Ionization sources in Titan's deep ionosphere

We analyze a multi-instrumental data set from four Titan encounters by the Cassini spacecraft to investigate in detail the formation of the ionosphere. The data set includes observations of thermospheric and ionospheric species and suprathermal electrons. A model describing the solar and electron energy deposition is used as an organizing element of the Cassini data set. We first compare the calculated secondary electron production rates with the rates inferred from suprathermal electron intensity measurements. We then calculate an effective electron dissociative recombination coefficient, applying three different approaches to the Cassini data set. Our findings are threefold: (1) The effective recombination coefficient derived under sunlit conditions in the deep ionosphere (<1200 km) is found to be independent of solar zenith angle and flyby. Its value ranges from 6.9 × 10^-7 cm³ s^-1 at 1200 km to 5.9 × 10^-6 cm³ s ^-1 at 970 km at 500 K. (2) The presence of an additional, minor source of ionization is revealed when the solar contribution is weak enough. The contribution by this non-solar sourceenergetic electrons most probably of magnetospheric originbecomes apparent for secondary electron production rates, due to solar illumination alone, close to or smaller than about 3 × 10^-1 cm^-3 s^-1. Such a threshold is reached near the solar terminator below the main solar-driven electron production peak (<1050 km). (3) Our ability to model the electron density in the deep ionosphere is very limited. Our findings highlight the need for more laboratory measurements of electron dissociative recombination coefficients for heavy ion species at high electron temperatures (especially near 500 K).