DPS 2001 meeting, November 2001
Session 49. Outer Planet Satellites
Oral, Chairs: R. Johnson, C. Dumas, Friday, November 30, 2001, 4:30-5:50pm, Regency GH

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[49.08] Triton's ionosphere: Impacts of H2 vibrational excitation

T. Majeed (Southwest Research Institute), J. C. McConnell Collaboration, G. R. Gladstone Collaboration, R. Link Collaboration

We explore the possible impact that vibrational excited H2 (expressed by an effective vibrational temperature, Tv) might have on the ionospheric structure of Triton. Although our approach assumes that individual vibrational levels are in collisional equilibrium, it provides a useful way to represent the H2(v) distribution in Triton's thermosphere and allows us to assess the opening of nominally endothermic ion-neutral reaction channels. Some recent models of Triton have identified C+ as the main thermospheric ion, ignoring reaction with H2 as a loss mechanism since it is endothermic. However, the reaction of C+ with H2(v) becomes exothermic for the levels with v >~1. The higher vibrational levels of H2 may be populated by electron impact excitation and solar-induced resonance fluorescence of Lyman and Werner bands of H2.

By including H2 vibrational excitation in our modified version of 1-D chemical diffusive model, together with a model of atmospheric temperatures and composition inferred from the Voyager-2 UVS solar occultation, we show that an ionosphere of mostly C+ or N+ ions is significantly modified for Tv exceeding 1000 K. For a model with Tv of 2000 K and an extra ionization source of magnetospheric origin (equivalent to that estimated by Majeed et al. GRL, vol. 17, 1721, 1990), excellent agreement is obtained between the measured and calculated electron density profiles. N+ is the main ion at and above the ionospheric peak. For the same model, the calculated densities of atomic nitrogen at two altitudes (200 and 400 km) are also in good agreement with measured densities. Further details of the model and related results will be discussed.

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