Sunlit aurora and the N2+ ion: a personal perspective

After a brief autobiographical summary, a description is given of observations of sunlit aurora and the corresponding twilight and dayglow effects, starting with the classic work of Stّrmer, which showed the phenomenon to extend as high as 1100 km. Spectroscopic studies found that the unusual intensity is caused by resonant scattering of sunlight by the N2+ ion, and that this ion has a very short lifetime at altitudes below 160 km. At these altitudes an ion-atom reaction with the oxygen atom converts it to NO+. Independent work by others led to the suggestion that the same reaction is important in production of the ionosphere. The great extent of the sunlit aurora to high altitudes can only be understood if the ions are transported upwards from their site of production at lower altitudes. More recent observations from rockets, satellites, and the Space Shuttle have confirmed and extended the picture obtained from ground-based remote sensing.