The length of a day oscillates every 6 years, and we finally know why

The way we think about the center of our planet may need a serious update.

New evidence suggests that instead of constantly spinning faster than Earth’s rotation, the solid inner core is oscillating – spinning first in one direction relative to the surface far above, then in the other, changing of management every six years.

Not only does this have implications for our understanding of the inner workings of our homeworld, but it may also perfectly explain a mystery that has puzzled scientists for some time: an oscillating variation in the length of Earth’s day, with a period 5.8 years old.

“Based on our findings, we can see changes in the Earth’s surface relative to its inner core, as people have been claiming for 20 years,” said geophysicist John E. Vidale of the University of California at dusk. South to Los Angeles (UCLA).

“However, our latest observations show that the inner core rotated slightly slower from 1969 to 1971, then moved in the other direction from 1971 to 1974. We also note that the length of a day has increased and decreased predictably. The coincidence of these two observations makes the oscillation the probable interpretation.”

Although our understanding of the Earth’s core has grown a lot over the past few decades, there is still a lot we don’t know. We can’t just go there and take a look; everything we know we have gleaned from indirect observations, such as seismic waves propagating and bouncing around the planet.

But it is still a very effective tool. Scientists were able to determine that Earth’s inner core is likely a hot, dense ball of solid iron, measuring about 2,440 kilometers (1,516 miles) in diameter, slightly larger than the size of Pluto. Evidence also suggests that it demonstrates superrotation, spinning faster than the Earth itself.

Researchers first detailed this phenomenon in 1996, with an estimated superrotation rate of 1 degree per year. Vidale and his colleague, Wei Wang, also from UCLA, later revised the rate to 0.29 degrees per year, using data from underground nuclear tests carried out at Russia’s Novaya Zemlya test site in the 1990s. 1970.

In the new research, they went back in time, adding two tests carried out under Amchitka Island in 1971 and 1969. And it revealed something strange. The data suggests that, rather than superrotating, Earth’s inner core was underrotating, that is, rotating slower than Earth’s rotation, by about 0.1 degrees per year.

A diagram illustrating the Vidale and Wang model. (Edward Sotelo/USC)

According to the researchers, this corresponded to an oscillation. When in full rotation, the inner core overrotates, but then it slows down before accelerating again.

“The idea of ​​the inner core oscillating was a model that existed, but the community was divided on its viability,” Vidale said.

“We went in expecting to see the same direction and rate of spin in the previous pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving the other way.”

The six-year periodicity of the oscillation fits perfectly with other oscillations for which we do not have a confirmed explanation.

Earth’s days also undergo time variations of plus or minus 0.2 seconds every six years or so, and the Earth’s magnetic field also oscillates over a six-year period. In amplitude and phase, they correspond to the periodicity of the model derived by Vidale and Wang for the oscillations of the inner core of the Earth.

All of this means it will take more data to sort out, which could be tricky. The facility that recorded nuclear test data, the US Air Force’s Large Aperture Seismic Array, closed in 1978, and underground nuclear testing is nowhere near as prolific as before.

But new advances in sensor technology could mean the detailed data needed to probe Earth’s inner core isn’t that far in the future; the results so far offer a tantalizing hint that the Earth’s innards are a bit more complex than we knew.

“The inner core is not fixed – it moves under our feet, and it seems [be] going back and forth a few miles every six years,” Vidale said.

“One of the questions we’ve been trying to answer is, is the inner core moving gradually or is it mostly locked to everything else in the long run? We’re trying to figure out how the inner core s is formed and how it moves through time – this is an important step in understanding this process better.”

The research has been published in Scientists progress.

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