UC Berkeley physicists propose laser-based method to map Earth's magnetic field

Feb. 21, 2011
Berkeley, CA--UC Berkeley physicists have devised a way to measure the Earth's magnetic field using only a ground-based laser, rather than expensive satellites.

Berkeley, CA--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. The method involves exciting sodium atoms in a layer 90 km (60 miles) above the surface--as is done for laser guide star applications--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." 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 this week in the journal Proceedings of the National Academy of Sciences at www.pnas.org/content/early/2011/02/07/1013641108.abstract.

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. 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.

The idea was sparked by a discussion Budker had with a colleague about of the lasers used by many modern telescopes to remove the twinkle from stars caused by atmospheric disturbance. That technique, called laser guide star adaptive optics, uses lasers to excite sodium atoms deposited in the upper atmosphere by meteorites. Once excited, the atoms fluoresce, emitting light that mimics a real star. Telescopes with such a laser guide star, including the Very Large Telescope in Chile and the Keck telescopes in Hawaii, adjust their "rubber mirrors" to cancel the laser guide star’s jiggle, and thus remove the jiggle for all nearby stars. In practice, a 20-50 W laser tuned to the orange sodium line (589 nm wavelength) would shine polarized light into the 10 km thick (approximately five miles) sodium layer in the mesosphere, which is about 90 km overhead. William Happer, a physicist who pioneered spin-polarized spectroscopy and the sodium laser guide stars, had thought of the idea, but had never published it.

The work was supported by NGA NURI, the University Research Initiatives program of the National Geospatial-Intelligence Agency, which is part of the U.S. Department of Defense.

SOURCE: UC Berkeley; http://newscenter.berkeley.edu/2011/02/14/lasers-vie-with-satellites-to-map-earth%e2%80%99s-magnetic-field/

Posted by:Gail Overton

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