Titan's upper atmosphere: Structure and ultraviolet emissions

The Voyager 1 ultraviolet spectrometer (UVS) solar occultation and airglow data obtained during the Titan flyby are analyzed for composition and thermal structure of the upper atmosphere and relative importance of airglow excitation processes. From optical depth profiles inferred by Smith et al. (1982, J. Geophys. Res. 87, 1351–1359) for the evening terminator, the entrance occulatation, an asymptotic temperature of T∞ = 175 K, and a CH4 mixing ratio increasing from 0.06 ± 0.01 at 1000 km to 0.20 ± 0.02 at 1400 km are derived. The optical depth profiles are not of sufficient accuracy to extrapolate and accurately infer the tropopause CH4 mixing ratio, but our preference is for high values ∼0.026–0.034. The homopause occurs high in the atmosphere (∼1000 km) with an eddy diffusion coefficient K0 ∼ (4–8) × 108 cm2 sec−1 at this level and an approximate altitude dependence K = K0 exp (ζ/1.6), where ζ is the normalized geopotential height. From the shape of the observed bright limb profile for N2 emission (924–998 Å) the magnetospheric power dissipation is, at most, 10% of the solar EUV power input to Titan's extended atmosphere (= 2 × 109 W for λ < 800 Å) and consistent with our estimates of magnetospheric power delivered to the ionopause by curvature drift to thermal and suprathermal electrons. This value of magnetospheric power input is a factor of 25 lower than the original estimate of Strobel and Shemansky (1982, J. Geophys. Res. 87, 1361–1368). Our calculated absolute intensities for the N+ (1085 Å) multiplet are consistent with the UVS observed intensities after downward revision by the Holberg et al. (1982, Astrophys. J., 257, 656–671) inflight calibration scheme, whereas the calculated N2 c′4 (0-0, 958 Å) plus (0–1, 981 Å) band intensities (the 970-Å feature) are only ∼50% of the observed values. The nonthermal N atom escape rate is estimated to be ≤1025 sec−1. An upper limit on the tropopause Ar mixing ratio of 0.14 is derived from a comparison of the calculated relative intensities of the Ar resonance lines at 1048 and 1067 Å with the N+ (1085-Å) multiplet to the observationally inferred upper limit of 0.5.