Sprites form at plasma irregularities in Earth's lower ionosphere

This is a photograph of a sprite. 

Credit: H. H. C. Stenbaek-Nielsen

Atmospheric sprites have been known for nearly a century, but their origins were a mystery.

Now, a team of researchers has evidence that sprites form at plasma irregularities and may be useful in remote sensing of the lower ionosphere.

Victor Pasko

"We are trying to understand the origins of this phenomenon," said Victor Pasko, professor of electrical engineering, Penn State.

"We would like to know how sprites are initiated and how they develop."

Sprites are an optical phenomenon that occur above thunderstorms in the D region of the ionosphere, the area of the atmosphere just above the dense lower atmosphere, about 37 to 56 miles above the Earth.

The ionosphere is important because it facilitates the long distance radio communication and any disturbances in the ionosphere can affect radio transmission.

Jianqi Qin

"In high-speed videos we can see the dynamics of sprite formation and then use that information to model and to reproduce the dynamics," said Jianqi Qin, postdoctoral fellow in electrical engineering, Penn State, who developed a model to study sprites.

Sprites occur above thunderstorms, but thunderstorms, while necessary for the appearance of a sprite, are not sufficient to initiate sprites. All thunderstorms and lightning strikes do not produce sprites.

Recent modeling studies show that plasma irregularities in the ionosphere are a necessary condition for the initiation of sprite streamers, but no solid proof of those irregularities existed.

This is a photograph of five plasma irregularities responsible for sprite initiation. 

Credit: H. H. C. Stenbaek-Niels

The researchers studied video observations of sprites, developed a model of how sprites evolve and disappear, and tested the model to see if they could recreate sprite-forming conditions.

They report their results (May 7) in Nature Communications.

Sprites resemble reddish orange jellyfish with bluish filamentary tendrils hanging down below.

Careful examination of videos of sprites forming showed that their downward hanging filaments form much more rapidly than in the horizontal spread, leading the researchers to suggest that localized plasma irregularities cause the streamers to propagate.

The researchers used a two-dimensional cylindrical symmetric plasma fluid model, a mathematical model of the ionization movements in the sprite, to study sprite dynamics.

They then used the model to recreate optical sprite creation. From this recreation, the researchers determined where the sprite streamers originated, and they could estimate the size of the plasma irregularity.

This is a sequence of black and white images of sprite initiation on July 20, 2012. 

Credit: H. H. C. Stenbaek-Nielsen

NASA ISS Crew seeing elusive Cosmic Sprites - Rare Video

A sprite glows red (inset) in this image captured by astronauts on the International Space Station on April 30, 2012.   

Credit: Image Science & Analysis Laboratory, NASA Johnson Space Center 

High above the clouds during thunderstorms, some 50 miles above Earth a different kind of lightning dances.

Bursts of red and blue light, known as "sprites," flash for a scant one thousandth of a second. They are often only visible to those in flight above a storm, and happen so quickly you might not even see it unless you chance to be looking directly at it.

One hard-to-reach place that gets a good view of sprites is the International Space Station. On April 30, 2012, astronauts on the ISS captured the signature red flash of a sprite, offering the world and researchers a rare opportunity to observe one.

Filmed at 10,000 frames per second by Japan's NHK television, movies like this of electromagnetic bursts called "sprites" will help scientists better understand how weather high in the atmosphere relates to weather on the ground. 

Credit: NHK

Indeed, sprites are so hard to catch on film, that pilots had claimed to see them for almost a century before scientists at the University of Minnesota accidentally caught one on camera in July of 1989.

Since then, researchers aboard planes have occasionally snapped a shot, but it continues to be difficult to methodically film them.

So a group of scientists, along with help from Japan's NHK television, sought them out regularly for two weeks in the summer of 2011.

Filming at 10,000 frames per second on two separate jets, the team recorded some of the best movies of sprites ever taken – movies that can be used to study this poorly understood phenomenon and the forces that create them.

By filming from two jets flying 12 miles apart, the team mapped out the 3-dimensional nature of the sprites. Ground-based measurements rounded out the picture.

Lightning Jets and Sprites

The ancient Greeks might have thought Zeus was furious with heaven itself. The power of lightning strikes and Sprites, that shoot upwards from storm clouds has been measured for the first time – and they turn out to be every bit as powerful as normal lightning.

First caught on camera in 2003, "gigantic jets" shoot upwards from thunderclouds and can reach altitudes above 80 kilometres. But it wasn't until 21 July last year that Steven Cummer at Duke University in Durham, North Carolina, and his colleagues managed to measure the electrical discharge from a single gigantic jet, released from tropical storm Cristobal.

"No one had been very close to one with the right radio instrumentation before," Cummer says. "So we didn't know whether they just petered out without doing anything much, or whether they actually took some charge and dumped it somewhere."