Diagram of Proposed Laser Magnetometer (IMAGE) University of California - Berkeley Caption To measure the Earth's magnetic field, an orange laser beam is directed at a layer of sodium 90 kilometers above the Earth. The beam is pulsed at a rate determined by the local magnetic field in order to excite spin polarization of the sodium atoms. The fluorescent emission from the sodium, which depends on the spin polarization, is detected by a ground-based telescope and analyzed to determine the strength of the magnetic field. Credit Dmitry Budker lab/UC Berkeley Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.