Sprite (lightning)

Sprites or red sprites are large-scale electric discharges that occur in the mesosphere, high above thunderstorm clouds, or cumulonimbus, giving rise to a varied range of visual shapes flickering in the night sky. They are usually triggered by the discharges of positive lightning between an underlying thundercloud and the ground.

Precis
Sprites appear as luminous red-orange flashes. They often occur in clusters above the troposphere at an altitude range of 50 - 90 km. Sporadic visual reports of sprites go back at least to 1886. They were first photographed on July 4, 1989, by scientists from the University of Minnesota and have subsequently been captured in video recordings thousands of times.

Sprites are sometimes inaccurately called upper-atmospheric lightning. However, they are cold plasma phenomena that lack the hot channel temperatures of tropospheric lightning, so they are more akin to fluorescent tube discharges than to lightning discharges. Sprites are associated with various other upper-atmospheric optical phenomena including blue jets and ELVES.

History
The earliest known report is by Toynbee and Mackenzie in 1886. Nobel laureate C. 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 the University of Minnesota, using a low-light video camera, accidentally captured the first image of what would subsequently become known as a sprite.

Several years after their discovery they were named sprites (air spirits) after their elusive nature. 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 featured high speed video that was captured by Thomas Ashcraft, Jacob L Harley, Matthew G McHarg, and Hans Nielsen in 2019 at about 100,000 frames per second is fast enough to provide better detailing of how sprites develop. However, according to NASA's APOD blog, despite being recorded in photographs and videos for the 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.

In 2016, sprites were observed during Hurricane Matthew's passage through the Caribbean. The role of sprites in the tropical cyclones is presently unknown.

Characteristics
Sprites have been observed over North America, Central America, South America, Europe, Central Africa (Zaire), Australia, the Sea of Japan and Asia and are believed to occur during most large thunderstorm systems.

Rodger (1999) categorized three types of sprites based on their visual appearance.
 * Jellyfish sprite – very large, up to 50 by.
 * Column sprite (C-sprite) – large-scale electrical discharges above the earth that are still not totally understood.
 * Carrot sprite – a column sprite with long tendrils.

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, and are usually triggered by the discharges of positive lightning between the thundercloud and the ground, although sprites generated by negative ground flashes have also been observed. They often occur in clusters of two or more, and typically span the altitude range 50 to 90 km, with what appear to be tendrils hanging below, and branches reaching above.

Optical imaging using a 10,000 frame-per-second high speed camera showed that sprites are actually clusters of small, decameter scale, (10 –) 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.



In order to film sprites from Earth, special conditions must be present: 150 - 500 km of clear view to a powerful thunderstorm with positive lightning between cloud and ground, red-sensitive recording equipment, and a black unlit sky.

Mechanism
Sprites occur near the top of the mesosphere 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 carries charges 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 strong electric field is generated in the region above the thunderstorm. In the low pressure of the upper mesosphere the breakdown voltage is drastically reduced, allowing for an electron avalanche to occur. Sprites get their characteristic red color from excitation of nitrogen in the low pressure environment of the upper mesosphere. At such low pressures quenching by atomic oxygen is much faster than that of nitrogen, allowing for nitrogen emissions to dominate despite no difference in composition.

Sprite halo
Sprites are sometimes preceded, by about 1 millisecond, by a sprite halo, a pancake-shaped region of weak, transient optical emissions approximately 50 km across and 10 km thick. The halo is centered at about 70 km 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.

Research carried out at Stanford 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. Research in 2004 by scientists from Tohoku University found that very low frequency emissions occur at the same time as the sprite, indicating that a discharge within the cloud may generate the sprites.

Related aircraft damage
Sprites have been blamed 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 120000 ft over a thunderstorm near Graham, Texas. Months after the accident, an investigation concluded that a "bolt of lightning" traveling upward from the clouds provoked the incident. The attribution of the accident to a sprite was made retroactively, since this term was not coined until late 1993.