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An ionosphere is the ionized part of the upper atmosphere of a planet or a moon, a transition layer between the space environment and the lower atmosphere. At Titan, the ionosphere was first detected by the Voyager 1 radio occultation experiment (Bird et al., 1997). As Titan is located within Saturn's magnetosphere for most of the time with occasional incursions into its magnetosheath (and even rarer incursions into the solar wind), its ionosphere is a key layer in coupling Titan with Saturn's space environment. The question of whether Titan's ionosphere is produced primarily by solar radiation or electron precipitation from Saturn's magnetosphere has been under debate for several decades (e.g., Nagy and Cravens, 1998). This is not surprising, bearing in mind the complex and dynamic nature of both the magnetospheric forcing and of the magnetic field line configuration at Titan (see Chapter 12). For instance, while Titan does not have any significant intrinsic magnetic field, Saturn's magnetic field lines drape around and permeate its ionosphere. The draping changes significantly with the angle between the solar direction and the co-rotating plasma direction, which varies as Titan orbits around Saturn.
The Cassini spacecraft, which arrived at Saturn in July 2004, has explored Titan's ionosphere in detail through many close fly-bys, the first of which took place in October 2004. The resulting rich datasets from many instruments, combined with comprehensive analyses, have revealed the chemically and dynamically most complex ionosphere in the solar system.
Titan, Mars, and Venus are three largely unmagnetized planetary bodies with dense atmospheres that are immersed in external and highly dynamic magnetized plasma flows. Mars and Venus interact with the solar wind, whereas Titan usually interacts with the rotating magnetosphere of Saturn, and only occasionally is subject to shocked solar wind during brief excursions into Saturn's magnetosheath (Figure 12.1). Titan's atmosphere is ionized by the energetic plasma flow, together with solar and cosmic ray radiation (see Chapter 11), and the resulting ionosphere provide a conductive environment with which the external plasma flow interacts. The ability of the ionosphere to carry an electrical current plays an important role in the dynamics and energetics of the ionosphere, and through collisions, to the deposition of energy and momentum into the neutral atmosphere. This magnetosphere/ionosphere interaction at Titan involves the formation of an induced magnetosphere around Titan with interaction boundaries that drapes the magnetic field lines into a long tail behind the moon, already detected by the instruments of the Voyager 1 spacecraft (e.g., Ness et al., 1982; Gurnett et al., 1982) during its swift fly-by of Titan's plasma wake. The interaction causes ionospheric convection and facilitates the escape of ionospheric plasma through the tail to the surrounding streaming magnetosphere past Titan. In addition, Titan's vast neutral gas environment becomes partly ionized; the created ions are picked up by the induced convection electric field by the streaming magnetospheric plasma and drift away in a gyrating motion, at the same time mass loading the streaming plasma so it slows down in the neighborhood of the moon.
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