Haze Seasonal Variations of Titan's Upper Atmosphere during the Cassini Mission

This study presents a 13 yr survey of haze UV extinction profiles, monitoring the temporal evolution of the detached haze layer (DHL) in Titan's upper atmosphere (350-600 km). As reported by West et al. (GRL vol.38, L06204) at the equator, we show that the DHL was present at all latitudes below 55°N during the northern winter (2004-2009). Subsequently, it globally sunk and disappeared in 2012. No permanent DHL was observed between 2012 and 2015. Only in late 2015 did a new structure emerge from the Northern hemisphere, and propagate to the equator. This new DHL is not as pronounced as that observed in 2004, and is much more complex. In one specific sequence, in 2005, we were able to investigate the short timescale variability of the DHL, and no major changes were observed. When both sides of the limb were visible (dawn/dusk), we notice that the extinction of the DHL is slightly higher on the dawn side. Moreover, during a polar flyby in 2009, we observed the longitudinal variability of the DHL and spotted some local inhomogeneities. Finally, comparisons with UVIS stellar occultations and General Climate Models are both consistent with our findings. However, we noticed that the timing of the DHL main pattern predicted by the GMCs can be off by up to 30° in solar longitude. All these observations offer new perspectives on the seasonal cycle of Titan's upper atmosphere, the evolution of the DHL and its interaction with the dynamics.