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

The way we think about the center of our planet may need to be seriously updated.

New evidence suggests that instead of consistently spinning faster than Earth’s rotation, the solid inner core oscillates β€” rotating first in one direction relative to the surface far above, then in the other, changing direction every six years.

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

“From our findings, we can see changes to the Earth’s surface compared to its inner core, as people have claimed for 20 years,” said geophysicist John E. Vidale of the University of Southern California, Los Angeles (UCLA). ).

“However, our latest observations show that the inner core rotated a little slower from 1969-71 and then shifted in the other direction from 1971-74. We also noticed that the length of a day grew and shrank as would be predicted. two observations make the wobble the likely interpretation.”

Although our understanding of the Earth’s core has developed greatly in recent decades, there is still much we don’t know. We can’t just go there and look at it; everything we know, we gleaned from indirect observations, such as seismic waves propagating and bouncing across the planet.

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

The researchers first detailed this phenomenon in 1996, with an estimated superspin rate of 1 degree per year. Vidale and his colleague, Wei Wang, also at UCLA, later revised the rate to 0.29 degrees per year, using data from underground nuclear tests conducted at the Russian Novaya Zemlya test site in the 1970s.

In the new research, they went back in time, adding two tests carried out below Amchitka Island in 1971 and 1969. And that revealed something strange. The data suggested that instead of superrotation, Earth’s inner core was subrotating β€” that is, rotating more slowly than Earth’s rotation, about 0.1 degrees per year.

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

This, the researchers said, was consistent with the wobble. When at full rotation, the inner core super-rotates, but then decelerates before accelerating again.

β€œThe idea that the inner core oscillates was a model that was out there, but the community was divided on whether it was viable,” Vidale said.

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

The oscillation’s six-year periodicity matches perfectly with other oscillations for which we have no confirmed explanation.

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

All of these means will require more data to unravel, which can be tricky. The facility that recorded the nuclear test data, the US Air Force’s Large Aperture Seismic Array, closed in 1978, and underground nuclear tests are nowhere near as prolific as they used to be.

But further advances in sensor technology could mean the detailed data needed to probe Earth’s inner core isn’t that far off in the future; the results so far offer a tantalizing clue that the Earth’s interior is a little more complex than we knew.

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

“One of the questions we’re trying to answer is, is the inner core progressively moving or is it locked up compared to everything else over the long term? We’re trying to understand how the inner core formed and how it moves over time – this is a important step to better understand this process.”

The research was published in advances in science.

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