Ion densities and composition of Titan's upper atmosphere derived from the Cassini Ion Neutral Mass Spectrometer: Analysis methods and comparison of measured ion densities to photochemical model simulations

Abstract

.[1] The Cassini Ion Neutral Mass Spectrometer (INMS) has measured both neutral and ion species in Titan's upper atmosphere and ionosphere and the Enceladus plumes. Ion densities derived from INMS measurements are essential data for constraining photochemical models of Titan's ionosphere. The objective of this paper is to present an optimized method for converting raw data measured by INMS to ion densities. To do this, we conduct a detailed analysis of ground and in-flight calibration to constrain the instrument response to ion energy, the critical parameter on which the calibration is based. Data taken by the Cassini Radio Plasma Wave Science Langmuir Probe and the Cassini Plasma Spectrometer Ion Beam Spectrometer are used as independent measurement constraints in this analysis. Total ion densities derived with this method show good agreement with these data sets in the altitude region ($1100-1400 km) where ion drift velocities are low and the mass of the ions is within the measurement range of the INMS (1-99 Daltons). Although ion densities calculated by the method presented here differ slightly from those presented in previous INMS publications, we find that the implications for the science presented in previous publications is mostly negligible. We demonstrate the role of the INMS ion densities in constraining photochemical models and find that (1) cross sections having high resolution as a function of wavelength are necessary for calculating the initial photoionization products and (2) there are disagreements between the measured ion densities representative of the initial steps in Titan photochemistry that require further investigation. , et al. (2012), Ion densities and composition of Titan's upper atmosphere derived from the Cassini Ion Neutral Mass Spectrometer: Analysis methods and comparison of measured ion densities to photochemical model simulations,

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

“…Thus, in addition to very large observational uncertainties, 40 Ar is most likely not truly inert. Mandt et al [2012] suggested that the major stable isotopes of 14 N/ 15 significantly improved counting statistics, which is evident in Figure 2d. As can be seen in Figure 2d and in Table 4, the models using a lower methane homopause, models B and B (HE), calculate isotope ratios outside the INMS uncertainties.…”

“…As seen in this figure and as quantified in Table 4, model A represents a superior fit to these isotopic ratios. Moreover, as noted in Mandt et al [2012], these isotope ratios possess lower counting statistical uncertainties and may function as a more reliable diagnostic for eddy diffusion than 40 Ar.…”

“…In order to do this, we consider two broad cases of eddy diffusion: (1) a low-methane homopause case near 880 km (consistent with Strobel [2009], Yelle et al [2008], and Cui et al [2012]) and (2) a high-methane homopause case near 1000 km (consistent with Bell et al [2010b] and Mandt et al [2012]). To test these methane homopause cases, we introduce three T-GITM simulations: model A, a high-homopause simulation; model B, a low-homopause simulation; and model B (HE), a low-homopause simulation with hydrodynamic escape imposed as a boundary condition because T-GITM cannot self-consistently calculate it.…”

“…Thus, understanding Titan's atmospheric evolution is key to determining how Earth's atmosphere may have transitioned into its current state [Yung and Demore, 1999]. Models that describe Titan's atmospheric evolution are sensitive to the assumed atmospheric escape rates of methane (CH 4 ) and molecular hydrogen (H 2 ) over geologic time [e.g., Mandt et al, 2009;Mandt et al, 2012;Lorenz et al, 1999]. Therefore, quantifying current atmospheric escape rates is central to understanding the evolutionary trajectory of Titan's atmosphere.…”

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

“…The ionosphere, peaking typically at altitudes of 1000-1100 km, is composed of hydrocarbon ions, nitrile ions, and other complex organic molecular ions, including ones being negatively charged, and is now known to be an important source region for Titanʼs organic aerosols (e.g., Coates et al 2007;Vuitton et al 2007;Waite et al 2007;Crary et al 2009;Wahlund et al 2009;Mandt et al 2012;Lavvas et al 2013;Wellbrock et al 2013;Shebanits et al 2013). The electron number density profile displays the basic variations anticipated from Chapman theory.…”