Spectroscopy of the O I 989‐ and 7990‐Å multiplets in the dayglow and aurora

The Spectroscopy of the 2p4 ³P‐2p³3s′ ³D° (989 Å) transition of atomic oxygen and the intermediate branch transition 2p³3p ³P‐2p³3s′ ³D° (7990 Å) has been modelled for an optically thick thermosphere using Monte Carlo simulation. The model reproduces rocket observations of 989 Å in the dayglow and aurora if the e + O electron impact excitation cross section is 8.4×10−18 cm² at 25 eV. The individual 989‐Å multiplet strengths agree with dayglow observations which partially resolve the line structure. The 7990‐Å singlet to doublet ratio observed in auroras is also consistent with the model. No evidence is found for selective absorption of individual triplet and doublet lines of the 989‐Å transition through accidental resonances with rotational lines of N2. This is compatible with the latest wavelength data which indicate that the O I and N2 lines are off resonance by two Doppler widths. The 7990/989 intensity ratio in an aurora is sensitive to the abundance of atomic oxygen and the height distribution of the initial excitation rate and, to a lesser extent, to absorption by O2. Ratios of the 7990‐Å multiplets primarily depend on the O abundance with second‐order dependence on O2 absorption and the characteristic energy of the aurora. Accurate ground‐based observations of the 7990‐Å multiplet ratios may yield the atomic oxygen abundance in auroras to within a factor of 2.