  We report experimental results on the first on-sky observation of atomic spin
precession of mesospheric sodium driven by polarization modulation of a
continuous-wave laser. The 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 km and 100 km altitude. The experiment was performed at La Palma,
and the uncertainty in the measured Larmor frequency ($\approx$260 kHz)
corresponded to an error in the geomagnetic field of 0.4 mG. The results are
consistent with geomagnetic field models and with the theory of light-atom
interaction in the mesosphere.
