Journal Club

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Earth’s contrasting inner core rotation and magnetic field rotation linked

For more than three centuries, analysis of Earth’s magnetic field has revealed it is drifting westward along with the Earth’s liquid outer core, rotating slower than the rest of the planet. At the same time, seismic probes of the Earth’s deep interior over the past few decades have suggested that its inner solid core spins a bit faster — that is, eastward — than the rest of the planet. Now a model of the Earth’s core run on a supercomputer hints these phenomena might be linked, report findings in the Proceedings of the National Academy of Sciences.

Earth’s magnetic field results from the planet’s dynamo, which arises from churning in Earth’s liquid outer core. The magnetic field’s slow westward drift is one of its best- and longest-known features, first revealed in 1692 by Edmund Halley, who also discovered the famous comet. Past analysis of archaeological artifacts and ancient rocks suggested the field has not always drifted westward, but has also strayed eastward over the past 3,000 years.

Many potential explanations for this drift involved flows of material in the fluid outer core known as thermal winds, but these are tied to the lowermost mantle of the Earth, which only changes on a time scale of 10 million to 100 million years, too slow to explain the relatively quick changes other scientists have seen in the direction of the drift.

A seemingly unrelated phenomenon is the inner core’s apparent superrotation — the assumption that the inner core, which is about the size of Earth’s moon, whirls slightly faster than the rest of the planet. Now, after modeling the Earth’s magnetic field on the supercomputer Monte Rosa in Switzerland, geophysicist Philip Livermore at the University of Leeds in England and his colleagues suggest the inner core and outer core are linked by the Earth’s magnetic field.

“You can think of this link in terms of equal and opposite action,” Livermore says. “The magnetic field pushes eastwards on the inner core, causing it to rotate faster than the rest of Earth. However, it also pushes in the opposite direction on the liquid outer core, making it drift westward.”

Over the course of decades and longer, changes occur in the Earth’s magnetic field, which may cause fluctuations of this proposed torque. This could help explain recent observations suggesting variations in the rate at which the inner core rotates from 1961 to 2007.

Categories: Earth, Atmospheric, and Planetary Sciences
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