Molecular nitrogen Carroll-Yoshino v′=0 emission in the thermospheric dayglow as seen by the Far Ultraviolet Spectroscopic Explorer

Molecular nitrogen (N-2) Carroll- Yoshino excitation is of considerable interest in the analysis of EUV- FUV observations obtained from several solar system objects (Earth, Titan, and Triton). The fundamental c4 '(0, 0) band at 958 angstrom is a strong transition excited by photoelectron impact and hence is bright under optically thin conditions. On Earth c4 '(0, 0) is optically thick near the altitude of peak photoelectron excitation and the emergent (0, 0) emission is weak. Past analyses of EUV airglow spectra focused on the roles of (0, v '' <= 2) photon redistribution and predissociation enhancement resulting from multiple scattering in accounting for reduced emission. The role of (0, v '' > 2) fluorescence was deemed minor. Now, high- resolution dayglow measurements by the Far Ultraviolet Spectroscopic Explorer (FUSE) made in September 1999 reveal c4 '(0, v '' = 6 - 9) fluorescence bands much brighter than expected. The results of an analysis of FUSE dayside disk measurements are presented, and the observed (0, v '') emission is discussed in the context of results from a multiple scattering model. The c4 '(0, 9) band in particular stands out as a potentially useful remote sensing observable.