  This chapter describes the scientific motivations that led to the development
of the SMILE (Solar wind Magnetosphere Ionosphere Link Explorer) mission. The
solar wind coupling with the terrestrial magnetosphere is a key link in
Sun-Earth interactions. In-situ missions can provide detailed observations of
plasma and magnetic field conditions in the solar wind and the magnetosphere,
but leave us still unable to fully quantify the global effects of the drivers
of Sun-Earth connections, and to monitor their evolution. This information is
essential to develop a comprehensive understanding of how the Sun controls the
Earth's plasma environment and space weather. SMILE offers a new approach to
global monitoring of geospace by imaging the magnetosheath and cusps in X-rays
emitted when high charge-state solar wind ions exchange charges with exospheric
neutrals. SMILE combines this with simultaneous UV imaging of the northern
aurora and in-situ plasma and magnetic field measurements in the magnetosheath
and solar wind from a highly elliptical northern polar orbit. In this chapter
the science that SMILE will explore and the scientific preparations that will
ensure the optimal exploitation of SMILE measurements are presented.
