Atomic and molecular emissions in the middle ultraviolet dayglow

Abstract

Dayglow spectra in the middle ultraviolet, obtained during a sounding rocket flight from White Sands Missile Range in 1992, have been analyzed to determine the altitude distributions of thermospheric atomic and molecular species and to address a number of problems related to airglow excitation mechanisms. Among the atomic and molecular profiles retrieved are the N2 second positive, N2 Vegard‐Kaplan and NO gamma band systems, and the OI 297.2 nm, OII 247.0 nm, and NII 214.3 nm emissions. A self‐consistent study of the emission profiles was conducted by comparing observed intensities with one another and to forward models. Model photoelectron and photon fluxes were generated by the field line interhemispheric plasma model (FLIP) and two solar flux models. Neutral densities were obtained from mass‐spectrometer/incoherent scatter (MSIS)‐90. The results from the data analysis suggest that the major species' densities are within 40% of MSIS values. Evidence for the accuracy of the modeled densities and fluxes is seen in the close agreement between the calculated and observed intensities of the N2 second positive emission. Analysis of the OI 297.2 nm emission shows that the reaction N2(A) + O is the dominant source of O(1S) in the daytime thermosphere. The data imply that the vibrationally averaged yield of 0(1S) from the reaction is 0.43±0.12, which is smaller than the laboratory value measured for the N2(A, v′ = O) level. The cause of a disagreement between model and data for the Vegard‐Kaplan emission is unclear, but the discrepancy can be eliminated if the N2(A)+O quenching coefficient or the A state lifetime is increased by a factor between 2 and 4. The observed intensity of OII 247.0 nm is greater than expected by a factor of 2, implying possible inadequacies in the EUVAC and/or EUV91 solar models used in the analysis.