Hydrocarbon Ions in the Lower Ionosphere of Saturn

Radio occultation measurements have shown that persistent electron density layers are observed in the lower ionospheres of Saturn. H ₂ is the major component of the Saturn lower thermosphere, and in the photon range 845 to 1116 A, it absorbs in discrete transitions from the ground state (X) to vibrational levels of excited electronic states. The cross sections for these absorptions vary greatly from the centers to the wings of the H ₂ absorption lines. We model here the ionization rates of hydrocarbons by photons that penetrate to low altitudes in the wings and gaps in the H ₂ absorption spectrum in this wavelength range. The calculation requires construction of very high resolution cross sections for H ₂ , and a similarly high resolution solar flux spectrum. We have constructed a photoabsorption spectrum of H ₂ at a resolution of 0.001 A, using a new set of cross sections for rovibrational lines in H ₂ Lyman (B - X), Werner (C - X), and the Rydberg B' - X and D - X band systems. We combined the high resolution cross sections with high resolution solar spectra that were constructed from the SOHO/SUMER (Curdt et al., 2001) measured spectrum for the quiet solar disk and a SOLAR2000 spectrum (e.g., Tobiska, 2004) for low solar activity. We compute the photo- and photo-electron-impact processes for H ₂ , H, He, and CH ₄ , and photo processes for 15 of the most important neutral species in the lower ionosphere of Saturn, including C ₂ H ₂ , C ₂ H ₄ , C ₂ H ₆ , CH ₃ , CH ₃ C ₂ H, H ₂ O, CO, C, CH, C ₂ , O, O ₂ , CO ₂ , H ₂ CO, and CH ₃ OH, with updated cross sections for these processes. We find that significant solar fluxes in this wavelength range, especially at the CIII 977.02 and OVI 1031.91 solar lines, penetrate to near the hydrocarbon homopause, resulting in photoionization of hydrocarbons. For the ionospheric chemistry, we compiled from the literature a set of 727 rate coefficients for ion-neutral, neutral-neutral, and dissociative recombination reactions of 47 ion species and 26 neutral species. The reaction set is more-or-less complete for hydrocarbon ions with up to 4 carbon atoms, and those containing one oxygen atom. The model for low solar activity shows a hydrocarbon ion layer in the altitude range of 600 - 900 km above 1 bar level, where the peak densities are 4×10 ³ , 7×10 ² , and 1×10 ² cm ^-3 at noon dusk and dawn, respectively. The hydrocarbon ion layer is composed mainly of CH _{5 ⁺} , C ₂ H _{3 ⁺} , C ₃ H _{5 ⁺} , C ₅ H _{n ⁺} , and C ₆ H _{n ⁺} at daytime, but at night time CH _{5 ⁺} and C ₂ H _{3 ⁺} , as intermediate ions, disappear more quickly than others due to reactions with ambient hydrocarbon neutrals in addition to their own recombination with electrons. The model also shows layers of H ₃ O ⁺ and H _{3 ⁺} above the hydrocarbon ion layer, and H ⁺ ions form the main peak of the ionosphere, as has been shown by many other modelers. We will discuss sensitivities of our models to input solar spectra, background neutral atmosphere models, vibrational temperatures of H ₂ , and recombination coefficient sets for the ions.