Upper-atmospheric lightning

Upper-atmospheric lightning is an early term sometimes invoked by researchers to refer to a family of electrical breakdown phenomena that occurs well above the altitudes of normal lightning. Also known as "Transient Luminous Events," these include sprites, sprite halos, blue jets, gigantic jets, and ELVES.

Representation of upper atmospheric lightning and electrical discharge phenomena

Researchers have speculated that upper atmospheric lightning may play a role in the formation of the ozone layer. Alternatively, they may be due to differences in potential that result in current from the ozone layer.

History

Reports by scientists of unusual lightning phenomena above thunderstorms date back to at least 1886. However, it is only in recent years that further investigations have been made.

Jets

Blue starters

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

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). 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; less than a hundred images have been obtained to date (2007). The majority of these images, which include the first color imagery, were obtained by researchers from the University of Alaska during a single aircraft flight in 1994 to study an intense Arkansas thunderstorm.

Gigantic Jets

On September 14, 2001, scientists at the Arecibo Observatory photographed a huge jet double the height of those previously observed, reaching around 80 km (50 miles) into the atmosphere. 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. The jets lasted under a second, with shapes likened by the researchers to giant trees and carrots.

Sprites

History

Red sprite

In the 1920s, the Scottish physicist C. T. R. Wilson predicted that electrical breakdown should occur in the atmosphere high above large thunderstorms. 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.'

Since then, sprites have been imaged tens-of-thousands of times, from the ground, from aircraft, and from satellites, the Space Shuttle and the International Space Station. They have been observed over North, Central and South America, Europe, Australia, the Sea of Japan, and Asia. Sprites are believed to be a ubiquitous feature of most large thunderstorm systems, and to have occurred along with lightning from the earliest days of the formation of the earth's atmosphere.

Description

Sprites are large scale electrical discharges that occur high above the trailing stratoform regions of active thunderstorm systems. They are a particular type of electrical breakdown characteristic of the early stages of lightning discharges (the streamer stage), but lacking the later stages of leaders and return strokes. Sprites appear as luminous reddish-orange, plasma-like flashes, that last longer (typically a few milliseconds) than normal lower stratospheric discharges (typically about 0.1-0.2 milliseconds). The red color of sprites has been determined from spectrographic measurements to be due to optical fluorescence from neutral molecular nitrogen in the air. They are triggered by the transient electric fields associated with discharges of positive lightning between the cloud and the ground, and are generally delayed behind the stroke by a few milliseconds. Sprites are typically centered above the causative lightning strokes, but may be laterally displaced from them by up to 50 km (30 miles).

Sprites possess complex, vertically striated internal structure, giving rise to a quite varied range of visual shapes. Their ignition altitude is about 75 km (47 mi), and from here they often develop both downward "tendrils" that extend to about 50 km (31 mi), and upward "branches" that extend to about 90 km (56 mi). Optical imaging using a 10,000 frames per second high speed camera shows (Geophysical Research Lett. Vol.34 (2007)) that sprites are actually clusters of small, decameter-sized (10-100 m, 30-300 ft) balls of ionization that move downward, followed a few milliseconds later by a separate set of upward moving balls of ionization, and traveling up to ten percent of the speed of light.

Sprites are sometimes, but not always, preceded, by about 1 millisecond, by a sprite halo, a pancake-shaped region of weak, transient optical emissions approximately 50 km (37 mi) across and 10 km (6 mi) thick centered at about 70 km (44 mi) altitude above the same lightning stroke that produces sprites. Sprite 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. Sprite halos have previously been mistaken for ELVES, due to their visual similarity and short duration.

Sprite-implicated damage

"Sprites" are occasionally 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 over a thunderstorm near Graham, Texas. Months after the accident, a post-flight investigation concluded that a "lighting bolt" traveling upwards from the clouds provoked the incident . 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 (120,000 ft = 37 km = 23 mi), 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 (50-90 km) sprites.

ELVES

ELVES often appear as a dim, flattened, expanding glow around 400 km (250 miles) in diameter that lasts for, typically, just one millisecond. 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. 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 the Ionosphere).

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