WINDII/UARS observation of twilight behaviour of the hydroxyl airglow, at mid-latitude equinox

Measurements are presented of the height profile of the volume emission rate of the P₁(3) line of the (8-3) band of the hydroxyl airglow at latitudes of 40° N and 40° S as observed with the WINDII instrument on board the UARS satellite during the northern spring equinox in 1993. The emission layer peaks near 88 km during most of the night, with a half width of 6–8 km. The profile is slightly asymmetric with a more rapid decrease on the bottom side. During the early hours after sunset, the volume emission rate on the bottom side of the layer appears to decay exponentially with a time constant which varies from 1.1 h at 78 km to 2.9 h at 82 km. These decay rates are faster by a factor of three or four than those expected on the basis of the removal of atomic oxygen by three-body recombination. As a result it is concluded that the observed decay results from a combination of tidal modulation of the emission rate along with chemical removal of atomic oxygen.