Titan airglow spectra from Cassini Ultraviolet Imaging Spectrograph (UVIS): EUV analysis

We present the first UV airglow observations of Titan's atmosphere by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini. Using one spectral channel in the EUV from 561–1182 Å and one in the FUV from 1115–1913 Å, UVIS observed the disk on 13 December, 2004 at low solar activity. The EUV spectrum consists of three band systems of N 2 (b 1 ∏ u , b′ 1 ∑ u + , c 4 ′ 1 ∑ u + → X 1 ∑ g + ), while the FUV spectrum consists of one (a 1 ∏ g → X 1 ∑ g + ). Both the EUV and FUV spectra contain many N I and N II multiplets that are produced primarily by photodissociative ionization. Spectral intensities of the N 2 c 4 ′ 1 ∑ u + (v′ = 0) → X 1 ∑ g + (v″ = 0–2) progression from 950–1010 Å are resolved for the first time. The UVIS observations reveal that the c 4 ′ 1 ∑ u + (0) → X 1 ∑ g + (0) vibrational band near 958 Å is weak and undetectable, and that N I multiplets near 953.2 and 964.5 Å are present instead. Magnetospheric particle excitation may be weak or sporadic, since the nightside EUV spectrum on this orbit shows no observable nitrogen emission features and only H Ly‐ β .