University of Kansas
The Ionosphere of Titan:
Image courtesy of NASA/JPL-Caltech.
The model for the current paper is essentially the same as the earlier model(s) described by Gan et al. (1992), Keller et al. (1992), and Keller et al. (1998), albeit with a number of improvements as described below. The "background" neutral model of the thermosphere used in this model was described in these earlier papers. The altitude variation of the major species (N2 and CH4) is based on Voyager ultraviolet occultation data, and the density profiles of the hydrocarbon species came from photochemical models of either Yung et al. (1987) or Toublanc et al. (1995); the former is adopted for the current paper. The model adopts "standard" aeronomical methods (cf. Schunk and Nagy, 2000) and is strictly an ionospheric model. For example, a solar flux as a function of wavelength is used together with photoionization cross sections to generate ion production rates and photoelectron production rates as a function of altitude, solar zenith angle, and electron energy. Photoelectron fluxes are calculated using the 2-stream transport method (Nagy and Banks, 1970) plus appropriate electron impact cross sections (cf. Gan et al., 1993). For the current study superthermal electrons are permitted to transport vertically and the incoming flux at the top of the ionosphere is assumed to be zero. Such a scenario would apply to a sunlit Titan facing away from the flow direction of Saturn's magnetospheric plasma. The ionospheric densities are produced using the photochemical model of Keller et al. (1998). The following improvements were made to these models:
1. A higher resolution solar flux was adopted for the soft x-ray portion of the spectrum. We adopted the soft x-ray spectrum used by Maurellis et al. (2000) but adjusted it for the solar activity level at the time of the Voyager encounter with Titan. The number of wavelength intervals was increased from 100 to 320. Further improvements will still be needed to obtain self-consistent photon spectra for a range of solar activity levels.
2. Solar photons with energies greater than K-shell thresholds can ionize from the K-shell. We now include the appropriate photoionization cross sections (i.e., K-shell "edges" included) for this process (e.g., Chantler, 1995) and also the associated production of Auger electrons. The K-shell thresholds are 402 eV for N and 284 eV for C, although the Auger electrons will have energies slightly lower than these energies.
3. The improved model produces ionization rates beyond the terminator-- that is, for solar zenith angles (SZA) considerably greater than 90 deg. This requires using an appropriate Chapman function with accurate spherical geometry in the determination of the optical depth.
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