Of molten iron and magnetism
by Carolyn Gramling Thursday January 5th, 2012
Since 1999, the German satellite CHAMP (CHallenging Mini-satellite Payload) has been swirling around the Earth, monitoring the growth and decline of the planet’s magnetic field over time. FIELD’s continuous measurements of the earth’s field have created a finely detailed picture of how the field changes in both space and time – and by extension, how the movement of molten iron in the Earth’s outer core Earth comes and goes. And with this data, the researchers report, they can now track even rapid, small-scale fluctuations in the strength of the field around the planet.
Molten iron moving through the Earth’s core generates the planet’s magnetic field, which protects the planet against solar flares and other space weather. The strength of the Earth’s magnetic field is known to fluctuate over very long time scales; on rare occasions, the field has even reversed polarity, in which the magnetic north pole and magnetic south pole reverse. (The last such reversal was around 780,000 years ago.)
The nine years of continuous magnetic field data from CHAMP and the Ørsted and SAC-C satellites paint a startling picture of small-scale fluctuations in the field, report researchers Nils Olsen of the Danish National Space Center in Copenhagen and Mioara Mandea of the GFZ. Potsdam in Germany on May 18 in Nature Geoscience. The strongest magnetic shifts, they found, are not global, but are in the southern hemisphere, spanning the Atlantic and Indian oceans.
The changes in fluid motion that are responsible for the field fluctuations occur right at the boundary between the core and the mantle, about 2,900 kilometers below the Earth’s crust, the researchers say. With the new model, the researchers followed the movement of this fluid deep inside the Earth: for example, the model showed that the flow accelerated southward under Asia in May 2003, then took a sharp westerly turn across the Indian Ocean before weakening.
The causes of these short-term local fluctuations are not yet clear, they say. But thanks to these satellite observations, scientists now have a new window into motion in the core – and what drives Earth’s geodynamo.