EUV emission from Titan's upper atmosphere: Voyager 1 encounter

Abstract

Analysis of Titan's EUV emission spectra obtained at the Voyager 1 encounter demonstrates that electron impact on N2 above 3600 km accounts for the bulk of the observed emission short of Lyman α. In conjunction with the UVS solar occultation data it is concluded that N2 is the major component of Titan's upper atmosphere, with upper limit mixing ratios at 3900 km on NeI, ArI, CO, H2, and HI of 0.01, 0.06, 0.05, 0.06, and 0.1, respectively. Magnetospheric electrons interact with Titan's sunlit hemisphere to produce a power dissipation rate of ≃2 × 109 W in the exosphere and ≃3 × 109 W below the exobase, with optical signatures from numerous N2 bands, NI, and NII multiplets. The N2 C4′ (0‐0) Rydberg band at 958 Å acts as an optical probe of Titan's exosphere because of transmission losses caused by fluorescence and predissociation. Magnetospheric electron precipitation produces an average dayside electron density of ≃3 × 10³ cm−3 between 3600 and 4000 km, the region of bright limb emission. When Titan is within Saturn's magnetosphere, magnetospheric electron impact dissociation of N2 generates an N atom escape rate of ≃3 × 1026 s−1 from Titan's exosphere. A nonthermal H atom escape rate of ≃2 × 1026 s−1 is estimated from magnetospheric electron impact ionization of N2 followed by reactions with CH4 and H2 and recombination to produce hot H atoms.