Ionization and energy exchange processes

Abstract

Solar extreme ultraviolet (EUV) radiation and particle, mostly electron, precipitation are the two major sources of energy input into the thermospheres and ionospheres in the solar system. Aschematic diagram showing the energy flow in a thermosphere–ionosphere system caused by solar EUV radiation is shown in Figure 9.1. Relatively long wavelength photons (≳900 A) generally cause dissociation, while shorter wavelengths cause ionization; the exact distribution of these different outcomes depends on the relevant cross sections and the atmospheric species. The only true sinks of energy, as far as the ionospheres are concerned, are airglow and neutral heating of the thermosphere. Even the escaping photoelectron flux can be reflected or become the incoming flux for a conjugate ionosphere. The specific distribution of the way that energy flows through the system is very important in determining the composition and thermal structure of the ionospheric plasmas. This chapter begins with a discussion of the absorption of the ionizing and dissociating solar radiation and the presentation of information needed to calculate ionization and energy deposition rates. This material is followed by a description of particle transport processes. The chapter ends with a presentation of electron and ion heating and cooling rates that can be used in practical applications. Absorption of solar radiation Radiative transfer calculations of the solar EUV energy deposition into the thermosphere are relatively simple because absorption is the only dominant process.