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Vibrationally Excited N₂ Molecules in Triton's Thermosphere, 15-9124

Principal Investigator
Tariq Majeed

Inclusive Dates: 02/18/99 - 07/31/99

Background - The major task of this project was to modify SwRI’s one-dimensional chemical-diffusive model to investigate the effects of vibrationally excited N₂ molecules on the distribution of Voyager measurements of electron densities in the thermosphere of Triton, one of the brightest moons of planet Neptune.

Approach - SwRI has used N₂ vibrational excitation in its one-dimensional chemical diffusive model, coupled with a model of the atmospheric temperature and composition inferred from Voyager’s ultraviolet spectrometer (UVS) solar occultation data, to interpret the measurements of Triton's ionospheric structure. Because of the exploratory nature of these calculations, the research team has assumed a Boltzmann distribution of N₂ vibrational excitation and allowed a single vibrational temperature to be an effective enhancer of atomic ion recombination in the model. The ionospheric model has 35 neutral and ion species and 150 reactions and includes transport of ions and neutrals in vertical direction by the effects of ambipolar molecular and eddy diffusion coefficients. The upper boundary in the model is chosen to be at an 800-kilometer altitude, while the lower boundary is located at a 40-kilometer altitude. The model assumes charge neutrality condition and solves the continuity and flow equations self-consistently for each ion and neutral species. The finite difference approximations to the solution of these equations are based on a generalized Newton's method.

Accomplishments - The project results demonstrate that the impact on an ionosphere of mostly C⁺ or N⁺ ions is significant for the N₂ vibrational temperatures exceeding 4000 K. For the model with the N₂ vibrational temperature of 4500 K and an extra ionization source of magnetospheric origin, an excellent agreement is obtained between the measured and calculated electron density profiles. N⁺ is the main ion at and above the ionospheric peak (see first illustration ). No vertical drift of plasma is required to fit the altitude of the measured electron density peak. The team has also investigated the sensitivity of charge-exchange reaction of N₂⁺ + CC⁺ + N₂ (represented by a rate coefficient, k10). For models with N₂ vibrational excitation included, the effect of k10 on the ionospheric electron densities is not important (see the second illustration).

In the third illustration, the results of calculated neutral densities for the three models are shown. The important point is that the measured densities of atomic nitrogen at two altitudes (200 and 400 kilometers) agree well with the calculated densities from the model that uses an extra ionization source and vibrational temperature of 4500 K. The calculated electron densities from this model can also explain the measured electron densities on Triton (shown in the first illustration).

In conclusion, the results of these exploratory calculations have shown that the presence of vibrationally excited N₂ (v) molecules in the thermosphere of Triton is important for the ionospheric sink. Thus an accurate modeling of each of the 40 vibrational levels as a function of altitude self-consistently with the ionospheric calculations is required. A proposal to carry out such calculations has been submitted to NASA's Planetary Atmospheres Program, and a paper discussing the project results has been prepared for possible publication in the "Journal of Geophysical Research."

A comparison is shown between the model (solid line) and Voyager measurements (symbols) of electron density profiles.  The model profiles of ionospheric ion densities are also shown.

The sensitivity of N₂⁺ + C  C⁺ + N₂ is shown for three models with N₂ vibrational excitation included.

The model density profiles of atomic nitrogen are shown for three model in comparison with Voyager measurements at 200 km and 400 km altitudes with error bars.

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