Thunderstorms fill the sky with lights above as well as beneath the clouds. Since 1990 there's been an explosion of interest in these glows and flashes in the high sky. They bear whimsical names like sprites, elves, gnomes and more.
These transient luminous events or TLEs are similar to lightning. Just as the solid earth conducts electricity and attracts lightning, so does the ionosphere, the layer above the stratosphere. A large lightning stroke launches a rising electromagnetic pulse (EMP) that excites the thin air until it emits light. (See more theory at the Stanford VLF site.)
The most common TLE is the sprite—a flash of red light directly above large thunderstorms. Sprites occur a fraction of a second after strong lightning strokes, soaring upward to an altitude of nearly 100 kilometers. David Sentman of the University of Alaska at Fairbanks named them sprites as a way to talk about them without presupposing their cause and mechanism.
Sprites are plentiful in the American Midwest, where great thunderstorms are common, but they are reported in many other places. The Sprite Watchers home page gives advice on how to look for them.
Sprites in detail are bundles of luminous tendrils that spread outward above and below a central bright ball. Simple ones are called carrot sprites. Large sprite clusters may resemble jellyfish, or angels. Groups of "dancing" sprites sometimes appear. A gallery of sprites published in Physics Today gives a good picture of these flashing creatures.
Blue Jets and Blue Starters
Blue jets are cones of dim blue light that begin around 15 km altitude and rise to around 45 km like a quick puff of smoke. They're rather rare. They may be associated with heavy hailstorms in the clouds beneath them.
Blue jets are hard to study from the ground, being at lower altitudes than sprites. Also, blue light doesn't travel through air as well as red, and high-speed cameras are less sensitive to blue. Blue jets are best studied from aircraft, but those flights are costly. So we must wait to learn more about blue jets.
Blue starters are rare low-altitude flashes and dots that don't grow into blue jets. First sighted in 1994 and described the next year, starters may be related to the same conditions that trigger blue jets.
Elves and Sprite Haloes
Elves are extremely brief disks of dim light (and very low frequency radio emissions) that appear at around 100 km. Sometimes they appear with sprites, but usually not. Elves were predicted before they were first observed in 1994. The name stands for "Emissions of Light and VLF from EMP Sources."
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.
Trolls, Gnomes and Pixies
Trolls (for Transient Red Optical Luminous Lineament) occur after an especially strong sprite, down in the lowest tendrils 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. This is a typical pattern in science: looking at the same old thing with better instruments always reveals something new and unexpected.
Gnomes are small, very brief white spikes of light that point upward from the top of a large thundercloud's anvil top, 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.
Pixies are so small that they appear as points, making them less than 100 m across. In the video that first documented them they appear scattered across the overshoot dome, flashing seemingly at random. Pixies and gnomes appear to be a pure white color, like ordinary lightning, and they do not accompany lightning strokes.
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 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." See a picture in the 2003 sprite report.
PS: TLEs are one more clue to the behavior of the upper atmosphere and its role in the global electrical circuit. A recent issue of the Newsletter on Atmospheric Electricity presents a mind-boggling range of research in this area. The state of the global circuit, for instance, is a promising way to monitor global warming.
Next: Studying Sprites