Abstract
A four-fold decrease has been observed between 1960 and 1964 in the twilight brightness of the 0–1 1Δ g − 3Σ g − O 2 band as observed during the winter months. There is some indication that the brightness may depend on the monthly average sunspot number. Airborne observations of the 0-0 band, reported by Noxon and Vallance Jones, are presented in detail. The 1963 solar eclipse observations of the band obtained by Noxon and Markham are also presented in full. The calculations previously reported by Vallance Jones and Gattinger of the predicted brightness of the emission on the basis of the Hartley dissociation of ozone have been extended to cover the case of the solar eclipse. The predictions can be reconciled with the dayglow eclipse observations if the dayglow brightness before the eclipse was unusually great. The results to date suggest that the dayglow may not show the strong seasonal minimum in the summer observed for the twilight emission. Further observations made during the morning twilight confirm that the emission is much weaker that that predicted. No explanation has been found of the seasonal, annual or evening-morning variation despite the consideration of several alternative excitation hypotheses.
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