Comparison of Planetary UV Dayglow: Analysis of Titan-Cassini and Earth-GOLD UV Airglow Observations By Laboratory Spectroscopy of N2

The far ultraviolet (FUV) dayglow emissions have been observed from Titan and Earth by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and Global-scale Observations of the Limb and Disk (GOLD), respectively, operating in the same wavelength range between 135–165 nm. Both Solar System Objects (SSO) are fascinating environments where N₂ is the most abundant thermosphere-ionosphere (T-I) species.  They both exhibit similar UV spectra.  The UV spectra consist of strong emissions from the Lyman-Birge-Hopfield (LBH) band system a ¹Π_g → X ¹Σ_g⁺ of N₂ excited from photoelectrons. The dayglow’s of both SSOs are most intense at its own ionosphere altitude [900 km at Titan vs 120-150 km at Earth] with similar thermodynamic conditions for atmospheric N₂ density [10¹⁰-10¹¹ cm^-3]. Titan and Earth have vastly different T-I temperatures (150 K vs ~700K) and the presence of minor constituents [ CH₄ on Titan vs O, O₂ on Earth]. The FUV dayglow spectra of both SSOs are shown over plotted in Fig. 1. Titan’s nitrogen dominated atmosphere is the densest of any moon in the solar system and Earth’s nitrogen dominated atmosphere has the densest N₂ content of any planet in the solar system. We propose to present a new model analysis of the full FUV (115–190 nm) disk airglow observations of Titan’s and Earth’s atmosphere based on UV laboratory optically thin electron impact fluorescence studies of the LBH band system. The experiments will be performed at SSO atmospheric T-I density and temperature that must occur to accurately determine the relevant emission cross sections. The experimental apparatus at the University of Colorado  has installed a new high energy resolution electron gun to study the threshold energy (10-30 eV) behavior of the LBH band system, where photoelectron distribution functions peak in energy. 

This work is under contract to the University of Colorado by NSF Aeronomy and NASA Cassini Data Analysis Program Offices.

Fig. 1 A comparison of GOLD (2019) and Cassini UVIS (2009)

FUV dayglow spectra between 136-155 nm, both containing the LBH bands.

The rotational temperatures of both SSO are very different and represent

the upper atmosphere T-I kinetics temperature.