By processing seismograms of nuclear tests at the turn of the 1970s, scientists have shown that the rotation period of the Earth's inner core changes cyclically.
All cosmic bodies rotate around their axis. The solid bodies to which our Earth belongs rotate as a whole and, as a rule, very evenly. Therefore, a second, previously defined as one 86400 th fraction of an Earth day, has long served as a standard unit of time. At any point on the Earth's surface, the result of its measurement will be the same.
Giant planets and stars that do not have hardness exhibit differential rotation: the period of rotation around the axis is different for different parts of the body. At the solar equator, matter makes one revolution around the axis in 25 days, and near the poles — in 34.
Upon closer examination, everything turns out to be not so unambiguous. If there are molten layers in the bowels of a celestial body with a solid surface, the movement of liquid in them can affect the rotation of the entire body. The rotation of the Earth is affected by many processes associated with the redistribution of mass in the atmosphere and in the depths of the planet, which is why 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 braking of the Earth's rotation by tidal forces.
The inner core of the Earth, consisting 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 is the case, but attempts to clarify the difference in rotation speed led to contradictory results: from 0.1 to one degree per year.
In a new paper, scientists from the University of Southern California (USC), led by John Vidale, clarified the rotation parameters of the Earth's inner core and found a more complex picture: it is ahead of the rotation of the planet itself, then lags behind it. The full results of their work are published in open form here.
How can we even find out what is happening to the inner core of the Earth, if it is located about 5,200 kilometers under our feet? 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, as well as scattered and deflected by inhomogeneities within the layers themselves.
The study of seismograms made it possible to discover areas of reduced velocity of seismic waves in the lower mantle of the Earth and even consider a structure in them with a thickness of only a couple of tens of kilometers. When the inner core rotates, its inhomogeneities shift, which is expressed in a changing shift of seismic waves that have rounded the center of the Earth from different sides.
Measuring the waves that have passed through the inner core is a very difficult task: it is necessary to isolate weak and repeatedly distorted seismic signals from the noise and analyze them. Underground nuclear tests of the 1960s and 1970s helped scientists here. Earthquakes form complex waves with a source distributed in space (hypocenter). Even the uncertainty of the source position of several kilometers is comparable to the calculated difference in the course of seismic waves and can greatly complicate the analysis of seismograms. On the contrary, an underground nuclear explosion is an "ideal" source of probing waves. It has precisely known coordinates and amplitude, and besides, it is point-like and one-time, and not extended in space and time.
Earlier, scientists from the same group analyzed seismic waves from nuclear tests on Novaya Zemlya in 1971 and 1974. In their new work, they included the analysis of waves from two other nuclear tests of sufficient power, which were conducted on Amchitka Island 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 dissipates seismic waves. When the scientists were convinced that they had identified the desired signals on all seismograms, they found the following picture.
From 1969 to 1971, the core shifted by 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 data obtained with the duration of the Earth's day and found that its changes during the same period confirm their results.
Thus, fluctuations 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 in the graphs is due to the duration of the processes that cause this exchange. The mechanism causing the oscillations is most likely related to convective flows in the outer core, but its details have yet to be studied.