Polarization-driven spin precession of mesospheric sodium atoms

We report experimental on-sky observations of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. A magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 and 100 km altitude. The experiment was performed at La Palma, and the uncertainty (0.2 kHz) in the measured Larmor frequency (≈260 kHz) corresponded to an error in the geomagnetic field of 0.3 mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere.