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Sprite (lightning)

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First color image of a sprite, taken from an aircraft.
Differents types of electrical phenomena in the atmosphere.

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 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 within the altitude range 50–90 km above the Earth's surface. 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.

History

Allusions to transient optical phenomena above thunderclouds can be found in anecdotal reports from as early as 1730 (see Johann Georg Estor). 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 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.[1]

Several years after their discovery they were named sprites (air spirits) after the mischievous character Puck in Shakespeare's Midsummer Night's Dream.[2] Since their 1989 discovery, sprites have been imaged tens-of-thousands of times, from the ground, from aircraft, and from space, and have become the subject of intensive investigations.

Characteristics

Sprites have been observed over North America,[3] Central America, South America,[4] Europe, Southern Africa (Zaire), Australia, the Sea of Japan and Asia and are believed to occur during most large thunderstorm systems. Sprites are colored reddish-orange[2] 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 triggered by the discharges of positive lightning between the thundercloud and the ground.[5] They often occur in clusters of two or more, and typically span the altitude range 50 kilometres (31 mi) to 90 kilometres (56 mi), with what appear to be tendrils hanging below, and branches reaching above.[2]

Optical imaging using a 10,000 frames per second high speed camera shows that sprites are actually clusters of small, decameter-sized (10–100 m, 30–300 ft) 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.[6] 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.

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.[7][8]

Recent research carried out at the University of Houston in 2002 indicates that some normal (negative) lightning discharges produce a sprite halo, and that every lightning bolt between cloud and ground attempts to produce a sprite or a sprite halo.[9] 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.[10]

Sprites have erroneously been held responsible 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, a post-flight investigation concluded[citation needed] that a "bolt of lightning" traveling upward from the clouds provoked the incident.[11] The attribution of the accident to a sprite was evidently made retroactively by several years, since this term was not coined until late 1993. Because of the comparatively low altitude of the balloon, whatever thunderstorm-related discharge may have been a causative factor in the accident, it was more likely to have been one of several other types of stratospheric discharges known to occur, such as blue jets, rather than the higher altitude sprites.[citation needed]

See also

References

  1. ^ Walter A. Lyons and Michey D. Schmidt (2003). P1.39 The Discovery of Red Sprites as an Opportunity For Informal Science Education. American Meteorological Society. Retrieved on 2009-02-18.
  2. ^ a b c Sentman, D.D.; Wescott, E. M.; Osborne, D. L.; Hampton, D. L.; Heavner, M. J.; et al. (1995). "Preliminary results from the Sprites94 aircraft campaign: 1. Red Sprites". Geophys. Res. Lett. 22 (10): 1205–1208. Bibcode:1995GeoRL..22.1205S. doi:10.1029/95GL00583. {{cite journal}}: Explicit use of et al. in: |first= (help)
  3. ^ Kathy Berry (1994). Spectacular Color Flashes Recorded Above Electrical Storms. NASA. Retrieved on 2009-02-18.
  4. ^ Don Savage and Kathy Berry (1995). Sprites Confirmed Over Storms Outside U.S. For First Time. NASA. Retrieved on 2009-02-18.
  5. ^ Boccippio, D. J.; Williams, ER; Heckman, SJ; Lyons, WA; Baker, IT; Boldi, R; et al. (1995). "Sprites, ELF Transients, and Positive Ground Strokes". Science. 269 (5227): 1088–1091. Bibcode:1995Sci...269.1088B. doi:10.1126/science.269.5227.1088. PMID 17755531. {{cite journal}}: Explicit use of et al. in: |first= (help); Unknown parameter |month= ignored (help)
  6. ^ Stenbaek-Nielsen, H. C.; McHarg, M.G.; Kanmae, T.; Sentman, D.D. (June 6, 2007). "Observed emission rates in sprite streamer heads". Geophys. Res. Lett. 34 (11): L11105. Bibcode:2007GeoRL..3411105S. doi:10.1029/2007GL029881. L11105.
  7. ^ Rina Miyasato, Hiroshi Fukunishi, Yukihiro Takahashi, Michael J. Taylor, Hans. C. Stenbaek-Nielsen (2002). Characteristics of Lightning-induced Sprite Halos and Their Generation Mechanisms. Academic Society Home Village. Retrieved on 2009-02-18.
  8. ^ Christopher Barrington Leigh (2000). Sprite halos. Stanford University. Retrieved on 2008-02-18.
  9. ^ Amanda Siegfried (2002). New thinking needed on atmospheric physics, study suggests. University of Houston. Retrieved on 2009-02-18.
  10. ^ A. Ohkubo, H. Fukunishi, Y. Takahashi, T. Adachi (2005). "VLF/ELF sferic evidence for in-cloud discharge activity producing sprites". Department of Geophysics, Tohoku University, Sendai, Japan. doi:10.1029%2F2004GL021943. {{cite web}}: |access-date= requires |url= (help); Check |doi= value (help); Missing or empty |url= (help)CS1 maint: multiple names: authors list (link)
  11. ^ STRATOCAT (2009). "Data of the stratospheric balloon launched on 6/5/1989 from [[Columbia Scientific Balloon Facility]], Palestine, Texas, US for Molecules observation made fluorescent with a Laser". Retrieved 2009-02-18. {{cite web}}: URL–wikilink conflict (help)
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