Transient Luminous Events

For at least a century there have been occassional eye-witness reports of transient optical flashes above thunderstorms. Wilson (1956) described discharges that he observed above a thunderstorm as ``diffuse fan-shaped flashes of greenish colour extending up into a clear sky''. There are numerous reports from pilots who have observed flashes extending above thunderstorms (Vonnegut, 1980; Vaughan and Vonnegut, 1989). Many other observations of brief optical flashes above thunderstorms have been reported in the literature going back to 1885, as summarized by Rodger (1999).

The first official record of an optical discharge above a thunderstorm was obtained only relatively recently when, quite by accident, such an event was documented by a low-light-level camera during the SKYFLASH program in 1989 (Franz et al., 1990). Within only a few years, additional such discharges had been documented from the space shuttle (Boeck et al., 1995; Vaughan et al., 1992), from the air (Sentman et al., 1995; Sentman and Wescott, 1993), and from the ground (Lyons, 1994; Winckler, 1995).

Various terms were first used to describe high altitude luminous flashes, such as ``cloud-to-ionosphere'' or ``cloud-to-stratosphere'' discharges. However, these terms implied vertical extents which were not accurately known in the earliest reports and also hinted at a direction-of-motion which could not be measured until recently. Consequently, the unassuming term ``sprites'' was adopted to describe the elusive and mysterious nature of the high altitude luminous structures (Sentman et al., 1995).

The first color imagery and unambiguously triangulated extents of sprites were obtained during the Sprites94 aircraft campaign by Sentman et al. (1995). They found that sprites have a main red body located between $\sim\,$50-90 km altitude with faint bluish tendrils often extending to lower altitudes. During the Sprites94 aircraft campaign, Wescott et al. (1995) documented another type of optical emission which appeared to propagate upwards from the tops of thunderstorms at $\sim$20 km altitude to about 40-50 km altitude at an average velocity of $1\times10^5$ m/s. They termed these events ``blue jets'' due to their bluish color and development characteristics.

Similar to blue jets are ``blue starters'' (Wescott et al., 1996). Blue starters differ from blue jets only in their terminal altitude. Wescott et al. (1996) observed that blue starters propagated up from the tops of storms at 17-18 km altitude to a maximum altitude of only $\simeq$25 km. It is likely that most of the pilot reports (Vonnegut, 1980; Vaughan and Vonnegut, 1989) of discharges propagating up from the tops of storms were either blue starters or blue jets and not sprites, which appear nearly instantaneously at higher altitudes.

Boeck et al. (1992) observed a third major class of transient luminosity above thunderstorms. Using a Space Shuttle payload bay TV camera, they noted a significant brightening of the airglow layer at about 95 km altitude directly above a luminous discharge in a storm below. This brightening was attributed to the heating of nighttime ionospheric electrons and the increased ionization associated with the passage of an intense electromagnetic pulse (EMP), as described by Inan et al. (1991).

Theoretical models indicate that the peak emissions associated with the passage of an EMP pulse through the lower ionosphere would be between 85 and 95 km altitude (Taranenko et al., 1992; Taranenko et al., 1993), which is similar to the altitude observed by Boeck et al. (1992). These events were termed ``elves'', an acronym for ``emissions of light and VLF perturbations from EMP sources''. Observations from the ground showed that elves are horizontally extensive ($>100$ km) and have short durations ($<1$ ms) (Inan et al., 1997; Fukunishi et al., 1996). Elves are correlated with large peak-current CGs ($>100$ kA) of both positive (Lyons, 1996; Inan et al., 1997; Fukunishi et al., 1996) and negative (Barrington-Leigh and Inan, 1999) polarities. All of these observations are consistent with an EMP-based mechanism.

Sprites, blue jets and starters, and elves all are examples of transient luminous events (TLEs) above thunderstorms. The remainder of this dissertation will focus on sprites and their parent discharges.