elastic anisotropy in the Earth's inner core has been attributed to a preferred lattice orientation1, which may be acquired during solidification of the inner core2 or developed subsequent to solidification as a result of plastic deformation3,4,5. But solidification texturing alone cannot explain the observed depth dependence of anisotropy6,7,8, and previous suggestions for possible deformation processes have all relied on radial flow, which is inhibited by thermal9 and chemical stratification10. Here we investigate the development of anisotropy as the inner core deforms plastically under the influence of electromagnetic (Maxwell) shear stresses. We estimate the flow caused by a representative magnetic field using polycrystal plasticity simulations for ε-iron, where the imposed deformation is accommodated by basal and prismatic slip11. We find that individual grains in an initially random polycrystal become preferentially oriented with their c axes parallel to the equatorial plane. This pattern is accentuated if deformation is accompanied by recrystallization. Using the single-crystal elastic properties of ε-iron at core pressure and temperature12, we average over the simulated orientation distribution to obtain a pattern of elastic anisotropy which is similar to that observed seismologically13,14.