Eddies in the deep Earth

Blips in the Earth's magnetic field are windows to the planet's interior

The flow of molten material in our planet’s outer core is the prime source of Earth’s magnetic field. That flow can fluctuate rapidly over large areas, recent data suggest, so geophysical models that estimate how the magnetic field evolves over time should account for such variations.

Earth’s outer core, a molten mix of iron and other metals that is no more viscous than water, flows at an average speed of about 20 kilometers per year. Because that material contains charged particles, its flow produces the planet’s magnetic field, says Nils Olsen, a geophysicist at the DanishNationalSpaceCenter in Copenhagen.

Most large-scale variations in Earth’s magnetic field take place over long periods. Data gathered by satellites since 1999, however, as well as measurements taken by ground-based observatories, indicate that changes in the magnetic field’s strength can take place over small areas at a rather quick pace — twitches that researchers have dubbed geomagnetic jerks.

Instruments have detected two such jerks in the past decade. In one, which occurred in mid-2003, changes in the magnetic field were most evident in southern Asia, says Olsen. Variations noted during the other, in late 2004, were strongest in the region stretching from the South Atlantic to the southwestern Indian Ocean. The rate of field change in each case measured, at its peak, just 0.004 percent per year, he notes.

These small blips nevertheless betray large and rapid changes in the flow of molten material along the surface of the outer core, Olsen and Mioara Mandea, a geophysicist at the NationalResearchCenter for Geosciences in Potsdam, Germany, report online May 18 in Nature Geoscience.

Each of the flow changes inferred from the magnetic field data covered less than a quarter of the Earth’s surface and lasted no more than a few months. Such changes in the flow of outer core material are akin to ocean eddies, says Olsen. The new findings suggest that geophysical models used to simulate variations in the planet’s magnetic field strength should include the possibility of short-term changes in the flow of core material.

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