Upper atmospheres and ionospheres of planets and satellites

The upper atmospheres of the planets and their satellites are more directly exposed to sunlight and solar-wind particles than the surface or the deeper atmospheric layers. At the altitudes where the associated energy is deposited, the atmospheres may become ionized and are referred to as ionospheres. The details of the photon and particle interactions with the upper atmosphere depend strongly on whether the object has an intrinsic magnetic field that may channel the precipitating particles into the atmosphere or drive the atmospheric gas out to space. Important implications of these interactions include atmospheric loss over diverse timescales, photochemistry, and the formation of aerosols, which affect the evolution, composition, and remote sensing of the planets (satellites). The upper atmosphere connects the planet (satellite) bulk composition to the nearplanet (-satellite) environment. Understanding the relevant physics and chemistry provides insight to the past and future conditions of these objects, which is critical for understanding their evolution. This chapter introduces the basic concepts of upper atmospheres and ionospheres in our solar system and discusses aspects of their neutral and ion composition, wind dynamics, and energy budget. This knowledge is key to putting in context the observations of upper atmospheres and haze on exoplanets and to devise a theory that explains exoplanet demographics.