Model May Explain "Super-Rotation" Of Earth's Core

January 23, 1997

Geophysicists at The Johns Hopkins University have developed a model that may help explain recent findings that suggest the Earth's solid inner core rotates faster than the rest of the planet.

The model shows how this so-called "super-rotation" of the inner core could be caused by electromagnetic forces and the fluid motions of an outer, liquid core, which surrounds the inner solid core. The outer core is composed primarily of liquid iron, while the inner core is a ball of solid iron measuring 2,440 kilometers (1,500 miles) across.

The model could help to provide insights into the mysterious processes, originating deep inside the Earth, that generate the planet's magnetic field.

The Johns Hopkins geophysicists developed their model after researchers at other institutions discovered the apparent super-rotation. Those scientists inferred the super-rotation by analyzing past data from seismic waves traveling through the Earth's core. One team found that the inner core rotates roughly 1 degree per year faster than the Earth's mantle, the part of the planet that lies between the core and the crust. Another team of seismologists concluded that the inner core rotates about 3 degrees per year faster than the mantle.

If it were possible to project some location on the Earth's surface down onto the surface of the inner core and color it with a red mark, then over the years one would observe the red mark moving to the east. At a super-rotation rate of 1 degree per year, the inner core would lap the mantle within 360 years.

Edmund Halley first observed in the late 17th century that the magnetic field on the Earth's surface tends to drift westward; the drift is believed to be caused by fluid motions in the outermost regions of the liquid outer core. With the inner core seismic data, scientists now have information that the previously unobservable flow is moving in the opposite direction, to the east, and at speeds five to 15 times faster than previously thought.

Three geophysicists at Johns Hopkins have proposed a mechanism to explain the apparent super-rotation of the inner core. They have developed a model based primarily on the structure of convective motions of the fluid in the outer core. The model was created by graduate students Jonathan Aurnou and Daniel Brito, working with Professor Peter Olson, in the Department of Earth and Planetary Sciences.

Fluid moving in the southern hemisphere, just below the solid inner core, and the complementary motion of fluid in the northern hemisphere, just above the solid core, combine to speed up the inner core's rotation so that it spins slightly faster than the rest of the planet.

Olson has compared the motion of the fluid to that of a hurricane. "Like a hurricane, it creates a very strong vortex," he said.

The fluid flow has been shown in computer models produced in 1995 by Gary Glatzmaier at the Los Alamos National Laboratory and Paul Roberts at the University of California, Los Angeles.

Another key factor speeding up the inner core's rotation is the set of strong electromagnetic forces that act to lock the inner core to the surrounding outer core fluid. This strong electromagnetic "coupling" causes the inner core to rotate to the east, closely following the motion of the surrounding outer core fluid.

The Hopkins model also is providing insights into another subject of keen interest: the magnetic field at the planet's core. The Earth's main geomagnetic field is generated at the core, but scientists have little direct information about the exact method by which the magnetic field is generated. Scientists do know that the flow of the metallic fluid of the outer core reshapes and alters the Earth's magnetic field in the region around the inner core. Understanding more about the inner core super-rotation allows scientists to roughly estimate the state of the magnetic field deep within the Earth's core.

"If these recent seismic findings stand up to further scrutiny, then the super-rotation rate of the Earth's inner core may be used as a `back door' route to understanding the physical state of the deepest parts of the Earth," Aurnou said.

The Johns Hopkins model was developed after two teams of scientists discovered the apparent super-rotation. Xiadong Song and Paul Richards, scientists at the Lamont-Doherty Earth Observatory, were the researchers who concluded that the inner core rotates roughly 1 degree per year faster than the Earth's mantle. Wu-Jei Su and Adam Dziewonski at Harvard University and Raymond Jeanloz at the University of California, Berkeley, found that the inner core rotates about 3 degrees per year faster than the mantle.
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Johns Hopkins University

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