Upper-atmospheric lightning

Sprites are large scale electrical discharges above the earth that are still not totally understood. They occur high above thunderstorm clouds (cumulonimbus), giving rise to a quite varied range of visual shapes flickering in the night sky. They are triggered by the discharges of positive lightning between an underlying thundercloud and the ground. Sprites appear as luminous reddish-orange flashes.

Sporadic visual reports of sprites go back at least to 1886, but they were first photographed in 1989 by scientists from the University of Minnesota. Sprites are sometimes inaccurately called upper-atmospheric lightning. However, sprites are cold plasma phenomena that lack the hot channel temperatures of tropospheric (lower-atmospheric) lightning, so they are more akin to fluorescent tube discharges than to lightning discharges. Several years after their discovery they were named sprites (air spirits) after their elusive nature.

Three types of sprites have been categorized by Dr. Jeff Mcgarg of the US Airforce Research Academy. Using an image intensifier on the front of super slow motion camera Mcgarg and his researchers have named the sprites based on their visual appearance: ‘Jellyfish’ sprite (very large, up to 30 miles by 30 miles); ‘Carrot’ sprite; and ‘C’ or ‘Column’ sprite. Sprites are colored reddish-orange in their upper regions, with bluish hanging tendrils below, and can be preceded by a reddish halo. They last longer than normal lower stratospheric discharges, which last typically a few milliseconds. They often occur in clusters of two or more, and typically span the altitude range 50 kilometers (31 mi) to 90 kilometers (56 mi), with what appear to be tendrils hanging below, and branches reaching above.

Optical imaging using a 10,000 frames per second high speed camera shows that sprites are actually clusters of small, decameter-sized (10–100 m, 30–300 ft) balls of ionization that are launched at an altitude of about 80 km and then move downward at speeds of up to ten percent the speed of light, followed a few milliseconds later by a separate set of upward moving balls of ionization. Sprites may be horizontally displaced by up to 50 km from the location of the underlying lightning strike, with a time delay following the lightning that is typically a few milliseconds, but on rare occasions may be up to 100 milliseconds. Sprites are sometimes preceded, by about 1 millisecond, by a sprite halo, a pancake-shaped region of weak, transient optical emissions approximately 50 kilometers (31 mi) across and 10 kilometers (6.2 mi) thick. The halo is centered at about 70 kilometers (43 mi) altitude above the initiating lightning strike. These halos are thought to be produced by the same physical process that produces sprites, but for which the ionization is too weak to cross the threshold required for streamer formation. They are sometimes mistaken for elves, due to their visual similarity and short duration.

Sprites have erroneously been held responsible for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms. For example, the malfunction of a NASA stratospheric balloon launched in 1989 from Palestine, Texas suffered an uncommanded payload release while flying at 120,000 feet (37,000 m) over a thunderstorm near Graham, Texas. Months after the accident, a post-flight investigation concluded that a ‘bolt of lightning’ traveling upward from the clouds provoked the incident. The attribution of the accident to a sprite was evidently made retroactively by several years, since this term was not coined until late 1993. Because of the comparatively low altitude of the balloon, whatever thunderstorm-related discharge may have been a causative factor in the accident, it was more likely to have been one of several other types of stratospheric discharges known to occur, such as blue jets, rather than the higher altitude sprites.

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