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University of KansasTitan Studies |
DraftTitan's Ionosphere:
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Image courtesy of NASA/JPL-Caltech.
We obtained reasonable agreement between theoretical and measured electron density time histories only when the model included ionization both from solar radiation and from incident magnetospheric (suprathermal) electrons. Furthermore, we can account for about two-thirds of the maximum electron density measured by the RPWS Langmuir probe on the inbound (dayside) trajectory with only solar radiation, with magnetospheric electrons accounting for the remaining 33 percent contribution. However, not surprisingly, the magnetospheric electron contribution to the ionosphere is much more important on the nightside (outbound).
The model results also show some sensitivity (about 10 - 25 percent) to plausible variations (or uncertainties) in the solar flux and/or the electron temperature. A simple photochemical equilibrium expression for the electron density is: ne = [P/a]1/2 =[P/a0]1/2 [Te/300 K]1/4, where P is the total ion production rate at a given location and where the effective dissociative recombination coefficient is given by a=a0(300 K/Te)1/4. The effective recombination coefficient near 1200 km estimated from the model is a0 approx.= 6 x 10-7 cm3 s-1. From this expression we can see why a factor of 1.6 higher solar flux (Solar Flux 1 and 2 cases) produces electron densities which are larger by a factor of (1.6)1/2 (25 percent) (Table 1). Similarly, factors of 2 in the electron temperature produce factors of (2)1/4 (19 percent) differences in the electron density.
Small-scale structures evident in the measured electron density (e.g., the sharp peak at t approx.= +250 s -- outbound) were not reproduced by our model. However, the incident magnetospheric electron distribution in the model was independent of time, and it would not be surprising if the incident electron flux in the region of the magnetosphere linked to the ionosphere by the magnetic field varied with time. For example, magnetospheric flux tubes hung up in Titan's ionosphere for long times could be depleted of their electrons [cf. Gan et al., 1993]. CAPS data should eventually shed more light on such time variations.
Plasma transport effects (not included in this model) will also become important at higher altitudes (above about 1400 - 1500 km according to Ma et al. [2004] or Cravens et al. [1998]) where photochemical equilibrium ceases to be a good approximation. In this transport-dominated regime, the dynamical role of the magnetic field becomes important, as demonstrated by many MHD and hybrid models [e.g., Cravens et al., 1998; Ledvina and Cravens, 1998; Brecht et al., 2000; Kabin et al., 1999; Nagy et al., 2001; Ma et al., 2004; Ledvina et al., 2004].
The current paper focused on the overall structure of Titan's ionosphere and the role of different ionization mechanisms. The next step will be to study the ion composition. The INMS and CAPS experiments just measured the ion composition in Titan's ionosphere [e.g., Keller et al., 1998; Ma et al., 2004; Fox and Yelle, 1997].
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Tizby Hunt-Ward tizby@ku.edu |