Analysis of terrestrial thermospheric N2c′41Σu+(0) ∼ b′ 1Σu+(1) − X1Σg+ dayglow emission observed by the Far Ultraviolet Spectroscopic Explorer

Abstract

Terrestrial thermospheric dayglow emission from the coupled and overlapping c′41Σu+(0) and b′ 1Σu+(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 c′41Σu+(0) ∼ b′ 1Σu+(1) − X1Σg+(vi = 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 N2 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 c′41Σu+(0) ∼ b′ 1Σu+(1) − X1Σg+(0) band is ∼1% of that inferred from the emission rates to X1Σg+(vi > 2) levels. A qualitative explanation is given for the drastically reduced intensity and band shape distortion observed in the c′41Σu+(0) − X1Σg+(0) emission band. Estimates of the total electron excitation rates for the nominal b′ 1Σu+(1) and c′41Σu+(0) levels are determined from the spectrum by extrapolating the model through regions containing unmeasured and/or resonantly absorbed bands.