USC researchers have discovered evidence suggesting the Earth's inner core oscillates, defying widely held beliefs that it spins at a higher pace than the planet's surface.
According to seismic data analysis, the inner core shifted direction in the six years from 1969 to 1974.
The findings were published today in Science Advances.
The researchers claimed that their concept of inner core movement also explains the change in day duration, which has been seen to vary for decades.
The rate and direction of rotation are determined using atomic testing
Ethan Miller/Getty Images
In the last 30 years, our knowledge of the inner core has vastly improved.
Over decades, the inner core, a hot, compact ball of solid iron the size of Pluto, has been demonstrated to move and/or alter, as per ScienceDaily.
It is hard to witness directly, so researchers must rely on indirect data to explain the movement and changes' pattern, pace, and reason.
The initial study, published in 1996, proposed that the planet's inner core spins at a rate of 1 degree per year faster than the rest of the world; a phenomenon known as super-rotation.
Co-author of the study and Dean's Professor of Earth Sciences at USC Dornsife College of Letters, Arts and Sciences, John E. Vidale's subsequent discoveries confirmed that the inner core super-rotates, although at a slower pace.
The data came from the Large Aperture Seismic Array (LASA), a U.S.-based seismic array.
Wei Wang and Vidale of the Air Force laboratory in Montana discovered that the inner core rotated 0.1 degrees per year slower than originally estimated.
The study used a unique beamforming technique developed by Vidale to evaluate waves created by Soviet underground nuclear bomb explosions on the Arctic island Novaya Zemlya from 1971 to 1974.
Wang and Vidale discovered the new results by using the same methods in two previous atomic experiments conducted beneath Amchitka Island, near the extremity of the Alaskan archipelago, Milrow in 1969 and Cannikin in 1971.
They determined the inner core has reversed direction, sub-rotating at least a tenth of a degree every year, by measuring the compressional waves caused by nuclear explosions.
The inner core's relevance to our understanding of Earth's structure is much beyond both its relative size and our knowledge of its attributes, with a radius of 1220 km and so contains less than 1% of Earth's volume.
Through electromagnetic coupling, decadal fluctuations in Earth's magnetic field associated with torsional oscillations of the fluid outer core can create similar oscillations of the inner core.
Because the inner core is gravitationally tightly connected to the mantle, torsional oscillations might be constrained by variations in LOD and polar motion of the mantle over a decade duration.
Free oscillations
Free oscillations (also known as normal modes) are Earth's vibrating patterns.
Based on the circumstances and pattern of vibration, normal modes detect long-wavelength features in the Earth's interior, as per Earth Sciences.
For instance, 13S2, a spheroidal mode with radial order 13 and angular order 2, samples the Earth from the mantle to the inner core along its radius.
Splitting function estimate from normal mode observations is highly non-linear and essentially non-unique (many models explain data equally well).
If the seismic source model is known, the inversion is traditionally linearized and solved in an iterative method.
However, for major earthquakes employed in normal mode analysis, the point-source approximation is frequently insufficient.
A high number of earthquakes is also necessary to restrict the Earth's structure.
