Spotting Sprites
August 15, 2012

Catching Sprites In Action

[ Watch the Video: Sprites on Film ]

April Flowers for - Your Universe Online

Fifty miles above the Earth, dancing high above the clouds during thunderstorms, a different kind of lightning flashes.  Bursts of red and blue light, known as sprites, flash for one thousandth of a second. Unless you happen to be looking directly at the flash, flying high over the clouds during a thunderstorm, you probably won't even see the sprite.

One of the best places to get a good view of the phenomenon is rather hard to reach.  On April 30, 2012, astronauts on the International Space Station (ISS) captured the signature red flash of a sprite, offering researchers around the world a rare opportunity to observe one.

Sprites are so hard to catch on film that pilots claimed to see them for almost a century before scientists at the University of Minnesota accidentally caught one on camera in July of 1989.  Researchers have occasionally captured one on film from aboard planes, but it continues to be difficult to catch them with any regularity.

In 2011, a group of scientists, helped out by Japan's NHK television, sought out the elusive sprites for two weeks. The team filmed from two separate jets flying twelve miles apart to map out the 3-D nature of the sprites.  At 10,000 frames per second, these films are some of the best movies of sprites ever taken and can be used to study this poorly understood phenomenon. Together with the 3-D mapping, ground measurements helped to round out the picture.

"Seeing these are spectacular," says Hans C. Stenbaek-Nielsen, a geophysicist at the University of Alaska in Fairbanks. "But we need the movies, because not only are they so fast that you could blink and miss them, but they emit most of their light in red, where the human eye is relatively blind."

The research team, based out of Denver, Colorado, chased storm clouds every evening of those two weeks.  One team member had the unenviable, full time job of figuring out where to look next, aided only by a computer watching weather systems. However, once a hot zone of sprites was found, the researchers were usually able to film numerous sprites in a row.

A sprite's first flash is usually followed by a breakup into a number of streamers of light.  Figuring out what causes this divergence is one of the key elements that scientists will try to glean from these films.

Our current understanding is that sprites are related to lightning, in which a neutrally charged cloud discharges some of the electricity it holds to the ground.  Normally a negative charge is carried from the cloud to the ground, but in about one out every ten times, it is a positive charge that leaves the top of the cloud negatively charged.  The electric field above such a negatively charged cloud is the perfect breeding ground for sprites.

High atmosphere weather is typically considered to be a completely separate phenomenon from what we experience here on the ground, but the sprites show that some fundamental science connects these two regions, opening interesting physics questions about the interchange of energy between them.

Image 2 (below):  This is the first color image of a sprite ever captured. It was taken in 1994 by a NASA-sponsored project through the University of Alaska Fairbanks (UAF) that flew special cameras on two aircraft flown out of Oklahoma City. Credit: NASA/UAF