Study: Changes in Rotation of Earth’s Inner Core Occur on Decadal Scale

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The rotation of the Earth’s inner core may have recently paused and could be reversing, according to new research by geoscientists from Peking University.

Yang & Song analyze repeated seismic waves from the early 1990s and show that all of the paths that previously showed significant temporal changes have exhibited little change over the past decade. Image credit: NASA.

Yang & Song analyze repeated seismic waves from the early 1990s and show that all of the paths that previously showed significant temporal changes have exhibited little change over the past decade. Image credit: NASA.

The Earth’s inner core is separated from the rest of the solid Earth by the liquid outer core, enabling it to rotate differently from the rotation of the Earth itself.

The spin of the inner core is driven by the magnetic field generated in the outer core and balanced by the gravitational effects of the mantle.

Knowing how the inner core rotates could illuminate how these layers interact. However, the speed of this rotation, and whether it varies, is debated.

“Differential rotation of Earth’s inner core relative to the mantle is thought to occur under the effects of the geodynamo on core dynamics and gravitational core-mantle coupling,” said Peking University geoscientists Yi Yang and Xiaodong Song.

“This rotation has been inferred from temporal changes between repeated seismic waves that should traverse the same path through the inner core.”

In their study, the authors analyzed the difference in the waveform and travel time of seismic waves from near-identical earthquakes that have passed through the Earth’s inner core along similar paths since the 1960s.

They found that since around 2009, paths that previously showed significant temporal variation have exhibited little change, suggesting that the inner core rotation has paused.

They also identified that this may be associated with a reversal of the inner core rotation as part of a seven-decade oscillation with a previous turning point occurring in the early 1970s.

This variation correlates with changes in geophysical observations at the Earth’s surface, such as the magnetic field and the length of day.

“These observations provide evidence for dynamic interactions between the Earth’s layers, from the deepest interior to the surface, potentially due to gravitational coupling and the exchange of angular momentum from the core and mantle to the surface,” the researchers said.

The study was published in the journal Nature Geoscience.

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Y. Yang & X. Song. Multidecadal variation of the Earth’s inner-core rotation. Nat. Geosci, published online January 23, 2023; doi: 10.1038/s41561-022-01112-z

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