Thunderstorms are ferocious natural phenomena. Crackles of lightning and booms of thunder reign over the surface of the Earth. However, something else occasionally floats ethereally above the storm clouds: a short-lived sprite. These curious bursts of energy have been spotted not once, but twice in one evening by astronauts on the International Space Station (ISS).
A sprite is a weak flash of light that appears above a thunderstorm. They are usually red, with wispy red or purple tendrils that extend downwards toward the storm cloud. Their red color is thought to be from electrons in the vicinity being suddenly accelerated from the electrical charge from the storm. As the electrons whizz around, they collide with nitrogen molecules in the air and ionize them. This process produces red light.
The tendrils' purple color, on the other hand, is postulated to be caused by cosmic rays being accelerated by the electric discharges produced by the lightning storm, however, this is only a theory.
But what causes the sprites themselves? Scientists from the Florida Institute of Technology theorized that gravity waves create small perturbations in the atmosphere. These little disturbances can grow the electric field produced by lightning, initiating a sprite.
Finding the sprites in the ISS photographs may seem a little bit like a game of "Where's Waldo?" In both, faint red streaks will indicate what you're looking for. In this case you'll find them just above bright thunderstorm clouds, like a floating red marker saying "the thunderstorm is here!"
On 10 August, astronauts on the ISS photographed one such sprite floating above a thunderstorm over Illinois or Missouri (shown above). The red glow could be easily missed, outshone by many of the lights on Earth as well as the Moon.
Photograph taken by ISS where a sprite can be found. ISS/JSC/NASA.
Then, three minutes later, they were lucky enough to snap another one above El Salvador. The sprite is roughly 100 kilometers (60 miles) above the surface of the Earth, like a luminous jellyfish. The golden patches of light around the thundercloud are just from city lights breaking through the clouds.
It is often quite a challenge to see a sprite with the naked eye. Even though they are extremely luminous, they are brightest in the infrared range. They also only last a few milliseconds, so to photograph two in one evening is amazing.
Sprites are thought to have a disruptive effect on communication signals, so understanding more about their mysterious nature could help alleviate these problems.
Close up of sprite. ISS/JSC/ASA.