The production of Titan's ultraviolet nitrogen airglow

Abstract

[1] The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed Titan's dayside limb in the extreme ultraviolet (EUV) and far ultraviolet (FUV) on 22 June 2009 from a mean distance of 23 Titan radii. These high-quality observations reveal the same EUV and FUV emissions arising from photoelectron excitation and photofragmentation of molecular nitrogen (N2) as found on Earth. We investigate both of these solar driven processes with a terrestrial airglow model adapted to Titan and find that total predicted radiances for the two brightest N2 band systems agree with the observed peak radiances to within 5%. Using N2 densities constrained from in situ observations by the Ion Neutral Mass Spectrometer on Cassini, the altitude of the observed limb peak of the EUV and FUV emission bands is between 840 and 1060 km and generally consistent with model predictions. We find no evidence for carbon emissions in Titan's FUV airglow in contrast to previous Titan airglow studies using UVIS data. In their place, we identify several vibrational bands from the N2 Vegard-Kaplan system arising from photoelectron impact with predicted peak radiances in agreement with observations. These Titan UV airglow observations are therefore comprised of emissions arising only from solar processes on N2 with no detectable magnetospheric contribution. Weaker EUV Carroll-Yoshino N2 bands within the v′ = 3, 4, and 6 progressions between 870 and 1020 A are underpredicted by about a factor of five while the (0,1) band near 980 A is overpredicted by about a factor of three.