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 orientation(1), which may be acquired during solidification of the inner core(2) or developed subsequent to solidification as a result of plastic deformation(3-5). But solidification texturing alone cannot explain the observed depth dependence of anisotropy(6-8), and previous suggestions for possible deformation processes have all relied on radial flow, which is inhibited by thermal(9) and chemical stratification(10). 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 epsilon -iron, where the imposed deformation is accommodated by basal and prismatic slip(11). 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 epsilon -iron at core pressure and temperature(12), we average over the simulated orientation distribution to obtain a pattern of elastic anisotropy which is similar to that observed seismologically(13,14).