Analysis of terrestrial thermospheric N 2 c ′ 4 1 Σ u + (0) ∼ b ′ 1 Σ u + (1) − X 1 Σ g + dayglow emission observed by the Far Ultraviolet Spectroscopic Explorer

Terrestrial thermospheric dayglow emission from the coupled and overlapping c ′ 4 1 Σ u + (0) and b ′ 1 Σ u + (1) levels of molecular nitrogen, observed by the Far Ultraviolet Spectroscopic Explorer, is analyzed with the aid of a coupled channels quantum mechanical model of N 2 spectroscopy and predissociation dynamics. Model emission spectra for the mixed c ′ 4 1 Σ u + (0) ∼ b ′ 1 Σ u + (1) − X 1 Σ g + ( v i = 2, 6–9) transitions, calculated for the case of excitation by photoelectron impact, are in excellent agreement with the observations. While the principal excitation mechanism for N 2 in the thermosphere is photoelectron impact, evidence is also found in other transitions of resonant fluorescence, induced by lines in the solar atomic hydrogen Lyman series, atomic oxygen transitions, and other N 2 bands. The observed emission rate of the c ′ 4 1 Σ u + (0) ∼ b ′ 1 Σ u + (1) − X 1 Σ g + (0) band is ∼1% of that inferred from the emission rates to X 1 Σ g + ( v i > 2) levels. A qualitative explanation is given for the drastically reduced intensity and band shape distortion observed in the c ′ 4 1 Σ u + (0) − X 1 Σ g + (0) emission band. Estimates of the total electron excitation rates for the nominal b ′ 1 Σ u + (1) and c ′ 4 1 Σ u + (0) levels are determined from the spectrum by extrapolating the model through regions containing unmeasured and/or resonantly absorbed bands.