Length of the day fluctuates every 6 years and we finally know why

(ORDO NEWS) — Our ideas about the center of our planet may need a major update.

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

Not only does this affect our understanding of the inner workings of our home world, but it could also explain a mystery that has puzzled scientists for some time now: the 5.8-year fluctuations in the length of Earth’s day.

“Our results show that the Earth’s surface is shifting relative to its inner core, as people have been saying for 20 years,” said geophysicist John E. Vidale of the University of Southern California at Los Angeles (UCLA).

“However, our latest observations show that the inner core rotated slightly more slowly in 1969-71 and then shifted the other way in 1971-74. We also note that the length of the day grew and shortened, as one would expect. The coincidence of these two observation makes oscillation the most likely interpretation.”

Although our understanding of the Earth’s core has expanded 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 have received from indirect observations, such as seismic waves propagating and passing through the entire planet.

But it’s still a very effective tool. Scientists have determined that the Earth’s inner core is likely a hot, dense ball of solid iron about 2,440 kilometers (1,516 miles) across, slightly larger than Pluto. The data also suggests that it exhibits superrotation, spinning faster than the Earth itself.

Researchers first described the phenomenon in detail in 1996, estimating the superrotation rate at 1 degree per year. Vidale and colleague Wei Wang, also at the University of California, later revised that figure to 0.29 degrees per year using data from underground nuclear tests conducted at Russia‘s Novaya Zemlya test site in the 1970s.

In the new study, they went back in time, adding two tests conducted under the island of Amchitka in 1971 and 1969. And it revealed something strange. The data obtained indicated that the Earth’s inner core does not rotate, but subrotates, that is, it rotates more slowly than the Earth rotates, by about 0.1 degrees per year.

Length of the day fluctuates every 6 years and we finally know why 2

This, according to the researchers, corresponds to oscillations. At the moment of full rotation, the inner core is super-rotating, but then it slows down, and then accelerates again.

“The idea that the inner core oscillates was a model that existed, but the community was divided on whether it was viable,” Vidale said.

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

The six-year periodicity of fluctuations is clearly in line with other fluctuations for which we have no confirmed explanation.

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

Unraveling all this will require more data, which can be a daunting task. The US Air Force‘s Large Aperture Seismic Array, which recorded nuclear test data, closed in 1978, and underground nuclear testing is no longer conducted as often as it used to be.

But further advances in sensor technology may mean that the detailed data needed to probe the Earth’s inner core isn’t all that far off in the future; The results so far provide a tantalizing hint that the Earth’s interior is a bit more complex than we thought.

“The inner core is not stationary – it moves under our feet, and it seems to move a couple of kilometers back and forth every six years,” Vidale said.

“One of the questions we’ve been trying to answer is, does the inner core move gradually, or is it basically fixed 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 an important step in better understanding this process.”


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