During a thunderstorm, high in the ionosphere, you’ll find an odd variety of lightning that is far above the thunderstorm itself. Jellyfish lightning, also known as sprites, are red flashes of light that last for a few seconds. They can have a wide bell-shaped top and tentacle-like wisps of light at the bottom, resembling a jellyfish. Though scientists have been aware of their existence for decades, nobody knew where they came from. That mystery has finally been solved, as a team of researchers have determined how these phenomena occur with plasma irregularities. The announcement comes from lead author Jianqi Qin from Penn State University and the results were published in Nature Communications.

The ionosphere is the layer of the atmosphere where long-distance radio communication takes place. It is about 60-90 km (37-56 mi) above the surface of the Earth, above the dense lower atmosphere where most human activity takes place. As sprites occur in this region, they can disturb radio signals. 

"In high-speed videos we can see the dynamics of sprite formation and then use that information to model and to reproduce the dynamics," Qin stated in a press release. 

These sprites only occur during thunderstorms, though sprites are about three times higher up than storms. Though the storms are necessary for sprites to occur, they aren’t quite sufficient enough to cause them on their own, otherwise sprites would happen during every single storm. It had previously been suggested that plasma irregularities in the ionosphere coupled with the thunderstorm to produce sprites, though there was no hard evidence to support this at the time. 

The team analyzed several videos of sprites in order to understand their formation and disappearance. The tentacle-like tendrils at the bottom were shown to form much faster than the bell-shaped top. The researchers suspected that plasma irregularity was the cause of this, and they were ultimately able to create a computer model. They found support for their hypothesis that localized plasma irregularities can spark the formation of a sprite. Depending on the level of irregularity, they could even predict how large the flash of light would be.

Once they determined plasma was the source of the sprite, it led to the question of what caused the plasma to become irregular. They ruled out other sprites as a source, as most sprites don’t occur in close enough groups to support that. Meteors could be responsible, as they don’t begin to burn up until they have dropped below into the lower atmosphere. Further study will be required to solve the next piece in this puzzle.

"This technique can be used for remote sensing in the ionosphere as well," said co-author Victor Pasko in the press release. "Using high speed videos and fluid models we may be able to see other things that go on in the ionosphere and better understand the effects of various natural phenomena on very low frequency radio communications."

[Header image credited to H.H.C Stenbaek-Nielsen, used via CC BY-NC-ND 2.0]