11.1 Introduction

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.