Laser Remote Magnetometry Using Mesospheric Sodium

We have demonstrated a remote magnetometer based on sodium atoms in the Earth's mesosphere, at a 106-km distance from our instrument. A 1.33-watt laser illuminated the atoms, and the magnetic field was inferred from backscattered light collected by a telescope with a 1.55-m-diameter aperture. We theoretically predict a shot noise limited measurement sensitivity of 19 nT/√Hz. The measured sensitivity was 162 nT/√Hz due to a smaller returned intensity and smaller resonance strength than expected. The value of magnetic field inferred from our measurement is consistent with several models of the Earth's field shape to within a fraction of a percent. Projected improvements in optics, plus the use of advanced lasers or a large telescope, could result in 1-nT/√Hz sensitivity.