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

Abstract

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 N2 (b 1∏u, b′ 1∑u+, c4′ 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 N2 c4′ 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 c4′ 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.