Economical Magnetic Field Mapping

Mapping the Earth"s magnetic field – to find oil, track storms or probe the planet"s interior – typically requires expensive satellites.

University of California, Berkeley, physicists have now come up with a much cheaper way to measure the Earth"s magnetic field using only a ground-based laser.

Exciting Sodium Atom in the Atmosphere

The method involves exciting sodium atoms in a layer 90 kilometers (60 miles) above the surface and measuring the light they give off.

"Normally, the laser makes the sodium atom fluoresce," said Dmitry Budker, UC Berkeley professor of physics. "But if you modulate the laser light, when the modulation frequency matches the spin precession of the sodium atoms, the brightness of the spot changes."

Interactions with Earth"s Magnetic Field

Because the local magnetic field determines the frequency at which the atoms precess, this allows someone with a ground-based laser to map the magnetic field anywhere on Earth.

Budker and three current and former members of his laboratory, as well as colleagues with the European Southern Observatory (ESO), lay out their technique in a paper appearing online in the journal Proceedings of the National Academy of Sciences.

Various satellites, ranging from the Geostationary Operational Environmental Satellites, or GOES, to an upcoming European mission called SWARM, carry instruments to measure the Earth"s magnetic field, providing data to companies searching for oil or minerals, climatologists tracking currents in the atmosphere and oceans, geophysicists studying the planet"s interior and scientists tracking space weather.

Better than Satellite Data

Ground-based measurements, however, can avoid several problems associated with satellites, Budker said. Because these spacecraft are moving at high speed, it"s not always possible to tell whether a fluctuation in the magnetic field strength is real or a result of the spacecraft having moved to a new location. Also, metal and electronic instruments aboard the craft can affect magnetic field measurements.

"A ground-based remote sensing system allows you to measure when and where you want and avoids problems of spatial and temporal dependence caused by satellite movement," he said. "Initially, this is going to be competitive with the best satellite measurements, but it could be improved drastically."

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