Sprite (lightning)

Not to be confused withSprite (computer graphics).

A sprite at the horizon, with lightning below in thetroposphereand above the green line ofairglowat the uppermesopauseandborder to space(the bright light above is the Moon).

Spritesorred spritesare large-scaleelectric dischargesthat occur in themesosphere,high abovethunderstormclouds, orcumulonimbus,giving rise to a varied range of visual shapes flickering in the night sky. They are usually triggered by the discharges ofpositive lightningbetween an underlyingthundercloudand the ground.

Precis

• 
  First color image of a sprite, taken from an aircraft
• 
  A sprite overLaos,as seen from theInternational Space Station

Sprites appear as luminous red-orange flashes. They often occur in clusters above thetroposphereat an altitude range of 50–90 km (31–56 mi). Sporadic visual reports of sprites go back at least to 1886.They were first photographed on July 4, 1989,byscientistsfrom theUniversity of Minnesotaand have subsequently been captured in video recordings thousands of times.

Sprites are sometimes inaccurately calledupper-atmospheric lightning.However, they are coldplasmaphenomena that lack the hot channel temperatures oftroposphericlightning, so they are more akin tofluorescent tubedischarges than to lightning discharges. Sprites are associated with various other upper-atmosphericoptical phenomenaincludingblue jetsandELVES.

History

The earliest known report is by Toynbee and Mackenzie in 1886.Nobel laureateC. T. R. Wilsonhad 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.

Several years after their discovery they were namedsprites(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 byThomas 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 duringHurricane Matthew'spassage through the Caribbean.The role of sprites in the tropical cyclones is presently unknown.

Characteristics

• 
  Different types of electrical phenomena in the atmosphere
• 
  A sprite seen from the International Space Station (top right, faint red above the lightning).
• 
  ISS sprite image above; zoomed in
• 
  Another shot from the first color clip of the sprite.

Sprites have been observed overNorth America,Central America,South America,Europe,Central Africa(Zaire),Australia,theSea of JapanandAsiaand 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-orangein their upper regions, with bluish hangingtendrilsbelow, 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.

Optical imaging using a 10,000 frame-per-secondhigh speed camerashowed 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.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 theISSshows 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.

Mechanism

Sprites occur near the top of themesosphereat about 80 km altitude in response to the electric field generated by lightning flashes in underlying thunderstorms. When a sufficiently large positive lightning strike carrieschargesto 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 fieldis generated in the region above the thunderstorm. In the low pressure of the upper mesosphere thebreakdown voltageis drastically reduced, allowing for anelectron avalancheto occur.Sprites get their characteristic red color from excitation of nitrogen in the low pressure environment of the upper mesosphere. At such low pressuresquenchingby 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 spritehalo,a pancake-shaped region of weak, transientopticalemissionsapproximately 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.

Research carried out atStanford Universityin 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 fromTohoku Universityfound thatvery low frequencyemissions 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 aNASAstratosphericballoon 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.The attribution of the accident to a sprite was made retroactively, since this term was not coined until late 1993.

See also

• Upper-atmospheric lightning(includes Blue Jets)
• 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 theUniversity of Alaska Fairbanksthat popularized the terms
• "Red Sprites and Blue Jets".Archived fromthe originalon 2002-09-14.Retrieved2012-08-20.– webpage byUniversity of Alaska Fairbanks
• Ground and Balloon-Borne Observations of Sprites and Jets
• Darwin Sprites '97Space 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.Retrieved2012-07-13.{{cite web}}:CS1 maint: bot: original URL status unknown (link)
• Short professional bio of Dr 'Geoff' McHarg
• Photography websitePetapixelposted a link to a very rare and very clear photograph of a sprite taken by photographer Mike Hollingshead. Article atPhotographer Captures Rare Photograph of a Sprite with an Aurora
• At the Edge of Space– aNOVAprogram that examines the phenomenon of Sprites
• Red Sprites Over Adriatic SeaSeen from the Czech Republic (14 January 2019)