Suprathermal Electrons in Titan’s Sunlit Ionosphere: Model–observation Comparisons

Abstract

The dayside ionosphere of the Saturnian satellite Titan is generated mainly from photoionization of N 2 and CH 4 . We compare model-derived suprathermal electron intensities with spectra measured by the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) in Titanʼs 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 N 2 and CH 4 (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 Titanʼs dayside ionosphere. We compare the utilized dayside CAPS/ELS spectra with ones measured in Titanʼs 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.

Re‐Analysis of the Cassini RPWS/LP Data in Titan's Ionosphere: 1. Detection of Several Electron Populations

“…Several models have been developed to understand the processes at work in Titan's ionosphere (e.g., Galand et al., 2014 and references therein). Nevertheless, model results either predict too cold electrons (<0.02 eV; <200 K) at 900 km (Mukundan & Bhardwaj, 2018; Richard et al., 2011) or an overestimated electron density by a factor of two (Vigren et al., 2013, 2016). The addition of the negative ions to a photoionization model has been shown to decrease the discrepancy with the measurements (Shebanits, Vigren, Wahlund, Edberg, et al., 2017).…”

Titan's Variable Ionosphere During the T118 and T119 Cassini Flybys

“…Particle impact ionization also contributes and is the main ionization source on the nightside of the moon (Ågren et al, 2007;Cravens et al, 2008Cravens et al, , 2013Vigren et al, 2015). That the precipitation of magnetospheric electrons can vary significantly in Titan's ionosphere was demonstrated, for example, in Vigren et al (2016). They showed examples of when the electron impact ionization rate was comparable on the nightside and dayside, for similar ambient neutral conditions, despite the lack of impact ionization from newly created photoelectrons on the nightside, which are contributing significantly to the impact ionization on the dayside.…”

Photoionization Modeling of Titan’s Dayside Ionosphere

“…Third, Vigren et al (2016; again focusing on T40, T41, T42, and T48) showed that the suprathermal electron intensities calculated from the solar energy deposition model were typically a factor of 3 ± 1 higher than observed by CAPS/ELS for electron energies <60 eV, covering the bulk of the photoelectron energy distribution.…”