THE tiny magnetic fields created by ocean tides have been mapped globally for the first time. If the technique can be refined, it could provide a way to track ocean currents and reveal how they are influenced by, or could trigger, climate change.
The Earth’s magnetic field is created mostly by the flow of its molten iron core. But it is also affected by other factors, including the flow of the oceans. The salts dissolved in seawater exist as ions that carry either positive or negative charge, such as sodium and chloride. As water flows around the planet, the Earth’s magnetic field tends to move positive ions one way and negative ions the other. This gives rise to volumes of water in which there is a slight excess of either positive or negative ions. As these patches of charge move back towards each other they form electric currents, which create magnetic fields themselves.
Robert Tyler and his team of oceanographers at the University of Washington in Seattle looked at magnetic field data recorded by satellite to see if they could pick out the minuscule fields generated by the flow of water. Although they are only 1/6000th the strength of the Earth’s overall magnetic field, the researchers found that by focusing on changes that coincided with tidal frequencies they could isolate the oceans’ fields (Science, vol 299, p 239).
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Identifying non-tidal magnetic fields in the oceans will be much tougher, since they don’t have a steady frequency that can help to pick them out. “We’re not going to know if it’s possible until we try,” says Michael Purucker, a geophysicist at NASA’s Goddard Space Flight Center in Maryland.
But Tyler thinks it is theoretically possible. If he’s right, the technique would give oceanographers a valuable way of tracking movements of the oceans, such as deep water currents, that can’t be tracked otherwise.