Haze seasonal variations of Titan's upper atmosphere during the Cassini Mission

The 13 years of the Cassini mission provide a unique dataset monitoringthe evolution of Titan’s atmosphere during almost half of Titan's year,between the middle of the northern winter up to the northern summer solstice.In this work we present a survey of the haze vertical extinction profiles retrievedbetween 300 to 700 km altitude with an atmospheric limb multiple scattering model.We analyze 138 ISS NAC CL1-UV3 images taken between 2004 to 2017 covering theentire Cassini mission.We mainly focus on the temporal evolution of the haze contentin the upper atmosphere to track the evolution of the detached haze layer.As previously reported by West et al. 2011 (GRL vol. 38, L06204),during the northern winter the detached haze layer is presentat all latitudes below \ang{55}N and globally sunk to lower altitudemost rapidly at the vernal equinox in 2009.No permanent detached haze layer was observed in the UV3 filter between 2012 and 2015 butwe show that at the end of 2015, a new structure emerged fromthe summer hemisphere (North) and propagated to the equator. This new layerwas not as pronounced and was much more complex than the one observed at thebeginning of the mission and was likely to be the initiation of a new detached haze layer.We also investigate the short time scale variability of the detached hazelayer and no major changes is observed. Finally, we report some caseswhere the viewing geometry allows us to probe the longitudinal variabilityof the haze, highlighting some local inhomogeneities.We notice that the extinction in the detached haze layer is slightly higher in the dawnside compare to the dusk side.All these observations bring new perspectiveson the seasonal cycle of Titan's upper atmosphere, the evolution of the detachedhaze layer and its interaction with the dynamics.The main seasonal pattern predicted by the global circulation models is consistentwith our findings, but the details and the timing of the global collapseis not yet explained. Finally, comparisons with the UVIS instrument showsthat the haze extinction profiles retrieved during two stellar occultationsin 2009 are consistent with our results.