The world turns slightly faster
and slower on a regular 5.9-year cycle, a new study suggests. Researchers also
found small speed changes that happen at the same time as sudden alterations in
Earth’s magnetic field.
The
world’s rotation speed can change slightly, by up to milliseconds per day,
because of shifts in winds or the movement of fluid in Earth’s interior.
Scientists can measure how fast the Earth spins by observing distant objects in
space and timing how long they take to come back into view — that is one day length.
The new
study, published in the July 11 Nature, found trends in
day length after subtracting the effect of weather, allowing researchers to
home in on the effect of Earth’s fluid core.
Scientists
have previously found hints of six-year oscillations in day length, which occur
at the same time as larger, slower changes. But the new analysis revealed that
the cycle is remarkably regular, with the maximum change in day length
occurring once every 5.9 years. Using decades’ worth of data, the researchers
found that the oscillations maintained this precise timing and strength for
half a century. “That’s got to be saying something important,” says
geophysicist Bruce Buffett of the University of California, Berkeley, who was
not involved in the study. It’s too early to say exactly what causes the
oscillations, he adds.
This
regularity undercuts one hypothesis for the cause of the cycles: fluctuations
in the sun’s energy, which are more variable, says study author Richard Holme
of the University of Liverpool in England. Instead, the cycle must be caused by
something inside the Earth.
Holme’s
team also detected sudden, tiny increases and decreases in the Earth’s rotation
speed that coincided with abrupt changes in the behavior of Earth’s magnetic
field, known as geomagnetic “jerks.” The new day-length data could help
scientists understand what causes the mysterious jerks, Buffett says.
Along
with hinting at what’s going on in Earth’s core, the research may help improve
geomagnetic forecasts, which are crucial in mining exploration and drilling.
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