Sprite (lightning)

First color image of a sprite, taken from an aircraft

A sprite seen from the International Space Station (top right, faint red above the lightning).

Sprites or red sprites are large-scale electrical discharges that occur high above thunderstorm clouds, or cumulonimbus, giving rise to a quite 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.

Sprites appear as luminous reddish-orange flashes. They often occur in clusters above the troposphere at an altitude range of 50–90 km (31–56 mi). Sporadic visual reports of sprites go back at least to 1886 but they were first photographed on July 6, 1989, by scientists from the University of Minnesota and have subsequently been captured in video recordings many thousands of times.

Sprites are sometimes inaccurately called upper-atmospheric lightning. However, sprites 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.

ISS sprite image above; zoomed in

History

The earliest known report of transient optical phenomena above thunderclouds is from Johann Georg Estor in 1730. Another early 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.

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

Different types of electrical phenomena in the atmosphere

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 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.

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 kilometres (31 to 56 mi), with what appear to be tendrils hanging below, and branches reaching above.

A sprite over Laos, as seen from the International Space Station

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

This footage from the ISS shows a red sprite over East Asia immediately before 0:07, directly above the large lightning flash towards the upper right of the frame.

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.

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 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 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 120,000 feet (37,000 m) 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.

See also

• Aurora (astronomy)
• Catatumbo lightning
• Cosmic ray visual phenomena

External links

• "Red Sprites & Blue Jets" – a digital capture of the VHS video distributed in 1994 by the University of Alaska Fairbanks that popularized the terms
• "Red Sprites and Blue Jets". Archived from the original on 2002-09-14. Retrieved 2012-08-20. – webpage by University of Alaska Fairbanks
• Ground and Balloon-Borne Observations of Sprites and Jets
• Darwin Sprites '97 Space Physics Group, University of Otago
• Sprites, jets and TLE pictures and articles
• Sprites in Europe: European contributors blog
• Kanmae, T.; Stenbaek-Nielsen, H.C.; McHarg, M.G.; Haaland, R.K. (2010). "Observation of blue sprite spectra at 10,000 fps". Geophys. Res. Lett. 37 (13): L13808. Bibcode:2010GeoRL..3713808K. doi:10.1029/2010GL043739.
• "Mysterious Sprite Photographed by ISS Astronaut: Big Pic". Archived from the original on 2012-07-14. Retrieved 2012-07-13.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
• Short professional bio of Dr 'Geoff' McHarg
• Photography website Petapixel posted a link to a very rare and very clear photograph of a sprite taken by photographer Mike Hollingshead. Article at Photographer Captures Rare Photograph of a Sprite with an Aurora
• At the Edge of Space – a NOVA program that examines the phenomenon of Sprites
• Red Sprites Over Adriatic Sea Seen from the Czech Republic (14 January 2019)