In recent decades, researchers have begun looking at some strange optical phenomena in the cloud tops. High above ordinary lightning, exotic forms known as sprites and elves shoot upwards to the electrically charged ionosphere. They are cool and colourful relatives to the sizzling bolts that plunge down towards the Earth.Once referred to as Upper-atmospheric lightning or Ionospheric Lightning, these short-lived electrical-breakdown phenomena occur well above the altitudes of normal lightning and storm clouds.The terminology that is now preferred is transient luminous event (TLE), because the various types of upper atmospheric electrical-discharge events lack several characteristics of the more familiar tropospheric lightning.TLEs are secondary phenomena that occur in association with underlying thunderstorm lightning.They generally last anywhere from less than a millisecond to more than 2 seconds.There are several types of TLEs, the most common being sprites. Sprites are flashes of bright red light that occur above storm systems. Other types of TLEs include sprite halos, blue jets, blue starters, elves, gnomes and trolls.
Sprites and Elves
Airline pilots first noticed these flashes while flying over severe thunderstorms and later, they appeared on videos taken from NASA’s space shuttles.
“Sprite” stands for Stratospheric/Mesospheric Perturbations Resulting from Intense Thunderstorm Electrification, and “elve” for Emissions of Light and VLF perturbations from EMP events.
Sprites have a distinctive delicate shape, much like a small human form, which flashes for only a fraction of a second in the sky. They are primarily red in colour, except sometimes there is a bluish tinge in the downward extending tendrils.
Current thinking is that sprites result when free electrons in the thin atmosphere are accelerated by the sudden change in electric field strength caused by the parent lightning discharge far below. When the electrons slam into molecules of nitrogen, they cause the nitrogen to glow. Certain energies result in primarily red optical emissions, but in the lower part of the sprite, blue colours can also be seen. The process is similar to the aurora, where the energetic particles are supplied by the solar wind.
Elves are disks of dim light that appear in the ionosphere above the ground over thunderstorms. They occur at a height of around 95-105 km (60-65 miles), and can expand outward to around 400 km (250 miles) in diameter.
Elves result from especially powerful electromagnetic radiation pulses (EMP) that emanate from certain lightning discharges. As the energy passes upwards through the base of the ionosphere it causes the gases to glow briefly. Though as bright as a sprite, elves last for less than a thousandth of a second. This makes them virtually impossible to see with the naked eye. Though if you could see them, they would look like giant expanding doughnuts.
Their color was a puzzle for some time, but is now believed to be a red hue.
Sprite haloes are disks of light, like elves, but are smaller and lower, beginning at about 85 km and moving down to 70 km. They last about a millisecond and are followed by sprites, which seem to grow right from their disks. Sprite haloes are thought to be an initial stage of sprites.
Sprites and elves occur separately or together, appearing above supercell thunderstorms or “mesoscale convective systems.
“Sprites occur in the middle atmosphere, above an active thunderstorm system, and are the result of extremely powerful lightning discharges periodically occurring within the storm.
Current evidence suggests that sprites tend to occur in decaying portions of thunderstorms and are usually triggered by a powerful positive cloud-to-ground (CG) flash which lowers massive amounts of electrical charge to the Earth. They may also be coincident with intracloud lightning strikes. Only a very small percentage (<10%) of positive CGs actually produce sprites.
When the electric field is momentarily increased beyond the point of “dielectric breakdown”, a giant spark occurs, usually starting around 70 km (45 miles) above the ground.
The Sprites are massive but weak luminous flashes. They may look rather “solid” in many images, but when viewed through telescopes, many sprites are actually composed of networks of thin channels of electrical streamers that race both downwards and upwards from that point.
These optical occurrences often start in the mesosphere at around 70 km (45 miles) high. The brightest region being in the altitude range of 65-75 km, above which there is often a faint red glow or wispy structure that extends upwards to the edge of the ionosphere at about 90 km. Below the bright red region, blue tendril-like lamentary structures often extend downward into the stratosphere to as low as 40 km (25 miles).
Aside from stretching vertically for as much as 70 km (45 miles), the sprite can often be tens of miles across.
Sprites rarely appear singly, usually occurring in clusters of two, three or more. Some of the very large events seem to be tightly packed clusters of many individual sprites. Other events are more loosely packed and may extend across horizontal distances of 50 km or more and occupy atmospheric volumes in excess of 10,000 cubic km.
Sprites and related phenomena can be detected in a variety of ways. Aside from the naked eye, low-light cameras and sensitive optical sensors are the main instruments used to detect sprites.
High-speed photometer measurements show that the duration of sprites is only a few (3-10) milliseconds.
The portion of the event that might be visible to the naked eye often lasts less than one hundredth of a second. When using a “night vision” camera, they are visible for longer periods, but rarely more than one tenth of a second.
The optical intensity of sprite clusters, estimated by comparison with tabulated stellar intensities, is comparable to a moderately bright auroral arc. However, to see them requires a dark-adapted eye and visual access to the region above the storm so they can be seen against a dark stellar background, unobstructed by intervening clouds.
Best viewing distance from storm is 200-300 km (100-200 miles). At these distances, sprites will subtend a vertical angular distance of 10-20 degrees. This is 2-4 times the separation of the pointer stars in the Big Dipper.
Because of their dimness, sprites cannot be viewed in the presence of nearby bright lights, as would be found in a city. One can also detect sprites, or more properly the radio emissions from their parent lightning by using extremely low frequency (ELF) and very low frequency (VLF) radio receivers.
When there is an electrical disturbance in the atmosphere, such as a lightning storm, EM waves travel from the centre of the disturbance outward, much like the ripples caused by dropping a pebble into still water. Recordings of the electromagnetic waves can be used to create a graphical representation of the phenomena, even from a long distance away.
Most lightning produces an electric signal that’s very sharp. Generally, these are short, quick, and bipolar. However, with some lightning, the sharp signal is followed by what scientists call a “slow tail,” a lasting impression on the screen that resembles a tail.When a slow tail is recorded, either or both a sprite and an elve would have occurred.
There is growing evidence that sprites – or their parent lightning – may produce sound waves at extremely low frequencies (around 1 Hertz). Below the range of human hearing, these infrasound waves can be picked up by special receivers at ranges of hundreds if not thousands of miles away. While not thunder in the usual sense of the word, these infrasound emissions are the acoustic signature of the processes that produce sprites.
Another TLE that might be related to sprites are referred to as Trolls (for Transient Red Optical Luminous Lineament). These red spots pop up near cloud tops after the flash of an extremely strong red sprite.
Trolls occur after an especially strong sprite, down near the cloud tops. Early recordings showed them as red spots with faint red tails, rising much like blue jets. Faster cameras show trolls to be a rapid series of events. Each event starts with a red glow that forms in a sprite tendril, then “drains” downward. Each following event starts higher, so that the series looks like an upward blur in slower videos.
The first documented blue jet was recorded by accident, on low-light cameras from the University of Alaska-Fairbanks onboard a NASA research jet in 1994.Blue jets are dim blue lights that appear to spurt from the top of thunderstorm clouds and shoot upwards through the stratosphere as narrow cones.They have been measured to have speeds of 80-160 km per second (50-100 miles per second),and to reach heights of up to 40 km (25 miles) before fading. They generally last less than a quarter of a second, but it is possible to perceive their upward motion with the unaided eye.
Even so, they are quite rare and usually can only be seen from airplanes. It is best if the viewer is within 100 or so miles of the storm as blue light does not transmit well through the atmosphere.
While generated by thunderstorms with clouds that are highly electrically active, blue jets do not appear to be related to specic cloud-to-ground lightning discharges. They also appear more likely to occur near the highest portion of intense thunderstorm cells, such as those which produce tornadoes and severe weather.
Blue starters were discovered on video from a nighttime research ight 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. They may be blue jets that never quite made it, i.e. less energetic.
Gigantic Blue Jets
These events were first described as “…a hybrid of blue jet and sprite. The upper part resembles a sprite while the lower half is jet-like.” These events visually span levels from the lower atmosphere to the E-layer ionosphere at 100 km. The luminous duration of these events ranges between 200 ms to 400 ms, which is much longer than that of typical sprites.A team from Taiwan found this new type of TLE that they call “gigantic blue jets. Research into the phenomena showed that they are apparently not related to positive CG lightning. Rather, they may arise from negative cloud-to-ionosphere discharges.
Upward lightning bolts
Researchers are beginning to think that true “upward lightning” may well exist. Regular lightning ashes can sometimes jump outside the parent cloud, but rarely extend more than a short distance (less than a mile) above storm tops. However, a growing number of reports describe brilliant white channels extending upwards many kilometres above storm tops. They resemble ordinary lightning channels, but also appear to last much longer – up to one or two seconds – and do not flicker. They also seem to grow upward out of the cloud, and upon reaching their maximum height, the entire channel dims away. They may well occur above the tops of explosively growing clouds. They often occur every few minutes, and episodes can last for a half hour or more.
The tiniest and quickest TLE flashes are gnomes. They are small very brief white spikes of light that emerge from the top of a large thundercloud’s anvil – specifically the “overshoot dome” caused as strong updrafts push rising moist air slightly above the anvil. They appear about 150 meters wide and about a kilometer high, and they last a few microseconds.
These TLEs are so small that they appear as points, making them less than 100 m across. In the video that rst documented them, they appear scattered across the overshoot dome, flashing seemingly at random. Pixies and gnomes appear to be a pure white colour, like ordinary lightning, and they do not accompany lightning strokes.