I keep reading about how every so often, Earth鈥檚 north and south magnetic poles 鈥渇lip鈥�. How long does it take for this to happen? Moments or years? And does the magnetosphere 鈥渢urn off鈥� when this change is taking place? Or is it more like turning a bar magnet through 180 degrees, with the poles moving across the planet鈥檚 surface and passing through the equator?
鈥� Reversals occur once every 300,000 years on average and it takes a few thousand years for the poles to switch places each time. Although the field diminishes, it never falls to zero. The last reversal took place 780,000 years ago, so perhaps the next one is somewhat overdue.
Earth鈥檚 magnetic field is generated because the planet behaves like a self-exciting dynamo. As the molten outer core freezes to become part of the solid inner core, latent heat is released, which drives convection currents in the outer core. Because the core is metallic, it contains delocalised electrons. The convection currents and Earth鈥檚 spin cause these electrons to move, creating electrical currents and generating a magnetic field.
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The magnetometer 鈥� which measures the strength and direction of magnetic fields 鈥� was only invented in 1832 by Carl Friedrich Gauss. So, in order to gauge Earth鈥檚 magnetic field longer ago than this, David Gubbins of the University of Leeds, UK, looked at ships鈥� logs kept by early ocean-going explorers. Navigating using a compass and sextant, these seafarers measured and recorded magnetic declination 鈥� the angle between true north and magnetic north. This allowed the strength of Earth鈥檚 field to be reconstructed and shows that it is about 10 per cent weaker than it was in 1860, suggesting we might be heading for a reversal.
In a further bid to understand what is going on, Daniel Lathrop and his team at the University of Maryland have constructed a spinning sphere of molten sodium called the . They are hoping to show that it acts as a dynamo, generating its own magnetic field. This continues a tradition begun by William Gilbert, who built a 鈥渢errella鈥�, a sphere made from a magnetic lodestone, to model Earth鈥檚 field. His work, published in 1600, was not surpassed for two centuries.
Mike Follows, Sutton Coldfield, West Midlands, UK
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This article appeared in print under the headline 鈥淩evolving core鈥�