The measurement of [O I> &lgr;5577-¿ airglow intensites in the morning and evening twilights with a Fabry-Perot interferometer has revealed an enhancement with an apparent seasonal variation. The enhancement is found to emanate from the F region of the ionosphere and is well accounted for in terms of the dissociative recombination of O2+ and F region electrons. It is shown that no other mechanism for the excitation of O(1S) atoms can be expected to give rise to a significant twilight enhancement. In additon, the results of earlier ground-based studies of the twilight green line airglow by other authors are reviewed critically with reference to the present results.