Analysis of terrestrial thermospheric N₂ c₄′; ¹Σ_u⁺(0) ∼ b′ ¹Σ_u⁺(1) - X ¹Σ _g⁺ dayglow emission observed by the far ultraviolet spectroscopic explorer

Terrestrial thermospheric dayglow emission from the coupled and overlapping c₄′ ¹Σ_u⁺-(0) and b′ ¹Σ_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 N₂ spectroscopy and predissociation dynamics. Model emission spectra for the mixed c₄′ ¹Σ_u¹(0) ∼ b′ ¹Σ_u⁺(1) - X¹ 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 N² bands.. The observed emission rate of the C₄′ ¹Σ_u¹(0) ∼ b′ ¹Σ_u⁺(1) - X ¹Σ _g⁺(0) band is ∼1% of that inferred from the emission rates to X ¹Σ_g⁺(v_i > 2) levels. A qualitative explanation is given for the drastically reduced intensity and band shape distortion observed in the C₄′ ¹Σ_u¹(0) - X ¹Σ _g⁺(0) emission band. Estimates of the total electron excitation rates for the nominal b′ ¹Σ_u ⁺(1) and c₄′ ¹Σ_u ⁺(0) levels are determined from the spectrum by extrapolating the model through regions containing unmeasured and/or resonantly absorbed bands.