An empirical approach to modeling ion production rates in Titan's ionosphere I: Ion production rates on the dayside and globally

Abstract

Titan's ionosphere is created when solar photons, energetic magnetospheric electrons or ions, and cosmic rays ionize the neutral atmosphere. Electron densities generated by current theoretical models are much larger than densities measured by instruments on board the Cassini orbiter. This model density overabundance must result either from overproduction or from insufficient loss of ions. This is the first of two papers that examines ion production rates in Titan's ionosphere, for the dayside and nightside ionosphere, respectively. The first (current) paper focuses on dayside ion production rates which are computed using solar ionization sources (photoionization and electron impact ionization by photoelectrons) between 1000 and 1400 km. In addition to theoretical ion production rates, empirical ion production rates are derived from CH 4 , CH 3 + , and CH 4 + densities measured by the INMS (Ion Neutral Mass Spectrometer) for many Titan passes.The modeled and empirical production rate profiles from measured densities of N 2 + and CH 4 + are found to be in good agreement (to within 20%) for solar zenith angles between 15 and 90°. This suggests that the overabundance of electrons in theoretical models of Titan's dayside ionosphere is not due to overproduction but to insufficient ion losses.

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

“…Similar model-observation comparisons have previously been performed by Robertson et Figure 4), particularly over the electron energy range 10-60 eV. Among several potential sources of differences (not only directly model related; see Section 3.1), we merely note that Robertson et al (2009) and Richard et al (2015a) used solar flux models in their calculations (based on the solar F 10.7 value), while we in the present work, similar to that done by Lavvas et al (2011), use solar EUV fluxes based on TIMED/SEE measurements. Regarding Lavvas et al (2011), their modeled electron spectra acquired along the T40 flyby agreed reasonably in shape with CAPS/ELS observations for energies <60 eV.…”

“…It was mentioned in Section 1.4.1 that the modeled electron fluxes by Robertson et al (2009) and Richard et al (2015a) were found to be in good agreement with CAPS/ELS observations. We plan to investigate in detail how the use of different solar EUV flux models affects the calculations of suprathermal electron intensities.…”

“…It follows, similar to the situation addressed in the previous paragraph, that in order for the dayside and nightside comparisons of + N 2 production rates to become similar (i.e., for S day and S night to become similar), the solar EUV fluxes would need to be reduced (which would bring an issue on dayside + CH 4 production rate comparisons) and/or the suprathermal electron fluxes would need to be somewhat enhanced with respect to the CAPS/ELS measurements across the energy bins contributing the greatest to the ionization of N 2 . We note that Richard et al (2015a) found better agreement between dayside + N 2 production rates calculated from their solar energy deposition model and empirical chemical model than did Sagnières et al (2015). This is most likely due to a combination of various factors: the calibration factor for INMS ion number densities (updated factor used by Sagnières et al), different CH 4 and N 2 number densities, the correction factor in the empirical model from the inclusion of minor reactions, and a different solar flux model.…”

“…We plan to investigate in detail how the use of different solar EUV flux models affects the calculations of suprathermal electron intensities. In particular, it is of interest to find out whether discrepancies between such solar EUV flux models and spectra from TIMED/SEE are a contributing cause as to why our model predicts electron fluxes typically a factor of 2-4 higher than observed by CAPS/ELS (for electron energies <60 eV) while the model results of Robertson et al (2009) and Richard et al (2015a) show good agreement with observations. The different levels of agreement have several other potential/known causes as well.…”

“…The solar zenith angle was ∼80°.5 and so markedly higher than the range of solar zenith angles encountered in the present study (only high-altitude points along the inbound of T40 were associated with solar zenith angles >50°). In Richard et al (2015a) the only presented model-observation comparison of electron fluxes was associated with the egress of T40 at ∼1020 km. The good agreement between modeled and observed electron fluxes, particularly for electron energies 10 eV < E < 25 eV, is reminiscent of what is shown in Figure 8 of Lavvas et al (2011).…”