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Sprites in Space

Sprites are large scale electrical discharges that occur high above the trailing stratiform anvil/precipitation regions of active thunderstorm systems. They are a particular type of electrical breakdown[3] characteristic of the early stages of lightning discharges (the streamer stage), but lacking the later stages of leaders and return strokes. Sprites appear as luminous reddish-orange, plasma-like flashes, that last longer (typically a few milliseconds) than normal lower stratospheric discharges (typically about 0.1-0.2 milliseconds). The red color of sprites has been determined from spectrographic measurements to be due to optical fluorescence from neutral molecular nitrogen in the air. They are triggered by the transient electric fields associated with discharges of positive lightning between the cloud and the ground, and are generally delayed behind the stroke by a few milliseconds. Sprites are typically centered above the causative lightning strokes, but may be laterally displaced from them by up to 50 km (30 miles). Sprites possess complex, vertically striated internal structure, giving rise to a quite varied range of visual shapes. Their apparent visible source altitude is about 75 km (47 mi), and from here they often develop both downward "tendrils" that extend to about 50 km (31 mi), and upward "branches" that extend to about 90 km (56 mi). Optical imaging using a 10,000 frames per second high speed camera shows (Geophysical Research Lett. Vol.34 (2007)) that sprites are actually clusters of small, decameter-sized (10-100 m, 30-300 ft) balls of ionization that move downward, followed a few milliseconds later by a separate set of upward moving balls of ionization, and traveling up to ten percent of the speed of light. Image:BigRed(thumb).jpg Sprites are sometimes, but not always, preceded, by about 1 millisecond, by a sprite halo, a pancake-shaped region of weak, transient optical emissions approximately 50 km (37 mi) across and 10 km (6 mi) thick centered at about 70 km (44 mi) altitude above the same lightning stroke that produces sprites. Sprite 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. Sprite halos have previously been mistaken for ELVES, due to their visual similarity and short duration.[4][5] Since their 1989 discovery, sprites have been imaged tens-of-thousands of times, from the ground, from aircraft, and from satellites, the Space Shuttle and the International Space Station. They have been observed over North, Central and South America, Europe, Australia, the Sea of Japan, and Asia. Sprites are believed to be a ubiquitous feature of most large thunderstorm systems, and to have occurred along with lightning from the earliest days of the formation of the earth's atmosphere. [edit] Sprite-implicated aircraft damage Sprites are occasionally held responsible for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms. One example of this is the malfunction of a NASA stratospheric balloon launched on June 6, 1989 from Palestine, Texas. The balloon suffered an uncommanded payload release while flying at 120,000 feet over a thunderstorm near Graham, Texas. Months after the accident, a post-flight investigation concluded that a "lighting bolt" traveling upwards from the clouds provoked the incident [6]. 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 (120,000 ft = 37 km = 23 mi), 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 (50-90 km) sprites.
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