Composition and Structure of the Ionosphere and Thermosphere

Abstract

Airglow emissions, radio and solar occultation data from the Voyager mission over a quarter of a century ago provided the main source of information on the composition and structure of Titan's upper atmosphere and ionosphere until October 2004, when the Cassini Orbiter first encountered Titan during the “Ta” fly-by. During this encounter, in situ measurements were made by many instruments onboard the Orbiter, including the Ion and Neutral Mass Spectrometer (INMS), the Radio Wave and Plasma Wave Spectrometer (RPWS), the Magnetometer (MAG), and the Cassini Plasma Spectrometer (CAPS). For example, INMS measurements confirmed that the major neutral species were molecular nitrogen and methane. Other species detected included mole cular hydrogen, acetylene, ethylene, benzene, and propane. The Langmuir probe part of the RPWS experiment observed substantial ionospheric electron densities and measured electron temperatures significantly exceeding the neutral temperature. A large set of data on the upper atmosphere and ionosphere has been collected during the many Titan encounters following Ta. The first composition measurements for the ionosphere were made by INMS during the outbound leg of the T5 pass in April 2005. A rich and complex ion-neutral chemistry scheme was predicted prior to the Cassini mission and the INMS composition data indeed revealed the presence of a very large number of ion species, both predicted and unpre-dicted. Stellar occultation measurements made by the Cassini Ultraviolet Spectrometer (UVIS) provided important information on the structure and composition of Titan's upper atmosphere, and radio occultation measurements made by the Radio Science Subsystem (RSS) revealed the existence of a substantial ionosphere even for altitudes below 1000 km. The discovery of negative ions in the ionosphere was also very exciting. A vigorous modeling effort aimed at explaining the structure and composition of the upper atmosphere and ionosphere is helping to put the data into a broader theoretical context. For example, solar extreme ultraviolet and x-ray radiation and energetic electrons from Saturn's magnetosphere interact with the upper atmosphere producing the ionosphere and initiating a complex neutral and ion chemistry that has important effects extending deep into the atmosphere.