Wednesday, September 18, 2013

300 Year Old Riddle Answered: What Direction Does The Solid Center Of Earth Spin

Scientists at the University of Leeds have solved a 300-year-old riddle about which direction the centre of Earth spins.

Seismic probing of the earth’s deep interior has shown that the inner core, the solid core of our planet, rotates slightly faster (i.e., eastward) than the rest of the earth. Quite independently, observations of the geomagnetic field provide evidence of westward-drifting features at the edge of the liquid outer core.  The associated electromagnetic torque currently is westward in the outermost outer core, whereas an equal and opposite torque is applied to the inner core. Decadal changes in the geomagnetic field may cause fluctuations in both these effects, consistent with recent observations of a quasi-oscillatory inner-core rotation rate.

Credit: University of Leeds

Earth's inner core, made up of solid iron, 'superrotates' in an eastward direction -- meaning it spins faster than the rest of the planet -- while the outer core, comprising mainly molten iron, spins westwards at a slower pace.

Although Edmund Halley -- who also discovered the famous comet -- showed the westward-drifting motion of Earth's geomagnetic field in 1692, it is the first time that scientists have been able to link the way the inner core spins to the behavior of the outer core. The planet behaves in this way because it is responding to Earth's geomagnetic field.

The findings, published today in Proceedings of the National Academy of Sciences, help scientists to interpret the dynamics of the core of Earth, the source of our planet's magnetic field.

In the last few decades, seismometers measuring earthquakes travelling through Earth's core have identified an eastwards, or superrotation of the solid inner core, relative to Earth's surface.

"The link is simply explained in terms of equal and opposite action," explains Dr Philip Livermore, of the School of Earth and Environment at the University of Leeds. "The magnetic field pushes eastwards on the inner core, causing it to spin faster than Earth, but it also pushes in the opposite direction in the liquid outer core, which creates a westward motion."

The solid iron inner core is about the size of the Moon. It is surrounded by the liquid outer core, an iron alloy, whose convection-driven movement generates the geomagnetic field.

The fact that Earth's internal magnetic field changes slowly, over a timescale of decades, means that the electromagnetic force responsible for pushing the inner and outer cores will itself change over time. This may explain fluctuations in the predominantly eastwards rotation of the inner core, a phenomenon reported for the last 50 years by Tkalčić et al. in a recent study published in Nature Geoscience.

Other previous research based on archeological artefacts and rocks, with ages of hundreds to thousands of years, suggests that the drift direction has not always been westwards: some periods of eastwards motion may have occurred in the last 3,000 years. Viewed within the conclusions of the new model, this suggests that the inner core may have undergone a westwards rotation in such periods.

The authors used a model of Earth's core which was run on the giant super-computer Monte Rosa, part of the Swiss National Supercomputing Centre in Lugano, Switzerland. Using a new method, they were able to simulate Earth's core with an accuracy about 100 times better than other models.

Contacts and sources:
University of Leeds

Citation: The full paper, P.W. Livermore, R. Hollerbach and A. Jackson, “Electromagnetically driven westward drift and inner-core superrotation in Earth’s core,” Proc. Natl. Acad. Sci. USA (2013), is available  This paper describes a computer model that suggests that the geomagnetic field itself may provide a link between them:

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