drChanges in the rotational speed of the Earth’s inner core may be related to differences in the length of the day UTC and the Earth’s magnetic field. Chinese researchers found a fluctuation cycle of about seven decades when assessing the travel time of the seismic waves.
While the Earth’s core rotated slightly faster than the mantle between 1980 and 2000, this difference has narrowed since then and may result in slightly slower rotation. A study of Yi Yang and Xiaodong Song from Beijing University In Beijing in Nature Geoscience.
Scientists distinguish between a solid inner core and a liquid outer core. The movement of fluids in the Earth’s outer core creates the Earth’s magnetic field, which prevents many harmful cosmic rays from reaching the Earth’s surface. Inside the Earth, a magnetic field drives the rotation of the inner core.
Torque imbalance
However, mantle gravity slows the rotation of the inner core. “A small imbalance between the electromagnetic torque and the gravitational moment is sufficient to alter the rotation of the inner core observed here,” the researchers wrote.
Yang and Song evaluate what are known as seismic couplings: these are recorded pairs of earthquakes of the same magnitude at roughly the same location in different years. The multiples from 1995 to 2020 come from eight different seismic stations that recorded earthquakes whose waves traveled at least part of the way through the Earth’s core. In addition, they managed to make several analog recordings university station In Alaska (USA), which recorded 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 pair, the waves were very different from each other, whereas for the 2009-2020 pair there was close agreement.
The turning point of the rotation was around 1972
From this the researchers concluded that the rotation of the Earth’s core has hardly changed compared to the rest of the Earth in recent years. The time difference between waves from the same earthquake traveling only through the outer core and waves traveling through the inner core also point in this direction.
Data from Coolidge Station in Alaska show that there was a time in the early 1970s when readings were similar to current readings. In the mid-1960s, the Earth’s core was probably rotating at a slightly slower speed than the Earth’s mantle. Since the mid seventies it has increased.
So Yang and Song see the period between 1971 and 1973 as one turning point and the period between 2009 and 2011 as another. If one resolves the missing measured values before 1964 according to known trends, one arrives at a cycle of more than seven decades.
“This multi-decade period coincides with changes in many other geophysical observations, notably day length and magnetic field,” the study authors wrote. They compared the cycles they found with the value of the length of the day in Universal Time.
Universal time is caused by the Earth’s rotation and can differ by milliseconds from the ordinary time measured by atomic clocks. The fluctuations in the length of the day correspond exactly to the fluctuations in the rotation of the Earth’s core. The researchers also found a comparable trend for changes in Earth’s magnetic field.
