University of Kansas Titan Studies

University of Kansas

Titan Studies

Draft

The Ionosphere of Titan:
An Updated Theoretical Model

T. E. Cravens et al.

Image courtesy of NASA/JPL-Caltech.

Results

The photoionization process generates photoelectrons. Photoelectron fluxes for the pure solar case were calculated using the 2-stream transport code. Figure 1 shows the calculated upward electron flux for SZA = 60 deg. and for an altitude of 1220 km (not far above the ionospheric peak). The results are almost identical to the Gan et al. (1993) results for this case, as they should be, except for electrons with energies in excess of about 100 eV produced by the solar soft x-ray flux. Gan et al. used a lower resolution x-ray flux and did not include Auger electrons. The most noticeable difference from the earlier model is the presence of Auger electrons. The clear peak at an energy a little less than 400 eV is due to nitrogen Auger electrons. The (carbon) Auger electron peak (energy somewhat less than 280 eV) is also present but is not as obvious.

The production rate profile for the N₂⁺ ion at a solar zenith angle of 60 deg. is shown in Figure 2. The primary production (i.e., directly from photoionization), the secondary production (i.e., from photoelectrons), and the total ionization rate associated with solar ionizing radiation are all shown. The ionization rate is almost the same as the results from our original model (Keller et al., 1992), especially near the main peak. The "ledge" in ionization rate on the bottomside (due mainly to the soft x-rays) is a few percent greater in the current version of the model than in the original model. Figure 3 shows total N₂⁺ production rate profiles for a range of SZA and the behavior is that of a typical "Chapman layer" with decreasing ionization rate and increasing peak altitude as SZA increases. Figure 4 shows CH₄⁺ production rate profiles. The production rate of this ion becomes increasingly important at higher altitudes or higher SZA because the neutral CH₄ has a larger scale height than the neutral N₂. The ion production rates for both ion species remain significant even for SZA well beyond the terminator.

Figure 1. Upward photoelectron flux as a function of electron energy at an altitude of 1220 km and for a solar zenith angle of 60 deg.

Figure 2. Production rate of N₂⁺ ions versus altitude for 60 deg. solar zenith angle. The primary (i.e., from photons), secondary (i.e., from superthermal photoelectrons), and total ionization rates are shown.

Figure 3. N₂⁺ production rate for a range of solar zenith angles. The curves are labeled with the solar zenith angle in units of degrees.

Figure 4. CH₄⁺ production rate for a range of solar zenith angles.

Figures 5 and 6 show electron density profiles and HCNH⁺ density profiles from the photochemical model (i.e., Keller et al., 1998) with the new ionization rates for a range of SZA. The main difference from the earlier (pure solar) results is the greater range of SZA. A substantial ionosphere (i.e., 1000 cm^-3 peak electron density) is present even 15 deg. beyond the terminator. A discussion of the ion composition is beyond the scope of this paper; see Keller et al. (1998) or Fox and Yelle (1997).

Figure 5. Electron density altitude profiles for several solar zenith angles for the solar-only conditions of this paper. The highest production rates are for 0 deg. solar zenith angle.

Figure 6. HCNH⁺ density profiles for several solar zenith angles.

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Last modified April 14, 2004

Tizby Hunt-Ward
tizby@ku.edu

	Figure 1. Upward photoelectron flux as a function of electron energy at an altitude of 1220 km and for a solar zenith angle of 60 deg.
	Figure 2. Production rate of N₂⁺ ions versus altitude for 60 deg. solar zenith angle. The primary (i.e., from photons), secondary (i.e., from superthermal photoelectrons), and total ionization rates are shown.
	Figure 3. N₂⁺ production rate for a range of solar zenith angles. The curves are labeled with the solar zenith angle in units of degrees.
	Figure 4. CH₄⁺ production rate for a range of solar zenith angles.

	Figure 5. Electron density altitude profiles for several solar zenith angles for the solar-only conditions of this paper. The highest production rates are for 0 deg. solar zenith angle.
	Figure 6. HCNH⁺ density profiles for several solar zenith angles.