Texturing of the Earth's inner core by Maxwell stresses

Elastic anisotropy in the Earth's inner core has been attributed to a prefe rred 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 explai n the observed depth dependence of anisotropy(6-8), and previous suggestion s for possible deformation processes have all relied on radial flow, which is inhibited by thermal(9) and chemical stratification(10). Here we investi gate the development of anisotropy as the inner core deforms plastically un der the influence of electromagnetic (Maxwell) shear stresses. We estimate the flow caused by a representative magnetic field using polycrystal plasti city simulations for epsilon -iron, where the imposed deformation is accomm odated by basal and prismatic slip(11). We find that individual grains in a n initially random polycrystal become preferentially oriented with their c axes parallel to the equatorial plane. This pattern is accentuated if defor mation is accompanied by recrystallization. Using the single-crystal elasti c properties of epsilon -iron at core pressure and temperature(12), we aver age over the simulated orientation distribution to obtain a pattern of elas tic anisotropy which is similar to that observed seismologically(13,14).