Influence of local ionization on ionospheric densities in Titan's upper atmosphere

Abstract

. We find that on the dayside the solar energy deposition model overestimates the INMS-derived N + 2 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 N + 2 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 N 2 local ionization frequency, CH + 5 has higher densities on the dayside than on the nightside. We explain that this is due to CH 4 being more efficiently ionized by solar photons than by magnetospheric electrons for a given amount of N 2 ionization.

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

“…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.…”

“…Modeled + N 2 production rates on the dayside are higher (by a factor of ∼1.5-2) than derived from an empirical chemical model. On the nightside the situation is reversed, with the modeled production rates being lower than values from the empirical chemical model by a multiplicative factor of ∼0.4-0.7 (Sagnières et al 2015). 3.…”

“…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. These differences are discussed in depth in Section 4.6 of Sagnières et al (2015).…”

“…Likewise P e,night changes linearly with systematic changes across the utilized suprathermal electron spectra (taken from measurements by CAPS/ELS). Sagnières et al (2015) compared + N 2 and + CH 4 production rates derived from their TIMED/SEE-based solar energy deposition model with production rates derived from an empirical chemical model driven by number densities of neutrals and (short-lived) ions measured by the INMS and laboratory-derived rate coefficients for key ion-neutral reactions. Below 1200 km the + N 2 production rates derived from the solar energy deposition model agreed in shape but were a factor of S day ∼ 1.5-2 higher than corresponding output values from the empirical chemical model (on the contrary, a good agreement was found between the model outputs of + CH 4 production rates).…”

“…The Solar Dynamics Observatory (SDO) was not yet operational during the investigated flybys, but it was for T83 and T86 (Woods et al 2012). Sagnières et al (2015) reported that for the latter flybys, the + N 2 (and + CH 4 ) production rates calculated along the Cassini trajectories were found to differ by less than 10% when using the solar flux from the SDO/EUV Variability Experiment instrument compared with TIMED/SEE. The extrapolation in distance of the solar EUV flux (measured) near Earth to Titan neglects EUV extinction (expected negligible) and does not correct for the phase difference between the objects.…”