Suprathermal Electrons in Titan’s Sunlit Ionosphere: Model-Observation Comparisons

E. Vigren , M. Galand , A. Wellbrock , A. J. Coates , J. Cui , N. J. T. Edberg , P. Lavvas (1) , L. Sagnières , D. Snowden , V. Vuitton (2) , J. -E. Wahlund

1 URCA - Université de Reims Champagne-Ardenne
2 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

The dayside ionosphere of the Saturnian satellite Titan is generated mainly from photoionization of N₂ and CH₄. We compare model-derived suprathermal electron intensities with spectra measured by the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) in Titan's sunlit ionosphere (altitudes of 970-1250 km) focusing on the T40, T41, T42, and T48 Titan flybys by the Cassini spacecraft. The model accounts only for photoelectrons and associated secondary electrons, with a main input being the impinging solar EUV spectra as measured by the Thermosphere Ionosphere Mesosphere Energy and Dynamics/Solar EUV Experiment and extrapolated to Saturn. Associated electron-impact electron production rates have been derived from ambient number densities of N₂ and CH₄ (measured by the Ion Neutral Mass Spectrometer/Closed Source Neutral mode) and related energy-dependent electron-impact ionization cross sections. When integrating up to electron energies of 60 eV, covering the bulk of the photoelectrons, the model-based values exceed the observationally based values typically by factors of ∼3 ± 1. This finding is possibly related to current difficulties in accurately reproducing the observed electron number densities in Titan's dayside ionosphere. We compare the utilized dayside CAPS/ELS spectra with ones measured in Titan's nightside ionosphere during the T55-T59 flybys. The investigated nightside locations were associated with higher fluxes of high-energy (>100 eV) electrons than the dayside locations. As expected, for similar neutral number densities, electrons with energies <60 eV give a higher relative contribution to the total electron-impact ionization rates on the dayside (due to the contribution from photoelectrons) than on the nightside.

Mots clés: molecular processes; planets and satellites: individual: Titan

Domaines: Planète et Univers [physics]

E. Vigren, M. Galand, A. Wellbrock, A. J. Coates, J. Cui, et al.. Suprathermal Electrons in Titan’s Sunlit Ionosphere: Model-Observation Comparisons. The Astrophysical Journal, 2016, 826, ⟨10.3847/0004-637X/826/2/131⟩. ⟨insu-03691530⟩