Method and data treatment Reanalysis of Titan INMS mass spectra Results About the homopause References Titan’s ionosphere was sampled by the mass spectrometer of the Cassini spacecraft many times during the course of the mission. Mass spectrometer analysis can be complicated, and identifying in which way a specie contribute to a mass peak is challenging. It is easier to fit the most abundant species, and trace species are often left unanalysed. Here we focus our work for now on 3 major species (N2,CH4, H2) and 1 trace specie (Ar). Consequences and prospectives Maélie Coutelier1, Thomas Gautier1, Koyena Das1, Joseph Serigano2 : maelie.coutelier@latmos.ipsl.fr 1 : LATMOS/IPSL, CNRS, Sorbonne Université, UVSQ, Guyancourt 2 : Morton K. Blaustein Department of Earth and Planetary Sciences, Baltimore 1: Cui, Jun, et al. "Analysis of Titan's neutral upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements." Icarus 200.2 (2009): 581-615. 2: Cui, J., et al. "The CH4 structure in Titan's upper atmosphere revisited." Journal of Geophysical Research: Planets 117.E11 (2012) 3: Mandt, Kathleen E., et al. "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." Journal of Geophysical Research: Planets 117.E10 (2012). 4: Magee, Brian A., et al. "INMS-derived composition of Titan's upper atmosphere: analysis methods and model comparison." Planetary and Space Science 57.14-15 (2009): 1895-1916. 5: Teolis, B. D., et al. "A revised sensitivity model for Cassini INMS: Results at Titan." Space Science Reviews 190.1 (2015): 47-84. 6: Gautier, Thomas, et al. "Decomposition of electron ionization mass spectra for space application using a Monte‐Carlo approach." Rapid Communications in Mass Spectrometry 34.8 (2020): e8684. Aknowledgment The funding for this research was provided by ANR (ANR “TOMTA”- ANR-20-CE49-0004-01) and PNP ●Calibration of INMS data using recommendations from previous works1,2,3,4,5 ●MS deconvolution code6 → Randomisation of the species fragmentation patterns between the allowed incertitudes → Monte-Carlo simulation. The code then try to fit at once all m/q peaks using the species in our database. → 100000 simulations → Allowed incertitude of 5% for the major species, and 30% for Argon. → Mean of the 5% best simulations. Global scale temporal variation clearly visible → Steady decrease of N2 mixing ratio and density until the vernal equinox. It increases until 2014 then decreases again. → Nightside/Dayside and Latitude/Longitude effects on the global scale variation are minor. Influence of the solar flux → N2 mixing ratio and density increase with periods of intense solar flux. → CH4 density increases with periods of intense solar flux, but proportionally less than N2. N2 → Isotope ratio 14N/15N = 197 ± 1.3 constant over the years. → Mixing ratio decreases when altitude increase. → molecular density changes with time. Figure 3: Nitrogen density (cm-3) as a function of altitude and time. The gray line is the solar F10.7 cm flux in solar flux units. N2 molecular density (cm-3) Vernal equinox 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Figure 2: Mixing ratios of Nitrogen, Methane, Argon and Hydrogen as a function of time and altitude. Vernal equinox 9.03 AU Increasing Saturn-Sun distance 10.04 AU Figure 1: 100000 simulations of gases mixing ratio results for flyby T126 at each altitude (low to high altitudes represented as a colour variation from yellow to blue). Figure 4: Mixing ratios of Nitrogen for flyby T27, T43, T59, and T83 used in our analysis. The gray area represents the altitude of the homopause graphically determined. Figure 5: Homopause altitude as a function of time. →Homompause altitude globally decreases over the years. → The altitude variations of the homopause follow the mixing ratio variations between each flyby. How to determine