Analysis of Titan airglow UV spectra from Cassini Ultraviolet Imaging Spectrograph (UVIS)

We present the analysis of FUV (1150-1750 Å) limb dayglow spectra of
Titan’s atmosphere obtained on 13 December 2004 at 5 Å resolution by the Ultraviolet
Imaging Spectrograph (UVIS) onboard Cassini. The fit to the data show that Titan’s
airglow consists of four principal emissions: 1) the N2 Lyman-Birge-Hopfield (LBH)
band system, which peaks in intensity at 1150 +/- 50 km, 2) N I multiplets peaking
at 1050km +/- 50 km, 3) sunlight reflected by N2 between 0 and 300 km and 4) H Ly
alpha which grows in intensity with increasing altitude. Comparisons with limb spectra
obtained by the Voyager 1 Ultraviolet spectrometer (V1/UVS) show that the vertically
integrated brightness are larger for V1/UVS than for UVIS by a factor of 3, consistent
with the XUV solar flux ratio at Titan at the time of these observations. The N2 LBH
and N I profiles obtained from the regression to the Titan data are compared to models
obtained by the Atmospheric Ultraviolet Radiance Integrated Code (AURIC), adapted
from Earth’s atmosphere to Titan’s.