Inner core super-rotation is a hypothesized eastward rotation of the inner core of Earth relative to its mantle, for a net rotation rate that is usually faster than Earth as a whole. A 1995 model of Earth's dynamo proposed super-rotations of up to 3 degrees per year; the following year, a seismic study claimed that the proposal was supported by observed discrepancies in the time that p-waves take to travel through the inner and outer core. However, the hypothesis of super-rotation is disputed in the later seismic studies. The seismic support of inner core super-rotations was based on the changes of seismic waves that transversed inside the inner core and free oscillations of Earth. The results are inconsistent between the studies. A localized temporal change of the inner core surface was discovered in 2006 and the temporal change of inner core surface also provided an explanation to the seismic evidence that was attributed to the hypothesis of inner core super-rotations. Recent studies indicated that a super-rotation of the inner core is inconsistent with the seismic data. Some studies proposed both the inner core super-rotation and localized temporal changes of inner core surface co-exist to consistently explain the seismic data, but other studies indicated that localized temporal changes of the inner core surface alone are enough to explain the seismic data.
Background At the center of Earth is the core, a ball with a mean radius of 3480 kilometres that is composed mostly of iron. The outer core is liquid while the inner core, with a radius of 1220 km, is solid. Because the outer core has a low viscosity, it could be rotating at a different rate from the mantle and crust. This possibility was first proposed in 1975 to explain a phenomenon of Earth's magnetic field called westward drift: some parts of the field rotate about 0.2 degrees per year westward relative to Earth's surface. In 1981, David Gubbins of Leeds University predicted that a differential rotation of the inner and outer core could generate a large toroidal magnetic field near the shared boundary, accelerating the inner core to the rate of westward drift. This would be in opposition to the Earth's rotation, which is eastwards, so the overall rotation would be slower. 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. The model predicted that the inner core rotates 3 degrees per year faster than the mantle, a phenomenon that became known as super-rotation. 1996, Xiaodong Song and Paul G. Richards, scientists at the Lamont–Doherty Earth Observatory, presented seismic evidence for a super-rotation of 0.4 to 1.8 degrees per year, while another study estimated the super-rotation to be 3 degrees per year.
Seismic observations
The main observational constraints on inner core rotation come from seismology. When an earthquake occurs, two kinds of seismic wave travel down through the Earth: those with ground motion in the direction the wave propagates (p-waves) and those with transverse motion (s-waves). S-waves do not travel through the outer core because they involve shear stress, a type of deformation that cannot occur in a liquid. In seismic notation, a p-wave is represented by the letter P when traveling through the crust and mantle and by the letter K when traveling through the outer core. A wave that travels through the mantle, core and mantle again before reaching the surface is represented by PKP. For geometric reasons, two branches of PKP are distinguished: PKP(AB) through the upper part of the outer core, and PKP(BC) through the lower part. A wave passing through the inner core is referred to as PKP(DF). (Alternate names for these phases are PKP1, PKP2 and PKIKP.) Seismic waves can travel multiple paths from an earthquake to a given sensor. PKP(BC) and PKP(DF) waves have similar paths in the mantle, so any difference in the overall travel time is mainly due to the difference in wave speeds between the outer and inner core. Song and Richards looked at how this difference changed over time. Waves traveling from south to north (emitted by earthquakes in the South Sandwich Islands and received at Fairbanks, Alaska) had a differential that changed by 0.4 seconds between 1967 and 1995. By contrast, waves traveling near the equatorial plane (e.g., between Tonga and Germany) showed no change. 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. This error can be reduced by using data for doublet earthquakes. These are earthquakes that have very similar waveforms, indicating that the earthquakes were very close to each other (within about a kilometer). Using doublet data from the South Sandwich Islands, a study in 2015 arrived at a new estimate of 0.41° per year. Seismic observations – in particular "temporal changes between repeated seismic waves that should traverse the same path through the inner core" – were used to reveal a core rotation slow-down around 2009. This is not thought to have major effects and one cycle of the oscillation in rotation is thought to be about seven decades, coinciding with several other geophysical periodicities, "especially the length of day and magnetic field".
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