Abstract: The polarization of emission lines is a noteworthy
observational parameter in astronomy. However, it has
never been detected without ambiguity in planetary upper
atmospheres. Theoretical considerations have suggested that
the polarization of the thermospheric oxygen red line
(630 nm) could exist in the polar cap region. We present
here its first successful measurement at Svalbard in January
2007, during active geophysical conditions. We assign its
origin and variability to complementary effects between
permanent low-energy electron precipitation and sporadic
auroral events. Implications in physics, geophysics and
planetary science are foreseen. In physics, it raises the
question of the polarization of a forbidden transition by
electron impact which is still unknown. In geophysics, it
provides a new parameter to constrain the thermospheric
models. In planetary science, it makes it possible to derive
the local configuration of the magnetic fields. It therefore
opens new perspectives for future space missions towards
other planets.