Hydrocarbon ions in the lower ionosphere of Saturn

Abstract

[1] 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 Å, 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 CH + 4 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 bar) with a maximum electron density of 3 10 3 cm -3 at low solar activity.

Plasma dynamics in Saturn's middle-latitude ionosphere and implications for magnetosphere-ionosphere coupling

“…The integrated values were between 0.4 S and 10 S, similar to the results of Moore et al (2010) . We perhaps underestimated the conductivity because we did not consider hydrocarbon ions in this model, even though they may be dominant in the lower ionosphere ( Kim et al, 2014 ). Moore et al (2010) showed the height-integrated Pedersen conductivity is 15.3 S when the electron density is 10 10 m −3 at all altitudes and the ionosphere consists only of C 3 H 5 + , which was just one examined case.…”

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

“…The ratio of atomic hydrogen to molecular hydrogen column densities is significantly larger in the extended EGP atmosphere than at Saturn or Jupiter. This means that absorption by H dominates absorption by H 2 , and as such, the resolution of σ abs H 2 has a much weaker effect for EGPs than for the solar system cases described by Kim & Fox (1994) and Kim et al (2014). The differences between using high-and low-resolution H 2 photo-absorption cross sections are minor (change lower than 2.5% in ion densities), therefore in blue, Eri (synthetic spectrum) in yellow, AD Leo (synthetic spectrum) in red, and AU Mic (synthetic spectrum) in purple.…”

“…Until now, these cross sections with a high spectral resolution have not been used in studies of EGP ionospheres. However, at Saturn (Kim et al 2014) and Jupiter (Kim & Fox 1994), using high-resolution H 2 photo-absorption cross sections led to the prediction of a larger and more extended layer of hydrocarbon ions in the lower ionosphere.…”

The chemistry of protonated species in the martian ionosphere

“…The history of the cross sections and rate coefficients for DR of H þ 3 has been critically reviewed recently by Johnsen and Guberman (2010), and we do not remark further on them here. We have adopted the DR coefficient that we have recently used for models of the ionosphere of Saturn (Kim et al, 2013), that of Pagani et al (2009), with branching ratios from Datz et al (1995).…”