Mapping the Earth’s magnetic field

Current astronomy technology could be used to more accurately measure the Earth’s magnetic field, physicists have found. This development could prove useful in tracking storms, finding oil and minerals, or studying the planet’s interior.

A group of scientists from the University of California has discovered a more effective way to measure the Earth’s magnetic field using ground-based lasers rather than satellites. They found that the same sodium atoms in the atmosphere used in laser guide stars – artificial stars used as a reference point to adjust telescope optics to remove atmospheric disturbances – could be employed to measure the Earth’s magnetic field. The findings appear in Proceedings of the National Academy of Sciences, March 1, 2011 (doi: 10.1073/pnas.1013641108).

The new method involves emitting an orange beam from a ground-based laser into the mesosphere, located about 90 km above the Earth’s surface, where there is a 10-km-thick layer of sodium atoms. The beam from the laser, which is small enough to load onto a truck or boat, is used to measure the rotation frequency of the sodium atoms, which are used as sensors to measure the strength of the Earth’s magnetic field.

To measure the Earth’s magnetic field, an orange laser beam is directed at a layer of sodium 90 km above the Earth. The beam is pulsed at a rate determined by the local magnetic field to excite spin polarization of the sodium atoms, producing a fluorescence emission. This is detected by a ground-based telescope and analyzed to determine the strength of the magnetic field. Courtesy of Dmitry Budker lab, UC Berkeley.

Less expensive than a satellite system, this method could lead to other ground-based laser techniques. It allows scientists to take measurements without the spatial and temporal dependence associated with satellite movement.

Satellites used currently to measure the Earth’s magnetic field are moving at high speeds, so it is not always possible to tell whether fluctuations are actual or a result of the spacecraft’s movement to a new location. In addition, metals and electronic instruments aboard the satellite could affect magnetic field measurements. Ground-based remote sensing systems, on the other hand, eliminate both problems.

Creating maps of the Earth’s magnetic field is important for geology, studies of currents in the atmosphere and oceans, climatology, and oil and mineral exploration. The group will continue its studies of how spin-polarized sodium atoms emit and absorb light, and it is building a 20-W modulated laser for the Very Large Telescope in Chile to test its theories.