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The Everyday Birth of a Mysterious Atmospheric Sprite

Atmospheric physicists offer new evidence for the mechanism behind a bizarre form of upper-atmospheric lightning.
Sprite seen from ISS. Image: NASA

On June 5, 1989, a NASA weather balloon was launched from the Columbia Scientific Balloon Facility outside of Palestine, Texas. The short flight was normal enough as the balloon passed safely above a nearby thunderstorm, eventually climbing to 120,000 feet. With no warning, the balloon released its payload, which, after two minutes of free-fall, crashed into a farmer's field at a speed of 700 miles per hour. Eighty miles away, at Possum Kingdom Lake, the balloon itself had a softer landing, albeit one that left a lonely fisherman thinking he'd somehow incurred the wrath of god (really).


If the fisherman had seen up close the actual cause of the balloon's tumble, a spectacular and mysterious phenomenon known as an atmospheric sprite, he would have likely been much more sure of that conclusion. An extended explosion of sparking bursts and cascading electrical showers, a sprite looks about like the ballistics of the divine should. The balloon event is considered to be the first hard evidence backing the theory that lightning can indeed travel upward from clouds to space. It did not, however, explain what a sprite is or how what comes into being.

In the intervening decades, the phenomenon has largely remained a mystery. Sprites, which are known to interfere with long-range communications signals in addition to weather balloons, are clearly triggered by conventional cloud-to-ground lightning (CGL), but the process by which the upper atmosphere and the lower atmosphere become "coupled" is elusive. A report published this week in Nature Communications and authored by atmospheric physicists at Florida Tech offers observational and simulation-based evidence for one somewhat subtle theorized mechanism: atmospheric gravity waves. These aren't gravity waves in the astrophysical sense, but are instead pressure waves that propagate from lower altitudes to higher altitudes as the result of some initiating event—disrupted airflow around a mountain, or, indeed, a thunderstorm.

Atmospheric sprites are grown from roots, in a sense, and these roots are streamers. Streamers are filaments of plasma (that do look an awful lot like roots) or ionized air molecules, for example molecules that have been stripped of their electrons such that instead of regular old neutral atoms there's a stew of electrons and positively charged atoms. This is plasma, and the the result of all of those freed electrons hanging around is that the ionized air is much, much more electrically conductive. The air itself becomes just a big old copper wire, and this wire is the basis for everyday lower-atmosphere lightning as well.


So, these filaments of ionization (streamers) eventually all get together and the result is a sprite. "The propagation characteristics of a single sprite streamer, such as acceleration, expansion and brightening, are relatively well understood," the FIT paper explains. "However, how sprite streamers are initiated, a critical question to understand the responses of the upper atmosphere to sudden, strong QE field impacts is still under active debate."

The FIT researchers were especially interested in a category of sprite known to occur after some additional delay (by several milliseconds) as the result of the formation of a halo, which is a very large pancake-shaped region of atmosphere with slightly higher-than usual "optical emissions," for example, light. A halo is the result of same general process that forms sprite streamers: some burst of energy strips away a bunch of electrons from air molecules. The difference with a halo is that the effect is a lot less so and the air doesn't pass the conductivity threshold needed for full-on sprite formation.

A halo gives rise to a full-on sprite with the addition of some perturbation and this is where our gravity waves come in. The waves meet the halo and act as "seeds," which grow in strength and eventually become plasma streamers and then sprites. As for where the gravity waves came from in the first place, Ningyu Liu, the current paper's first author, explained that they may originate within the very same thunderstorm, but perhaps also relatively distant phenomenon. It's possible for such a wave to travel along the surface of one atmospheric layer like a wave on a pond. By the time it hits a sprite halo, a gravity wave may have stretched out to the point that its wavelength spans many kilometers.

"Some of gravity waves can propagate all the way up to a few hundred kilometers above Earth's surface," Liu said, "and some may travel between two layers of upper atmosphere and spread horizontally, similar to radio waves bouncing between earth surface and the ionosphere and propagating away. "

"The theory proposed here implies that some cases of the observed long-delayed sprites are preceded by a relatively stationary, long-lasting halo around 70 km altitude," the current paper explains. "We present a unique observation of such an event. The event was recorded on 6 July 2011, from a close range of about 250 km, by two intensified high-speed cameras on two separate aircraft that flew at about 14 km altitude."

Without your own high-altitude aircraft, it's still possible to kind of see a sprite in action with help from "dark-adapted" eyes, thought it would more likely appear as a dim flicker. For now, the video above should suffice.