Altitude profiles of the photoelectron induced OD(λ6300Å) predawn enhancement by observation and theory

Emission profiles of the 6300Åline are determined from OGO 4 data in the dark ionosphere during conjugate sunrise. From Saint-Santin electron density profile measurements, it is shown that, for the two cases studied in December 1967, the recombination cannot account for the measured O¹D emission profiles. However, direct photoelectron-oxygen excitation can reproduce the data: if the photoelectron escape flux in the sunlit ionosphere, computed from standard photoelectron production, is transmitted through the field tube with an additional attenuation of 0.6 due to angular diffusion through photoelectron-electron and photoelectron-ion Coulomb collisions, the Hinteregger (1965) solar flux data must be increased by a factor 2, which agrees with previous results.