Rotation of Earth’s inner core oscillates back and forth over time compared to surface

The uncertainty of science: New data now suggests that the Earth’s inner core is no longer rotating faster than the planet’s outer layers — as had been measured in 1996 — but is now actually rotating slower.

Research published in 1996 was the first to propose the inner core rotates faster than the rest of the planet — also known as super-rotation — at roughly 1 degree per year. Subsequent findings from Vidale reinforced the idea that the inner core super-rotates, albeit at a slower rate.

Utilizing data from the Large Aperture Seismic Array, a U.S. Air Force facility in Montana, researcher Wei Wang and Vidale found the inner core rotated slower than previously predicted, approximately 0.1 degrees per year. The study analyzed waves generated from Soviet underground nuclear bomb tests from 1971-74 in the Arctic archipelago Novaya Zemlya using a novel beamforming technique developed by Vidale.

The new findings emerged when Wang and Vidale applied the same methodology to a pair of earlier atomic tests beneath Amchitka Island at the tip of the Alaskan archipelago — Milrow in 1969 and Cannikin in 1971. Measuring the compressional waves resulting from the nuclear explosions, they discovered the inner core had reversed direction, sub-rotating at least a tenth of a degree per year.

This latest study marked the first time the well-known six-year oscillation had been indicated through direct seismological observation.

What the data really suggests is that the core’s rotation is somewhat independent of the upper layers, so that each can vary relative to each other. This difference must carry with it some profound consequences, related to the length of the day, earthquakes, plate tectonics, and any number of other seismic phenomenon, but at the moment too little is known to tie everything together.

A bit of trivial for my readers: In 2004 I wrote an article for Natural History describing the discovery that the core rotated faster than the Earth’s upper layers. It was that article that prompted John Batchelor to invite me to appear on his show for the first time. The rest is history.