Sprites and Their Siblings

red sprite
Red sprite recorded 4 July 1994 over northern Texas. NASA

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. 

Sprites

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

The study of lights in the upper atmosphere pushes the capabilities of science, especially high-speed video. It also takes luck and friends in high places—like mountaintop observatories.

Sprite Observing

Special viewing sites are needed to see sprites, as they always hide above thunderstorms. At the Yucca Ridge Field Station, run by FMA Research in northern Colorado, sprite-watchers can see lightning from storms 1,000 kilometers away over the Great Plains.

A similar observatory is in the Pyrenees range of southern France. Other researchers take storm-jumper planes into the turbulent night skies to capture the elusive flashes.

The other major observing platform is in orbit. Important research has been done from the Space Shuttle, including the fateful flight of Columbia that crashed during reentry in 2003. And Taiwan's second satellite, launched in 2004, is dedicated to this field.

The Role of Luck

The hunt for sprites and their siblings has also depended on lucky breaks. Sprites were first recorded in 1989 when some University of Minnesota scientists, waiting to film a rocket launch, pointed the camera at a distant thunderstorm. One of them checked the wiring and fixed a loose cord. Minutes later the tape caught a flash so brief it occupied only two frames. Those two frames of video launched a whole new branch of Earth science.

On 22 July 2000, Walter Lyons was at Yucca Ridge shooting video of a huge "mesoscale" storm complex when a smaller isolated "supercell" thunderstorm drifted northward, blocking the view.

Supercells—the typical anvil-shaped cumulonimbus thunderstorms—do not produce sprites, but Lyons let the cameras roll. To his surprise, the recordings showed two new kinds of lights at the top of the supercell: gnomes and pixies.

Lyons is still looking for new lights. The scientific literature has eyewitness descriptions of lights in the high atmosphere dating back more than a century.

Most correspond to sprites and blue jets. But a tantalizing handful describe bright white streaks rising straight and unbranched from thunderstorm tops. A few photos give the further detail that the tops of these lights shade to blue.

Some day we will capture these on tape, analyze their spectra, and give them a name. Like sprites, elves, and trolls, they have always been here, but we never had eyes to see them with.

The Sprite Community

The annual December meetings of the American Geophysical Union have been reunions of the close-knit sprite community since 1994. At the 2001 session, the group in attendance paused to remember their late friend and mentor John Winckler (1917–2001), the geophysicist and collector of odd lightning stories who pointed the camera at that Minnesota thunderstorm in 1989. At the same time, talks by a European-African group and a sprite-hunting team from Taiwan were proof of the field's growth.

Every year brings advances in the study of sprites and their relatives. At the turn of the millennium this is what we were learning:

  • David Sentman, the man who gave sprites their name, documented gravity waves in the high atmosphere caused by sprites. In effect, sprites "splash" energy into the ionosphere like a swimmer flopping into a pool. Thus they join meteors, eruptions, thunderstorms and earthquakes as causes of ionospheric gravity waves.
  • The STEPS 2000 campaign recorded sprites as other observers detected distinctive infrasound signals—I would call these "sprite thunder." Walter Lyons prepared a PDF document on the subject.
  • The first sprites were observed in Brazil, a major thunderstorm playground, using instrumented balloons and planes. See them in this photo gallery.
  • Tohoku University's sprite group (apparently defunct as of 2009) looked for sprites in winter, monitoring cold-front storms in the ocean near Japan. This meteorological setting is quite different from the Great Plains of North America, yet sprites appear there too.
  • And a group at National Cheng Kung University in Taiwan observes over mainland China as well as the Pacific. They were the ones who discovered gigantic blue jets.

I try to keep tabs on this field each year, and I've reported new results from the 2003 and 2004 sessions.

There's also more to see in the Sprites category.

PS: This atmospheric research is also tied to the ongoing study of ordinary lightning. New networks are observing lightning in wonderful detail, yielding data that can give insight into the forces that cause sprites. To anyone who has ever watched heat lightning hidden deep in the high clouds, the resulting pictures are a magical glimpse at something never seen before.