Lightning is known to have delayed effects on the lower ionosphere both in the vicinity of the lightning stroke and in the conjugate magnetic hemisphere as a result of energetic coupling to the magnetosphere in the form of whistler-mode waves. These waves may resonantly interact with trapped energetic electrons, scattering them in pitch angle and causing them to ``precipitate'' into the ionosphere, where they produce secondary ionization and thus alter the electron density 0.5 to 1 s after the lightning [Lauben et al., 1999, and references therein]. This process, called lightning-induced electron precipitation, accounts for a class of perturbations in subionospherically propagating VLF signals and is considered to be an example of indirect coupling of lightning to the ionosphere.
In contrast, both sprites and elves involve direct modification of the electron population well above the troposphere by lightning-generated electric fields. This modification involves electron heating, electron density enhancement and depletion, and even propagation of filamentary waves of ionization in the tenuous upper atmosphere.
These processes are discussed below in Section 2.1. Sections 2.2 and 2.3 review the relevant theoretical and experimental literature concerning sprites and elves. Section 2.4 describes our own model for the electromagnetics, plasma heating, and optical emissions in sprites and elves, and Section 2.5 discusses the two-dimensional predictions of the model, including possible effects of the expected ionization changes. The model results are used in subsequent chapters to predict ground-based optical observations using three-dimensional geometry.