USC scientists found evidence that 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 over the six-year period from 1969 to 1974, according to analysis of seismic data. The scientists say their inner core motion pattern also explains the variation in day length, which has been shown to oscillate persistently over the past decades.
“Based on our findings, we can see changes in Earth’s surface relative to its inner core, as people have been claiming for 20 years,” said study co-author John E. Vidale and Dean Professor of Earth Sciences at USC Dornsife College of Letters, Arts and Sciences. “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 day length increased and decreased as we could foresee it.
“The coincidence of these two observations makes oscillation the likely interpretation.”
Atomic test analysis identifies spin rate and direction
Our understanding of the inner core has expanded dramatically over 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, which means researchers struggle to use indirect measurements to explain the pattern, speed, and cause of movement and change.
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 around 1 degree per year. Vidale’s later findings reinforced the idea that the inner core super-rotates, albeit at a slower rate.
Using data from the Large Aperture Seismic Array (LASA), a US Air Force facility in Montana, researchers Wei Wang and Vidale found that the inner core was rotating slower than expected, about 0.1 degrees per year. . The study analyzed waves generated by Soviet underground nuclear bomb tests from 1971 to 1974 in the Arctic archipelago of Novaya Zemlya using a new 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. By measuring compression waves resulting from nuclear explosions, they discovered the inner core had a reversed direction, underrotating at least a tenth of a degree per year.
This latest study marked the first time that the well-known six-year oscillation has been indicated by direct seismological observation.
“The idea that the inner core oscillates was a model that existed, but the community was divided on its viability,” says Vidale. “We started out hoping 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 went in the other direction.
Future research to dig deeper into why the inner core formed
Both Vidale and Wang noted that future research would depend on finding observations precise enough to compare to these results. Using seismological data from atomic tests in previous studies, they were able to pinpoint the exact location and time of the very simple seismic event, Wang says. However, Montana’s LASA closed in 1978 and the era of underground atomic testing in the United States is over, meaning researchers would have to rely on relatively inaccurate seismic data, even with recent advances in instrumentation.
The study supports speculation that the inner core oscillates with variations in day length – plus or minus 0.2 seconds over six years – and geomagnetic fields, both of which match the theory in amplitude and phase. . Vidale says the findings provide compelling theory for many questions posed by the research community.
“The inner core isn’t fixed – it moves under our feet and it seems to go 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 over time – this is an important step in understanding this process better.
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