Abstract
AbstractTitan has the most chemically complex ionosphere of the solar system. The main sources of ions on the dayside are ionization by EUV solar radiation and on the nightside include ionization by precipitated electrons from Saturn's magnetosphere and transport of ions from the dayside, but many questions remain open. How well do models predict local ionization rates? How strongly do the ionization processes drive the ionospheric densities locally? To address these questions, we have carried out an analysis of ion densities from the Ion and Neutral Mass Spectrometer (INMS) from 16 close flybys of Titan's upper atmosphere. Using a simple chemical model applied to the INMS data set, we have calculated the ion production rates and local ionization frequencies associated with primary ions and . We find that on the dayside the solar energy deposition model overestimates the INMS‐derived production rates by a factor of 2. On the nightside, however, the model driven by suprathermal electron intensities from the Cassini Plasma Spectrometer Electron Spectrometer sometimes agrees and other times underestimates the INMS‐derived production rates by a factor of up to 2–3. We find that below 1200 km, all ion number densities correlate with the local ionization frequency, although the correlation is significantly stronger for short‐lived ions than long‐lived ions. Furthermore, we find that, for a given N2 local ionization frequency, has higher densities on the dayside than on the nightside. We explain that this is due to CH4 being more efficiently ionized by solar photons than by magnetospheric electrons for a given amount of N2 ionization.
Highlights
The overestimation of the N+2 production rates from the solar energy deposition model is not new. It is reminiscent of the results found by Vigren et al [2013], whose modeled electron number densities were larger by a factor of 2 than the ones obtained from the Radio and Plasma Wave Spectrometer (RPWS)/Langmuir probe (LP) instrument aboard Cassini
We have compared the chemical model proposed based on Ion and Neutral Mass Spectrometer (INMS) ion and neutral number densities to a solar energy deposition model for dayside flybys and to an electron impact ionization model for nightside flybys
The chemical model reproduces the shape of the production rates very well, the N+2 production rates from the chemical model are smaller by a factor of 2 than the solar energy deposition model
Summary
Coates (2015), Influence of local ionization on ionospheri densities in Titan’s upper atmosphere, J. Influence of local ionization on ionospheric densities in Titan’s upper atmosphere. M. Sagnières , Marina Galand , Jun Cui , Panayotis P. Lavvas , Erik Vigren , Véronique Vuitton , Roger V. Yelle , Anne Wellbrock , and Andrew J. Moléculaire et Atmosphérique, Université de Reims, Reims, France, 4 Swedish Institute of Space Physics,Uppsala, Sweden, Institut de Planétologie et d’Astrophysique de Grenoble, Grenoble, France, 6 Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA, 7 Mullard Space Science Laboratory, University College London, Dorking, UK
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