Inner core super-rotation

When geodynamo models take into account gravitational coupling between the inner core and mantle, it lowers the predicted super-rotation to as little as 1 degree per million years.

[4] In 1995, Gary Glatzmeier at Los Alamos and Paul Roberts at UCLA published the first "self-consistent" three-dimensional model of the dynamo in the core.

[6] The model predicted that the inner core rotates 3 degrees per year faster than the mantle, a phenomenon that became known as super-rotation.

S-waves do not travel through the outer core because they involve shear stress, a type of deformation that cannot occur in a liquid.

[15] One of the criticisms of the early estimates of super-rotation was that uncertainties about the hypocenters of the earthquakes, particularly those in the earlier records, caused errors in the measurement of travel times.

[1] Song and Richards explained their observations in terms of the prevailing model of inner core anisotropy at the time.

A model for the inner core with uniform anisotropy had a direction of fastest travel tilted at an angle 10° from the spin axis of the Earth.

Those based on the Sandwich Island earthquakes have the fastest rates, although they also have a weaker signal, with PKP(DF) barely emerging above the noise.

[2] A study in 1997 revisited the Sandwich Islands data and came to a different conclusion about the origin of changes in travel times, attributing them to local heterogeneities in wave speeds.

[2] Another way of constraining the inner core rotation is using normal modes (standing waves in Earth), giving a global picture.

[24] However, their accuracy is limited by the shortage of seismic stations in the 1970s and 1980s,[8] and the inferred rotation can be positive or negative depending on the mode.

[2] In the 1995 model of Glatzmeier and Roberts, the inner core is rotated by a mechanism similar to an induction motor.

[25] The 1995 model did not include the effect of gravitational coupling between density variations in the mantle and topography on the inner core boundary.

[28][8] Geodynamo models that take into account gravitational locking and changes in the length of day predict a super-rotation rate of only 1° per million years.