By LEE BOWMAN
Scripps Howard News Service
August 26, 2005
The researchers first claimed nine years ago that the solid inner core was spinning faster than the outer part of the planet, but they were met with challenges by other seismologists who contended that their measurements had been mistaken.
"Extraordinary claims require extraordinary proof," said Xiadong Song, a professor of geology at the University of Illinois at Urbana-Champaign and a co-author of a study published Friday in the journal Science. 'We believe we have that proof."
Song and Paul Richards, a seismologist at Lamont-Doherty Earth Observatory of Columbia University, decided that to confirm the core was really spinning faster, they needed data from virtually identical earthquakes that occurred at different times.
They came up with 17 such similar sets of seismic waves - called waveform doublets - that passed through Earth from the South Sandwich Islands region of the South Atlantic to a recording station on the other side of the world in Alaska. The doublets, recorded at up to 58 seismic stations in and near Alaska and as much as 35 years apart, allowed the researchers to detect changes over time along their paths through Earth.
"The similar seismic waves that passed through the inner core show systematic changes in travel times and wave shapes when the two events of the doublet are separated in time by several years," Song said. "The only plausible explanation is a motion of the inner core."
Earth's iron core consists of a solid inner ball about 1,500 miles in diameter and a fluid outer layer about 4,350 miles in diameter. The inner ball is at the center of the geodynamo that generates Earth's magnetic field, and the electromagnetic torque from this force is thought to drive that solid core's slightly greater rotational speed.
The researchers calculate that the inner core is rotating about 0.3 to 0.5 degrees faster a year than the outer layers of the planet, which means that it takes the solid inner core about 900 years to get a full revolution ahead of the planet's outer areas.
The most likely explanation for why the inner core is rotating at a different speed, Song said, is electromagnetic coupling.
"The magnetic field generated in the outer core diffuses into the inner core, where it generates an electric current. The interaction of that electric current with the magnetic field causes the inner core to spin, like the armature (rotor) in an electric motor," he explained.
The research, funded by the National Science Foundations of the United States and China, promise better understanding of Earth's inner structures that may not only help explain how the planet formed and evolved, but could also answer other questions about long-term changes in the planet's spin and tilt that affected the length of a day on Earth or the onset of ice ages.
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