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Sprites orred sprites are large-scaleelectric discharges that occur in themesosphere, high abovethunderstorm clouds, orcumulonimbus, giving rise to a varied range of visual shapes flickering in the night sky. They are usually triggered by the discharges ofpositive lightning between an underlyingthundercloud and the ground.
Sprites appear as luminous red-orange flashes. They often occur in clusters above thetroposphere at an altitude range of 50–90 km (31–56 mi). Sporadic visual reports of sprites go back at least to 1886.[1] They were first photographed on July 4, 1989,[2] byscientists from theUniversity of Minnesota and have subsequently been captured in video recordings thousands of times.
Sprites are sometimes inaccurately calledupper-atmospheric lightning. However, they are coldplasma phenomena that lack the hot channel temperatures oftropospheric lightning, so they are more akin tofluorescent tube discharges than to lightning discharges. Sprites are associated with various other upper-atmosphericoptical phenomena includingblue jets andELVES.[1]
The earliest known report is by Toynbee and Mackenzie in 1886.[3] Nobel laureateC. T. R. Wilson had suggested in 1925, on theoretical grounds, that electrical breakdown could occur in the upper atmosphere, and in 1956 he witnessed what possibly could have been a sprite. They were first documented photographically on July 6, 1989, when scientists from theUniversity of Minnesota, using a low-light video camera, accidentally captured the first image of what would subsequently become known as a sprite.[4]
Several years after their discovery they were named sprites (air spirits) aftertheir namesake mythological entity based on their elusive nature.[5] Since the 1989 video capture, sprites have been imaged from the ground, from aircraft and from space, and have become the subject of intensive investigations. A high-speed video captured byThomas Ashcraft, Jacob L Harley, Matthew G McHarg, and Hans Nielsen in 2019 at about 100,000 frames per second provided some observations on how sprites develop. However, according to NASA's APOD blog, despite being recorded in photographs and videos for more than 30 years, the "root cause" of sprite lightning remains unknown, "apart from a general association with positive cloud-to-ground lightning." NASA also notes that not all storms exhibit sprite lightning.[6]
In 2016, sprites were observed duringHurricane Matthew's passage through the Caribbean.[7] The role of sprites in the tropical cyclones is presently unknown.[8]
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Sprites have been observed overNorth America,[9]Central America,South America,[10]Europe,[11]Central Africa (Zaire),Australia, theSea of Japan andAsia and are believed to occur during most large thunderstorm systems.
Rodger (1999) categorized three types of sprites based on their visual appearance.[1]
Sprites are colored reddish-orange[5] in their upper regions, with bluish hangingtendrils below, and can be preceded by a reddish halo. They last longer than normal lower stratospheric discharges, which last typically a few milliseconds, and are usually triggered by the discharges of positive lightning between the thundercloud and the ground,[12] although sprites generated by negative ground flashes have also been observed.[13] They often occur in clusters of two or more, and typically span the altitude range 50 to 90 kilometres (31 to 56 mi), with what appear to be tendrils hanging below, and branches reaching above.[5]
Optical imaging using a 10,000 frame-per-secondhigh speed camera showed that sprites are actually clusters of small, decameter scale, (10–100 m or 33–328 ft) balls of ionization that are launched at an altitude of about 80 km (50 mi) and then move downward at speeds of up to ten percent thespeed of light, followed a few milliseconds later by a separate set of upward moving balls of ionization.[14] Sprites may be horizontally displaced by up to 50 km (31 mi) 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.
In order to film sprites from Earth, special conditions must be present: 150–500 km (93–311 mi) of clear view to a powerful thunderstorm with positive lightning between cloud and ground, red-sensitive recording equipment, and a black unlit sky.[15]
Sprites occur near the top of themesosphere at about 80 km altitude in response to the electric field generated by lightning flashes in underlying thunderstorms. When a sufficiently large positive lightning strike carriescharges to the ground, the cloud top is left with a strongly negative net charge. This can be modeled as a quasi-static electric dipole and for less than 10 milliseconds a strongelectric field is generated in the region above the thunderstorm. In the low pressure of the upper mesosphere thebreakdown voltage is drastically reduced, allowing for anelectron avalanche to occur.[16][17] Sprites get their characteristic red color from excitation of nitrogen in the low pressure environment of the upper mesosphere. At such low pressuresquenching by atomic oxygen is much faster than that of nitrogen, allowing for nitrogen emissions to dominate despite no difference in composition.[18][19]
Sprites are sometimes preceded, by about 1 millisecond, by a spritehalo, a pancake-shaped region of weak, transientopticalemissions approximately 50 kilometres (31 mi) across and 10 kilometres (6.2 mi) thick. The halo is centered at about 70 kilometres (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 forELVES, due to their visual similarity and short duration.[20][21][22]
Research carried out atStanford University in 2000 indicates that, unlike sprites with bright vertical columnar structure, occurrence of sprite halos is not unusual in association with normal (negative) lightning discharges.[22]Research in 2004 by scientists fromTohoku University found thatvery low frequency emissions occur at the same time as the sprite, indicating that a discharge within the cloud may generate the sprites.[23]
Sprites have been blamed for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms. One example of this is the malfunction of aNASAstratospheric balloon launched on June 6, 1989, fromPalestine, Texas. The balloon suffered an uncommanded payload release while flying at 120,000 feet (37,000 m) over a thunderstorm nearGraham, Texas. Months after the accident, an investigation concluded that a "bolt of lightning" traveling upward from the clouds provoked the incident.[24] The attribution of the accident to a sprite was made retroactively, since this term was not coined until late 1993.
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