The ionosphere of Triton

The authors have used a model of the atmospheric temperature structure and composition inferred from the Voyager UVS solar occultations together with a 1-D chemical diffusive model to interpret the Voyager RSS ingress measurements of Triton's electron density. Although N+2 is the major ion created, N+ produced by dissociative ionization is the dominant ion. Reaction of thermospheric H2, produced by Lyman-α dissociation of CH4 in the lower atmosphere, is the major loss for N+ ions and maintains these ions in PCSS below 600 km. Solar EUV ionization cannot generate electron densities at the magnitude measured by the RSS experiment and an additional ionization source ∼ 3 x 108 ions cm-2 s-1 is required. The ionosphere may undergo a transition from PCSS to diffusive control if the N+ ion production rates were greater than the H2 flux derived from CH4. In this case the upward flowing H2 is totally converted to H by reaction with N+ and the remaining N+ ions recombine radiatively to create an ionosphere under diffusive control above the peak