Inner-Core Anisotropy and Rotation

Abstract

Key Words seismology of the core, inner-core boundary, inner-core shear waves, inner-core viscosity, inner-core attenuation s Abstract This paper reviews recent research focused on the Earth's inner core.Large inner-core traveltime anomalies and the anomalous splitting of core-sensitive free oscillations strongly suggest that the inner core is anisotropic. Initial models involved a simple, constant or depth-dependent cylindrical anisotropy at a level less than a few percent. Recent observations suggest that its eastern hemisphere is largely isotropic, whereas its western hemisphere is highly anisotropic, and there are indications that its top 100 km may be isotropic. The coda of inner-core reflected phases has been used to infer strong heterogeneities with a length scale of just a few kilometers. Thus, a complicated three-dimensional picture of the inner core is beginning to emerge, although it has been suggested that much of this complexity may be the misinterpretation of signals that have their origin in the lowermost mantle. Numerical models of the geodynamo suggest that the inner core may rotate at a slightly different rate than the mantle. Recent seismological estimates based upon traveltime and normal-mode data limit inner-core differential rotation to less than +0.2 degrees per year.

Candy Wrapper for the Earth's Inner Core

“…Solidification of the Earth's inner core (EIC) at the liquid-solid interface, also called the inner-core boundary (ICB), occurs under shear compression, a physicochemical process that indeed favors the development of axially oriented iron crystals. Although a number of mechanisms have been introduced to interpret the inner-core anisotropy5, most of them tightly relied on the lattice preferred orientation (LPO) of both hexagonal-close-packed ( hcp )67 and the body-centered-cubic ( bcc ) iron crystals8910. Cylindrical anisotropy of the order of 3% with the fast axis parallel to the rotation axis and the slow axis in the equatorial plane was the preferred explanation of early seismic observations1112.…”

Hemispherical anisotropic patterns of the Earth’s inner core

“…The growth of the IC from the freezing of the molten iron alloy at the inner-core boundary (ICB) (5) drives the convection in the outer core and provides the energy source for the geodynamo (6). From the analysis of the normal modes of the Earth's free oscillations and the body wave data, it has been found that compressional P waves travel about 3% faster along the Earth's spin axis than in the equatorial plane (7)(8)(9)(10). The growing evidence of an elastic anisotropic IC has continued to generate interesting understandings of the deepest part of our planet, though not all the scientific efforts have converged to the same scenario.…”

Stability and bifurcation diagram of Boussinesq thermal convection in a moderately rotating spherical shell allowing rotation of the inner sphere

“…Actually, it is discussed whether the Earth's inner core differentially rotates with respect to the mantle in this decade (e.g., Refs. [19][20][21]. Our previous study 22 also shows that the magnitude of the torque operating the co-rotating inner sphere induced by the stable traveling wave solutions is large enough to change the angular velocity of the inner sphere even near the critical regime.…”