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New Research Shows the Earth’s Inner Core Oscillates – Causes Variation in the Length of a Day

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Scientists have discovered evidence that the Earth’s inner core oscillates, contradicting previously held beliefs that it consistently rotates at a faster rate than the planet’s surface.

Scientists identify a six-year cycle of super- and sub-rotation that affected the length of a day based on their analysis of seismic data.

Earth’s structure is divided into layers, with the inner core at the center followed by the outer core, lower mantle, upper mantle, crust, and atmosphere. The inner core is the hottest part of the planet at about 10,000 °F (5400 °C), which is similar to the temperature of the surface of the sun!

Believed to consist mostly of an iron-nickel

“From our findings, we can see the Earth’s surface shifts compared to its inner core, as people have asserted for 20 years,” said John E. Vidale, co-author of the study and Dean’s Professor of Earth Sciences at USC Dornsife College of Letters, Arts and Sciences. “However, our latest observations show that the inner core spun slightly slower from 1969-71 and then moved the other direction from 1971-74. We also note that the length of a day grew and shrank as would be predicted.

“The coincidence of those two observations makes oscillation the likely interpretation.”

USC researchers identified a six-year cycle of super- and sub-rotation in the Earth’s inner core, contradicting previously accepted models that suggested it consistently rotates at a faster rate than the planet’s surface. Credit: Edward Sotelo/USC

Analysis of atomic tests pinpoints rotation rate and direction

Our understanding of the inner core has expanded dramatically in the past 30 years. The inner core — a hot, dense ball of solid iron the size of

Research published in 1996 was the first to propose that 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.

Earth’s layers and structure.

Utilizing data from the Large Aperture Seismic Array (LASA), 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.

“The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable,” Vidale says. “We went into this expecting to see the same rotation direction and rate in the earlier pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving in the other direction.”

Future research to dig deeper into why inner core formed

Vidale and Wang both noted future research would depend on finding sufficiently precise observations to compare against these results. By using seismological data from atomic tests in previous studies, they have been able to pinpoint the exact location and time of the very simple seismic event, says Wang. However, the Montana LASA closed in 1978 and the era of U.S. underground atomic testing is over, meaning that the researchers would need to rely on comparatively imprecise earthquake data, even with recent advances in instrumentation.

The study does support the speculation that the inner core oscillates based on variations in the length of day — plus or minus 0.2 seconds over six years — and geomagnetic fields, both of which match the theory in both amplitude and phase. Vidale says the findings provide a compelling theory for many questions posed by the research community.

“The inner core is not fixed — it’s moving under our feet, and it seems to going back and forth a couple of kilometers every six years,” Vidale said. “One of the questions we tried to answer is, does the inner core progressively move or is it mostly locked compared to everything else in 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.”

Reference: “Seismological observation of Earth’s oscillating inner core” by Wei Wang and John E. Vidale, 10 June 2022, Science Advances.
DOI: 10.1126/sciadv.abm9916



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Science

The Length of a Day Oscillates Every 6 Years, And We May Finally Know Why

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How we think about our planet’s center may need to be seriously updated.

New evidence suggests that, instead of consistently rotating faster than Earth’s spin, the solid inner core oscillates – spinning first in one direction with respect to the surface far above, then the other, changing direction every six years.

 

This not only has implications for our understanding of the inner workings of our home world, it can also neatly explain a mystery that has perplexed scientists for some time: an oscillating variation in the length of Earth’s day, with a period of 5.8 years.

“From our findings, we can see the Earth’s surface shifts compared to its inner core, as people have asserted 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 spun slightly slower from 1969-71 and then moved the other direction from 1971-74. We also note that the length of a day grew and shrank as would be predicted. The coincidence of those two observations makes oscillation the likely interpretation.”

Although our understanding of Earth’s core has developed a lot in recent decades, there’s still a lot we don’t know. We can’t just go there and take a gander at it; everything we know, we’ve gleaned from indirect observations, such as seismic waves propagating and bouncing through the entire planet.

 

But this is still a very effective tool. Scientists have been able to ascertain that Earth’s inner core is probably a hot, dense ball of solid iron, measuring roughly 2,440 kilometers (1,516 miles) across, a little bigger than the size of Pluto. Evidence also suggests that it demonstrates superrotation, rotating faster than 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 of UCLA, later revised the rate down to 0.29 degrees per year, using data from underground nuclear tests conducted at the Russian Novaya Zemlya testing site in the 1970s.

In the new research, they went back in time, adding two tests conducted below Amchitka Island in 1971 and 1969. And that revealed something odd. The data suggested that, rather than superrotating, Earth’s inner core was subrotating – that is, spinning more slowly than Earth’s rotation, by about 0.1 degrees per year.

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

This, the researchers said, was consistent with oscillation. When in the full swing of its spin, the inner core superrotates, but then it slows down before speeding up again.

“The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable,” Vidale said.

 

“We went into this expecting to see the same rotation direction and rate in the earlier 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 the oscillation neatly matches other oscillations for which we don’t have a confirmed explanation.

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

This all means will require more data to unravel, which could be tricky. The facility that recorded the data from the nuclear tests, the US Air Force’s Large Aperture Seismic Array, closed in 1978, and underground nuclear testing is nowhere near as prolific as it used to be.

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

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

“One of the questions we tried to answer is, does the inner core progressively move, or is it mostly locked compared to everything else in 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.”

The research has been published in Science Advances.

 

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Science

Earth’s inner core ‘oscillates’ over a mile every six years, study claims

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When Jules Verne wrote ‘A Journey to the Centre of the Earth’ over 150 years ago, he imagined a land of glowing crystals, turbulent seas, prehistoric animals and giant mushrooms.

But what actually lies beneath our feet remains a mystery – even today we know more about the rings of Saturn than the interior of our planet.

Over the past 30 years, however, our understanding of the Earth’s inner core has expanded dramatically, and it has been shown to move and change over decades.

But while it was previously thought to be rotating at a consistently faster rate than the planet’s surface, a new study shows that it oscillates, going back and forth over a mile every six years.

The cycle could explain variations in the length of days, which have been shown to oscillate persistently for the past several decades.

USC researchers identified a six-year cycle of super- and sub-rotation in the Earth’s inner core, contradicting previously accepted models that suggested it consistently rotates at a faster rate than the planet’s surface

FOUR LAYERS OF THE PLANET EARTH

Crust: To a depth of up to 70km, this is the outermost layer of the Earth, covering both ocean and land areas.

Mantle: Going down to 2,890km with the lower mantle, this is the planet’s thickest layer and made of silicate rocks richer in iron and magnesium than the crust overhead.

Outer core: Running from a depth of 2,890- 5,150km, this region is made of liquid iron and nickel with trace lighter elements.

Inner core: Going down to a depth of 6,370km at the very centre of planet Earth, this region has been thought to be made of solid iron and nickel. But this new study suggests that it contains both mushy and hard iron.

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The Earth’s inner core is a hot, dense ball of solid iron the size of Pluto – and as hot as our sun. 

It is impossible to observe directly, meaning researchers have to rely on indirect measurements to explain the pattern, speed and cause of its movement and changes. 

The US team used seismic data from 1969 to 1974 to create a computer model of the core’s movement.

Simulations confirmed the Earth’s surface shifts compared to its inner core, as scientists have asserted for 20 years. 

However, it contradicted previous theories suggesting the rate of rotation was consistently faster than the planet’s surface. 

‘The inner core is not fixed – it is moving under our feet, and it seems to going back and forth a couple of kilometres (1.25 miles) every six years,’ said lead author Professor John Vidale of the University of Southern California. 

Research published in 1996 was the first to propose that 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 Prof Vidale reinforced the idea that the inner core super-rotates, albeit at a slower rate. 

Utilising data from the LASA (Large Aperture Seismic Array), a US Air Force facility in Montana, Prof Vidale found the inner core rotates approximately 0.1 degrees per year.

Lab staff developed a novel beam-forming technique to analyse waves generated from Soviet underground nuclear bomb tests from 1971 to 1974 in the Arctic archipelago Novaya Zemlya.

The latest results emerged when they 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.

‘Our latest observations show that the inner core spun slightly slower from 1969-71 and then moved the other direction from 1971-74,’ said Professor Vidale. 

‘We also note that the length of day grew and shrank as would be predicted.

‘The coincidence of those two observations makes oscillation the likely interpretation.’

Because the Earth’s inner core is so inaccessible, researchers had to rely on the only means available to probe the innermost Earth — seismic data (stock image)

Map A shows the location of LASA (triangle) and the two nuclear test pairs (stars). B ans C show the distribution of the predicted time shifts

This marks the first time the six-year oscillation has been indicated through direct seismological observation.

Using seismological data from atomic tests in previous studies, the researchers have been able to pinpoint the exact location and time of the seismic event. 

‘The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable,’ said Professor Vidale.

‘We went into this expecting to see the same rotation direction and rate in the earlier pair of atomic tests, but instead we saw the opposite. 

‘We were quite surprised to find that it was moving in the other direction.’

LASA closed in 1978, and the era of US underground atomic testing is now over, meaning researchers will need to rely on comparatively imprecise earthquake data for future research in this area, even with recent advances in instrumentation.

However, the study does support speculation the inner core oscillates based on variations in the length of day – plus or minus 0.2 seconds over six years – and geomagnetic fields, both of which match the theory in both amplitude and phase.

Vidale said the findings provide a compelling theory for many questions posed by the research community.  

‘One of the questions we tried to answer is, does the inner core progressively move or is it mostly locked compared to everything else in the long term?

‘We are trying to understand how the inner core formed and how it moves over time — this is an important step in better understanding this process.’

The study is published in Science Advances.

EARTH’S LIQUID IRON CORE CREATES THE MAGNETIC FIELD 

Our planet’s magnetic field is believed to be generated deep down in the Earth’s core.

Nobody has ever journeyed to the centre of the Earth, but by studying shockwaves from earthquakes, physicists have been able to work out its likely structure.

At the heart of the Earth is what was thought to be its solid inner core, two thirds of the size of the moon, and made mainly of iron. However, this new study disputes this.

At 5,700°C, this iron is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid.

Surrounding this is the outer core, which is a 1,242 mile (2,000 km) thick layer of iron, nickel, and small quantities of other metals.  

The metal here is fluid, because of the lower pressure than the inner core.

Differences in temperature, pressure and composition in the outer core cause convection currents in the molten metal as cool, dense matter sinks and warm matter rises.

The ‘Coriolis’ force, caused by the Earth’s spin, also causes swirling whirlpools.

This flow of liquid iron generates electric currents, which in turn create magnetic fields.

Charged metals passing through these fields go on to create electric currents of their own, and so the cycle continues.

This self-sustaining loop is known as the geodynamo.

The spiralling caused by the Coriolis force means the separate magnetic fields are roughly aligned in the same direction, their combined effect adding up to produce one vast magnetic field engulfing the planet.

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Science

A new study shows that the inner core oscillates

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USC researchers identified a six-year cycle of super- and sub-rotation in the Earth’s inner core, contradicting previously accepted models that suggested it consistently rotates at a faster rate than the planet’s surface. Credit: Edward Sotelo/USC

USC scientists have found evidence that the Earth’s inner core oscillates, contradicting previously accepted models that suggested it consistently rotates at a faster rate than the planet’s surface.

Their study, published today in Science Advances, shows that the inner core changed direction in the six-year period from 1969–74, according to the analysis of seismic data. The scientists say their model of inner core movement also explains the variation in the length of day, which has been shown to oscillate persistently for the past several decades.

“From our findings, we can see the Earth’s surface shifts compared to its inner core, as people have asserted for 20 years,” said John E. Vidale, co-author of the study and Dean’s Professor of Earth Sciences at USC Dornsife College of Letters, Arts and Sciences. “However, our latest observations show that the inner core spun slightly slower from 1969–71 and then moved the other direction from 1971–74. We also note that the length of day grew and shrank as would be predicted.

“The coincidence of those two observations makes oscillation the likely interpretation.”

Analysis of atomic tests pinpoints rotation rate and direction

Our understanding of the inner core has expanded dramatically in the past 30 years. The inner core—a hot, dense ball of solid iron the size of Pluto—has been shown to move and/or change over decades. It’s also impossible to observe directly, meaning researchers struggle through indirect measurements to explain the pattern, speed and cause of the movement and changes.

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 (LASA), 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.

“The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable,” Vidale says. “We went into this expecting to see the same rotation direction and rate in the earlier pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving in the other direction.”

Future research to dig deeper into why inner core formed

Vidale and Wang both noted future research would depend on finding sufficiently precise observations to compare against these results. By using seismological data from atomic tests in previous studies, they have been able to pinpoint the exact location and time of the very simple seismic event, says Wang. However, the Montana LASA closed in 1978 and the era of U.S. underground atomic testing is over, meaning that the researchers would need to rely on comparatively imprecise earthquake data, even with recent advances in instrumentation.

The study does support the speculation that the inner core oscillates based on variations in the length of day—plus or minus 0.2 seconds over six years—and geomagnetic fields, both of which match the theory in both amplitude and phase. Vidale says the findings provide a compelling theory for many questions posed by the research community.

“The inner core is not fixed—it’s moving under our feet, and it seems to going back and forth a couple of kilometers every six years,” Vidale said. “One of the questions we tried to answer is, does the inner core progressively move or is it mostly locked compared to everything else in 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.”

Earth’s inner core: A mixture of solid Fe and liquid-like light elements

More information:
Wei Wang et al, Seismological observation of Earth’s oscillating inner core, Science Advances (2022). DOI: 10.1126/sciadv.abm9916. www.science.org/doi/10.1126/sciadv.abm9916
Provided by
University of Southern California

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The Earth moves far under our feet: A new study shows that the inner core oscillates (2022, June 10)
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