TITAN'S UPPER ATMOSPHERE FROM CASSINI/UVIS SOLAR OCCULTATIONS

This is an author-created, un-copyedited version of an article accepted for publication in The Astrophysical Journal. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at 10.1088/0004-637X/814/2/86.This is a pre-print (pre-peer review) manuscript. It is moderately different from the accepted manuscript and from the published article. Citation of published article: Fernando J. Capalbo, Yves Bénilan, Roger V. Yelle, Tommi T. Koskinen. Titan's Upper Atmosphere from Cassini/UVIS Solar Occultations. The Astrophysical Journal, vol. 814, num. 2, p. 86. December 2015. doi: 10.1088/0004-637X/814/2/86.International audienceTitan’s atmosphere is composed mainly of molecular nitrogen, methane being the principal trace gas. From the analysis of 8 solar occultations measured by the Extreme Ultraviolet (EUV) channel of the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, we derived vertical profiles of N2 in the range 1100 - 1600 km and vertical profiles of CH4 in the range 850 - 1300 km. The correction of instrument effects and observational effects applied to the data are described. We present CH4 mole fractions, and average temperatures for the upper atmosphere obtained from the N2 profiles. The occultations correspond to different times and locations, and an analysis of variability of density and temperature is presented. The temperatures were analyzed as a function of geographical and temporal variables, without finding a clear correlation with any of them; although a trend of decreasing temperature towards the north pole was observed. The globally averaged temperature obtained is (150 ± 1) K. We compared our results from solar occultations with those derived form other UVIS observations, as well as studies performed with other instruments. The observational data we present confirm the atmospheric variability previously observed, add new information to the global picture of Titan’s upper atmosphere composition, variability and dynamics, and provide new constrains to photochemical models