Dhe varying rotational speed of the Earth’s inner core may be related to differences in the length of universal time days and the Earth’s magnetic field. Chinese researchers have discovered a fluctuation cycle of around seven decades when evaluating the travel times of earthquake waves.
While the Earth’s core rotated slightly faster than the Earth’s mantle between 1980 and 2000, this difference has since narrowed and could lead to a slightly slower rotation. The study of Yi Yang and Xiaodong Song of the Peking University in Beijing was published in the journal “Nature Geoscience”.
Scientists distinguish between a solid inner core and a liquid outer core. The fluid movements in the Earth’s outer core create the Earth’s magnetic field, which keeps much of the dangerous cosmic rays from reaching the Earth’s surface. Inside the earth, the magnetic field drives the rotation of the inner core.
Torque imbalance
However, the gravity of the mantle slows down the rotation of the inner core. “A small imbalance between the electromagnetic and gravitational torques is sufficient to change the rotation of the inner core observed here,” the researchers write.
Yang and Song evaluated what are known as seismic doublets: these are pairs of records of earthquakes of similar magnitude at almost the same location in different years. The doublets from 1995 to 2020 came from eight different seismological stations that registered earthquakes whose waves passed at least part of the way through the earth’s core. In addition, they were able to make some analogue recordings of the Collegiate Station in Alaska (USA), which registered earthquakes in the South Sandwich Islands in the South Atlantic from 1964 to 2021.
The study authors compared the waveforms of similar earthquakes recorded in different years. They found that for the 1995-2008 doublets, the waves differ significantly from each other, while there is close agreement between the 2009-2020 doublet waves.
Turning point of the rotation around 1972
The researchers conclude from this that the rotation of the earth’s core has hardly changed compared to the rest of the earth in recent years. The time differences between waves of the same earthquake that only travel through the outer core and waves that also travel through the inner core also point in this direction.
Data from Alaska’s College Station suggests there was a time in the early 1970s when readings were similar to today’s. In the mid-1960s, the Earth’s core may have been spinning at a slightly slower rate than the Earth’s mantle. From the mid-1970s it increased.
Yang and Song therefore see the period between 1971 and 1973 as one turning point and the period between 2009 and 2011 as another turning point. If one supplements the missing measured values before 1964 according to the recognizable trend, then one arrives at a cycle of a little more than seven decades.
“This multidecadal periodicity coincides with changes in several other geophysical observations, notably daylength and magnetic field,” the study authors write. They compared the cycle they found with the values for the length of days in universal time.
Universal time results from the rotation of the earth and can vary by a thousandth of a second from the uniform time measured with atomic clocks. The fluctuations in day length agree quite well with the fluctuations in the rotation of the Earth’s inner core. The researchers also found comparable trends for changes in the earth’s magnetic field.
