Findings: Mapping the magentic field

January 10, 2003

The magnetic field generated by the tides has been observed for the first time. The feat could help map changes in ocean flow and cast light on climate change, writes Alison Goddard.

The magnetic field is generated because salt dissolved in seawater forms electrically charged ions. As these ions are carried by the tides through the Earth's main magnetic field, their paths are deflected by the Lorentz force, causing electrical charges to build up and to short through surrounding sections of water or electrically conducting sediments.

This process generates a secondary magnetic field that was detected by Robert Tyler, a oceanographer at the University of Washington, with colleagues Stefan Maus and Hermann Lühr of the GeoForschungsZentrum Geoscience Centre at Potsdam in Germany.

Professor Tyler said: "The ocean flow generates a magnetic field and hence magnetic observations contain information about the ocean flow which might be extracted.

"Understanding ocean flow variability is a key feature in understanding climate variability.

"The problem is that the ocean is very big and is always undersampled by direct measurements.

"Methods such as satellite altimetry, which infer ocean variability from satellite data, have been hugely successful for this reason, because they give global pictures."

He said that his work implied that oceanic flow changes could be observed through studying the magnet fields generated by the tides.

Theorists had predicted that the tides would generate magnetic fields but this is the first time they have been detected.

While the ocean-generated contribution to the total magnetic field is very small, it makes a substantial contribution to the geomagnetic field at satellite altitude and accounts for much of the previously unexplained component of the Earth's magnetic field.

The team demonstrated its existence using data from the Champ minisatellite, which is used for gravity, magnetic and atmospheric studies.

Their work is reported in the journal Science this week.

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