On the thermal structure of Triton's thermosphere

The analysis of the Voyager 2 Ultraviolet Spectrometer (UVS) solar occultation data obtained at Triton is consistent with a spherically symmetric, isothermal thermosphere above 400 km at T ∞ . = 96 K. A detailed calculation of energy loss processes in a pure N 2 atmosphere, heating and cooling rates, and resultant thermal structure associated with solar UV irradiance and magnetospheric electron precipitation indicates that solar heating, with calculated T ∞ = 70 K, is insufficient to account for the inferred T ∞ = 96 K. The magnetosphere must deposit twice as much power as the sun (λ < 800 Å) to heat the thermosphere to 96 K and generate the observed N 2 tangential column densities above 450 km. The thermal escape of H and N atoms and the downward diffusion of N atoms to recombine below 130 km results in local ionospheric heating efficiency of 24%. An upper limit on the tropopause CO mixing ratio of 2 × 10 −4 is inferred in the absence of aerosol heating to balance its efficient cooling by LTE rotational line emission.