Hydrocarbon ions in the lower ionosphere of Saturn

Radio occultation measurements of the Saturn ionosphere have shown that persistent but variable electron density layers appear well below the major peaks. We model here the region of hydrocarbon ions that is below the main peak and is produced by absorption of solar photons in the wavelength range 842 to 1116 angstrom, which penetrate to altitudes below the methane homopause in the wings of the H-2 absorption lines, and in the gaps between groups of lines. In this wavelength range, H-2 absorbs photons in discrete transitions to rovibrational levels of electronically excited states, which then decay to a range of rovibrational levels of the electronic ground state, or to the continuum of the ground state. The cross sections for these discrete absorptions vary by several orders of magnitude from the peaks to the wings of the absorption lines. We find that the adoption of high resolution photoabsorption cross sections for the H-2 bands leads to different photoionization profiles for both the hydrocarbons and H atoms, and to peak CH4+ photoproduction profiles that are more than an order of magnitude larger than those computed with low resolution cross sections. For the present model, we find that ionization by energetic electrons that accompany the absorption of soft X-rays appears in the same altitude range. We predict that a broad region of hydrocarbon ions appears well below the main peak, in the altitude range 600 to 1000 km above the 1 bar level (2-0.04 mu bar) with a maximum electron density of approximate to 3x10(3)cm(-3) at low solar activity.