Upper-atmospheric lightning

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Upper-atmospheric lightning or upper-atmospheric discharge are terms sometimes used by researchers to refer to a family of electrical-breakdown phenomena that occur well above the altitudes of normal lightning. The preferred current usage is transient luminous events (TLEs) to refer to the various types of electrical-discharge phenomena in the upper atmosphere, because they lack several characteristics of the more familiar tropospheric lightning. TLEs include red sprites, sprite halos, blue jets, gigantic jets, and elves.

Contents 1 History 2 Sprites 3 Jets 3.1 Blue jets 3.2 Blue starters 3.3 Gigantic jets 4 Elves 5 See also 6 References 7 External links

[edit] History

In the 1920s, the Scottish physicist C. T. R. Wilson predicted that electrical breakdown should occur in the atmosphere high above large thunderstorms.^[1] However, it was not until July 6, 1989 that the first direct visual evidence of high altitude electrical discharges was documented by scientists from the University of Minnesota. Several years after their discovery, the optical signatures of these events were named 'sprites' by researchers at the University of Alaska to avoid inadvertently implying physical properties that were, at the time, still unknown. The terms red sprites and blue jets gained popularity after a video clip was circulated following an aircraft research campaign to study sprites in 1994.^[2]

[edit] Sprites

Sprites are large scale electrical discharges which occur high above a thunderstorm cloud, or cumulonimbus, giving rise to a quite varied range of visual shapes. They are triggered by the discharges of positive lightning between the thundercloud and the ground.^[3] The phenomena were named after the mischievous sprite (air spirit) Ariel in Shakespeare's The Tempest. They normally are colored reddish-orange or greenish-blue, with hanging tendrils below, and arcing branches above, their location, and can be preceded by a reddish halo.^[4] They often occur in clusters, lying 50 miles (80 km) to 90 miles (145 km) above the Earth's surface. Sprites were first photographed on July 6, 1989 by scientists from the University of Minnesota and have since been witnessed tens of thousands of times.^[5] Sprites have erroneously been held responsible for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms.^[6]

[edit] Jets

[edit] Blue jets

Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 40 to 50 km (25 to 30 miles) above the earth. In addition, whereas red sprites tend to be associated with significant lightning strikes, blue jets do not appear to be directly triggered by lightning (they do, however, appear to relate to strong hail activity in thunderstorms).^[7] They are also brighter than sprites and, as implied by their name, are blue in color. The color is believed to be due to a set of blue and near-ultraviolet emission lines from neutral and ionized molecular nitrogen. They were first recorded on October 21, 1989, on a monochrome video of a thunderstorm on the horizon taken from the Space Shuttle as it passed over Australia. Blue jets occur much less frequently than sprites; fewer than a hundred images have been obtained to date (2007). The majority of these images, which include the first color imagery, were associated with a single thunderstorm being studied by researchers from the University of Alaska in a series of 1994 aircraft flights to study sprites^[8].

[edit] Blue starters

Blue starters were discovered on video from a nighttime research flight around thunderstorms ^[9] and appear to be "an upward moving luminous phenomenon closely related to blue jets."^[10] They appear to be shorter and brighter than blue jets, reaching altitudes of only up to 20 km.^[11] "Blue starters appear to be blue jets that never quite make it," according to Dr. Victor P. Pasko, associate professor of electrical engineering.^[12]

[edit] Gigantic jets

On September 14, 2001, scientists at the Arecibo Observatory photographed a huge jet double the height of those previously observed, reaching around 70 km (43 miles) into the atmosphere.^[13] The jet was located above a thunderstorm over the ocean, and lasted under a second. Lightning was initially observed traveling up at around 50,000 m/s in a similar way to a typical blue jet, but then divided in two and sped at 250,000 m/s to the ionosphere, where they spread out in a bright burst of light.

On July 22, 2002, five gigantic jets between 60 and 70 km (35 to 45 miles) in length were observed over the South China Sea from Taiwan, reported in Nature.^[14][15] The jets lasted under a second, with shapes likened by the researchers to giant trees and carrots.

[edit] Elves

Elves often appear as a dim, flattened, expanding glow around 400 km (250 miles) in diameter that lasts for, typically, just one millisecond.^[16] They occur in the ionosphere 100 km (60 miles) above the ground over thunderstorms. Their color was a puzzle for some time, but is now believed to be a red hue. Elves were first recorded on another shuttle mission, this time recorded off French Guiana on October 7, 1990.

Elves is a frivolous acronym for Emissions of Light and Very Low Frequency Perturbations from Electromagnetic Pulse Sources.^[17] This refers to the process by which the light is generated; the excitation of nitrogen molecules due to electron collisions (the electrons possibly having been energized by the electromagnetic pulse caused by a discharge from an underlying thunderstorm).

[edit] See also

• Transient luminous event (TLE)
• Megalightning
• St. Elmo's fire
• Aurora (astronomy)
• sprite (lightning)

[edit] References

[edit] External links

• Homepage of the Eurosprite campaign, itself part of the CAL (Coupled Atmospheric Layers) research group
• March 2, 1999, University of Houston: UH Physicists Pursue Lightning-Like Mysteries Quote: "...Red sprites and blue jets are brief but powerful lightning-like flashes that appear at altitudes of 40-100 km (25-60 miles) above thunderstorms..."
  • Ground and Balloon-Borne Observations of Sprites and Jets
• Barrington-Leigh, C. P., "Elves : Ionospheric Heating By the Electromagnetic Pulses from Lightning (A primer)". Space Science Lab, Berkeley.
• "Darwin Sprites '97". Space Physics Group, University of Otago.
• Gibbs, W. Wayt, "Sprites and Elves : Lightning's strange cousins flicker faster than light itself". San Francisco. ScientificAmerican.com.
• Barrington-Leigh, Christopher, "VLF Research at Palmer Station".
• Sprites, jets and TLE pictures and articles
• High speed video (10,000 fps) taken by Hans Stenbaek-Nielsen, University of Alaska
• Video Reveals 'Sprite' Lightning Secrets, Livescience article, 2007.
  • Video evidence
• Gigantic Jets Over Oklahoma An Astronomy Picture of the Day article with pictures and video of two separate gigantic jets above Oklahoma
• Gigantic jets between a thundercloud and the ionosphere.
• Huge Mystery Flashes Seen In Outer Atmosphere
• Sprite Gallery