(ORDO NEWS) — Having processed seismograms of nuclear tests at the turn of the 1970s, scientists have shown that the period of rotation of the Earth‘s inner core changes cyclically.
All space bodies rotate around their axis. Solid bodies, to which our Earth belongs, rotate as a whole and, as a rule, very uniformly.
Therefore, the second, previously defined as one 86,400th fraction of an Earth day, served as a standard unit of time for a long time. At any point on the surface of the Earth, the result of its measurement will be the same.
Giant planets and stars that do not have solidity exhibit differential rotation : the period of revolution around the axis is different for different parts of the body. At the solar equator, matter makes one rotation around its axis in 25 days, and near the poles – in 34.
On closer examination, things are not so clear cut. If there are molten layers in the depths of a celestial body with a solid surface, the movement of fluid in them can affect the rotation of the entire body.
The rotation of the Earth is influenced by many processes associated with the redistribution of mass in the atmosphere and in the depths of the planet, due to which the duration of the Earth’s day fluctuates with an amplitude of several milliseconds. It also increases by 2.3 milliseconds per century due to the deceleration of the Earth’s rotation by tidal forces.
The Earth’s inner core , which is made of solid iron, is separated from the rest of the planet’s mass by a thick layer of molten iron, and scientists have long assumed that its rotation may differ from the daily one. In particular, it is logical to expect that it “does not keep up” with tidal deceleration and rotates slightly faster than the earth’s surface (this is called superrotation).
The first seismic studies showed that this was the case, but attempts to clarify the difference in rotation speed led to conflicting results: from 0.1 to one degree per year.
In a new paper, scientists from the University of Southern California (USC), led by John Vidal (John Vidale) refined the parameters of rotation of the Earth’s inner core and found a more complex picture: it is either ahead of the rotation of the planet itself, or behind it. The full results of their work are published in open form here .
How is it even possible to know what is happening to the inner core of the Earth if it is located about 5200 kilometers under our feet? By the same method by which its existence was discovered – seismology.
Waves passing through the Earth are reflected and refracted at the boundaries of the layers, and also scattered and deflected by inhomogeneities within the layers themselves.
The study of seismograms made it possible to discover regions of reduced seismic wave propagation velocity in the lower mantle of the Earth and even to consider a structure in them only a couple of tens of kilometers thick.
During the rotation of the inner core, its inhomogeneities are shifted, which is expressed in the changing shift of seismic waves that have circled the center of the Earth from different sides.
Measuring waves that have passed through the inner core is a very difficult task: you have to isolate and analyze weak and multiply distorted seismic signals from the noise. Here scientists were aided by underground nuclear tests in the 1960s and 1970s.
Earthquakes form complex waves with a spatially distributed source (hypocenter). Even a source position uncertainty of several kilometers is comparable to the calculated seismic wave path difference and can greatly complicate the analysis of seismograms.
On the contrary, an underground nuclear explosion is an “ideal” source of probing waves. It has exactly known coordinates and amplitude, and besides, it is point and single, and not extended in space and time.
Previously, scientists from the same group analyzed seismic waves from nuclear tests at Novaya Zemlya in 1971 and 1974. In their new work, they included analysis of waves from two other nuclear tests of sufficient power, which were carried out on the island of Amchitka in Alaska in 1969 and 1971.
It turned out to be much more difficult to process them, since the Aleutian subduction zone is located near the test site , which strongly scatters seismic waves. When the scientists were convinced that they had identified the desired signals on all seismograms, they discovered the following picture.
From 1969 to 1971, the core moved 0.05 degrees against the rotation of the Earth, and from 1971 to 1974, on the contrary, it turned forward by 0.25 degrees. For additional confirmation, they compared the obtained data with the duration of the Earth’s day and found that its changes in the same period confirm their results.
Thus, oscillations with a period of about six years are superimposed on the superrotation of the inner core, which are associated with the exchange of angular momentum between it and the upper shells of the Earth.
The delay between the curves on the graphs is due to the duration of the processes that cause this exchange. The mechanism causing the oscillations is most likely due to convective currents in the outer core, but the details are yet to be explored.
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