Comparison of Titan and Earth UV Dayglow: Cascade UV Spectrum of the Lyman-Birge-Hopfield Band System Excited from N2 by Electron Impact

Far UltraViolet (FUV) dayglow emissions in the 135-165 nm wavelength range have been observed from Titan and Earth, by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Global-scale Observations of the Limb and Disk (GOLD) mission, respectively. The FUV airglow spectra of both solar system objects (SSO) contain strong emissions from the Lyman-Birge-Hopfield (LBH) band system of N₂. Thus, their FUV dayside spectra are very similar. We have isolated in the laboratory the far ultraviolet (FUV: 125.0-170.0 nm) cascade spectrum of the LBH band system of N₂ excited by 30 eV electrons and determined vibrational intensities that arise with a unique vibrational distribution, different from direct excitation. Based on the laboratory spectra we have further developed the LBH model proposed by Eastes [Eastes, JGR, 105, 18557 (1990)] for analysis of the full FUV (115-190 nm) airglow observations of Titan's and Earth's atmosphere, including both direct excitation and cascading. The cascading transition arises from the (aʹ and w → a → X ¹ Σ⁺_g ) states, by two processes: radiative and Collision-Induced-Electronic Transitions (CIET). The past studies of the LBH band system in the laboratory have failed to measure and to provide a model of the LBH cascade spectrum. The observed vibrational populations for low vibrational levels from v'=0-2 of the a-state are enhanced by CIET on both Earth and Titan at low thermosphere altitudes from our preliminary studies as found in the laboratory for N₂ gas. Our recent laboratory work and Titan dayglow analysis shows cascading must be included in studies of the Titan dayglow as it can provide up to one-half the observed dayglow intensity. These new results are important for the Cassini mission; the UV signature of nitrogen and its dissociation products are the principal means of studying the chemistry, composition, atmospheric escape and energy input to Titan's thermosphere, the ultimate source of the lower atmospheric haze and surface composition. The strength of the radiative-CIET cascade process low v' enhancement is expected to increase with gas pressure, because of the CIET process, a secondary near-resonance collision. The cross section variation with pressure and energy is an input into state-of-the-art electron transport models for accurate dayglow and aurora analysis.