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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.

They often occur in clusters above the troposphere at an altitude range of 50–90 km (31–56 mi). 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. 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.

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. 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 by Thomas 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.

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. Jellyfish sprite – very large, up to 50 by 50 km (31 by 31 mi). 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. 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. 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.

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.

Sprites are sometimes preceded, by about 1 millisecond, by a sprite halo, a pancake-shaped region of weak, transient optical emissions 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. 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.

Quick Facts

• They are usually triggered by the discharges of positive lightning between an underlying thundercloud and the ground.
• 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.
• 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.
• 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.
• 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.

Source material: Wikipedia - "Sprite (lightning)". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.