SEAFARERS鈥� records dating back to 1590 have made it possible to visualise the behaviour of one of the planet鈥檚 most mysterious phenomena 鈥� its magnetic field. Watching how patterns have changed over 400 years may reveal what drives the field and how it might change in the future.
The Earth鈥檚 magnetic field provides us and other animals with a compass, as well as protecting all life from cosmic rays. But despite its importance, we do not understand what processes drive its changing patterns, including the catastrophic flips that seem to occur every few million years. Within the past century the field has weakened by around 10 per cent, prompting fears that we are about to experience the next flip, but there is currently no way of knowing whether this is likely to happen or not.
Satellite data gives a snapshot of what the field looks like today: little blips are superimposed on the background field that at present points to the north. And although records from sedimentary rocks and lava flows show how the overall field has changed over thousands or millions of years, we don鈥檛 know how the blips behave over timescales of tens or hundreds of years. Watching the blips in motion should provide clues to what is causing them.
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So Christopher Finlay, Andrew Jackson and their colleagues from the University of Leeds in the UK pieced together a mammoth jigsaw puzzle. They raided historical archives for magnetic readings from ships dating back to 1590, as well as collecting more modern magnetic surveys and satellite data. Putting all the data together gave them a record of the magnetic field at the Earth鈥檚 surface, stretching back 400 years.
They used computer models of the planet鈥檚 structure to zoom below the surface, calculating the field through the mantle to the surface of the Earth鈥檚 core, where the field is generated. Then they subtracted the background north-south field and removed long-standing variations to give a movie of the short-term deviations (see Graphic).
While most of the field was pretty quiet, they saw a region of particularly intense spots underneath the Atlantic Ocean, migrating westwards near the equator at about 17 kilometres per year. The anomaly lasted for the entire 400-year period. 鈥淲e were surprised that it was concentrated at the equator and that it was so persistent for the 400 years of study,鈥� says Finlay. He says he had expected to see more evenly distributed blips appearing and disappearing rapidly.
The finding could be vital for working out what is driving the blips. One hypothesis to describe the changing field patterns involves temperature differences in the Earth鈥檚 lowermost mantle. When tectonic plates collide, pieces of a plate may break off and fall down to the bottom of the mantle. These bits of plate would create cold spots at the boundary between the mantle and the core, and the resulting temperature differences could cause the liquid iron core to flow. 鈥淭he movement of the liquid iron would drag the magnetic field lines along with it,鈥� says Finlay.
Another idea involves a magnetic wave in the liquid core. A mixture of magnetic and buoyancy forces, together with the Earth鈥檚 spin, could drive variations in the field like a wave. Unlike the temperature difference hypothesis, this idea does not require the physical flow of the liquid core. 鈥淚t is similar to watching the ripples from a stone dropped into a pond,鈥� explains Finlay. 鈥淭he water molecules just pass on the ripple, rather than travelling with the ripples.鈥�
Finlay believes that both mechanisms are probably involved, and he now plans to plug the results of the study into computer models that simulate different combinations of processes, to see which fits best.