Titan's upper neutral atmosphere and ionosphere

Titan's extended upper atmosphere (mesosphere and thermosphere), commonly defined as the region above 400km (1Pa), hosts a complex environment composed of primarily N2 and CH4, followed by numerous secondary hydrocarbons. The region also hosts numerous positive ions (including HCNH+, C2H5+, and C3H5+), formed by solar radiation ionization and electron impact ionization from Saturn's magnetosphere. The ionospheric peak electron density ranges from around 3.5⋅10−3 cm−3 on the dayside to below 1.0⋅10−3 cm−3 on the nightside, and the peak altitude ranges from 1200 to 1400km. A surprise discovery by the Cassini observations was large densities of heavy negative ions. These combined observations by Cassini/Huygens have revealed Titan to be one of the most complex photochemical environments, including the generation of aerosols and polycyclic aromatic hydrocarbons (PAHs). The distribution of gases is found to be strongly affected by atmospheric winds, showing hemispheric and seasonal trends as well as considerable time variability. Measurements from the Huygens probe and Cassini orbiter have shown the atmosphere to be dominated by atmospheric waves reaching amplitudes of up to 30K (3%–22%) and vertical wavelengths between 100 and 400km, which may play a key role in driving the observed variability. H2 is found to escape at the diffusion-limited rate and relatively well understood, while the escape rate of CH4 is still not well understood. Recent ground-based observations by the Atacama Large Millimeter/submillimeter Array (ALMA) for the first time directly measured zonal winds in the thermosphere, which reached 340m/s near 1000km. Ongoing modeling studies investigate the dynamical coupling between the stratosphere and mesosphere/thermosphere region to explain the variability in Titan's thermosphere.