British scientists have answered the question about which direction the centre of our planet spins. The 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 showed the westward-drifting motion of the 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 the Earth’s geomagnetic field.

In the last few decades, seismometers measuring earthquakes traveling through the 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. The magnetic field pushes eastwards on the inner core, causing it to spin faster than the Earth, but it also pushes in the opposite direction in the liquid outer core, which creates a westward motion,” said Dr Philip Livermore from the University of Leeds, who is a lead author of a study published in the Proceedings of the National Academy of Sciences.

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 the 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.

Other previous research based on archeological artifacts 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.

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Bibliographic information: Philip W. Livermore et al. Electromagnetically driven westward drift and inner-core superrotation in Earth’s core. PNAS, published online September 16, 2013; doi: 10.1073/pnas.1307825110