Thunderstorms and lightning go hand in hand. Sometimes, they are accompanied by weak flashes of light called sprites which have been observed twice in one day by scientists on the International Space Station (ISS).
ISS captured a sprite above a thunderstorm over Illinois or Missouri. Photo credits: ISS/JSC/NASA.
A sprite is a burst of energy occurring above a thunderstorm. It is usually red: the colour is thought to be conferred by the interaction between electrons in the surroundings that accelerate from the electrical charge of the storm and nitrogen molecules in the air. Extending from a sprite are red or purple tendrils that move towards the storm cloud; they are believed to be the result of cosmic rays accelerated by electric discharges from the lightning storm.
The sprite pictured above was followed by another one above El Salvador. They are difficult to capture for they last only milliseconds.
According to a theory of scientists from the Florida Institute of Technology, gravity waves are at the origin of the sprites; the former cause small disturbances in the air that act on electric fields of lightning, thereby creating a sprite.