Introduction

For over a century prior to the scientific recordings of the 1990s, there were reports of visually observed lightning-like channels, pillars of light, and diffuse, upward-propagating, optical phenomena above thundercloud tops, sometimes apparently extending between the cloud tops and the ionosphere (e.g., Everett and Everett 1903; Boys 1926; Malan 1937; Ashmore 1950; Wright 1950; Wood 1951; Wilson 1956; Powell 1968; Corliss 1977, 1983; Vonnegut 1980; Vaughan and Vonnegut 1982; Vonnegut et al. 1989; Vaughan and Vonnegut 1989; Fisher 1990; Hammerstrom 1993). On theoretical grounds the existence of discharges in the rarified air between the thundercloud top and the lower ionosphere had been predicted by, for example, Wilson (1925a, b, 1956), Hoffman (1960), and Cole et al. (1966), all of whom viewed runaway electrons as the potential causative mechanism. Runaway electrons are defined as electrons whose initial energy is high enough that they gain more energy from the ambient electric field between collisions with air atoms than they lose in the collision process, and hence they continue to gain energy and increase in speed (Section 14.5). The first recording of luminous phenomena above cloud tops was serendipitously obtained in 1989 by Franz et al. (1990), while testing a new low-light video system.