High above thunderstorms, giant red 'sprites' flash for an instant, and they aren't lightning.
People reported strange flashes above storms from at least 1886, but nobody photographed one until 1989, and then by accident. Sprites are vast red-orange discharges up to 90 kilometres high, shaped like jellyfish, columns or carrots. Despite thousands of recordings since, their root cause is still not fully understood.
Sprites occur in the mesosphere, 50 to 90 kilometres up, above large thunderstorms. They are usually triggered by a powerful positive cloud-to-ground lightning strike. When such a strike carries charge to the ground, the top of the storm is left with a strong negative charge, creating for a few milliseconds an intense electric field in the thin air above. At such low pressure, far less voltage is needed for the air to break down, so an avalanche of electrons lights up the sky.
Though sometimes called upper-atmospheric lightning, sprites are something different: a cold plasma, closer to the glow of a fluorescent tube than to the white-hot channel of an ordinary bolt. Their red colour comes from excited nitrogen. The upper parts glow red-orange with bluish tendrils hanging below, and a single 'jellyfish' sprite can span 50 by 50 kilometres. High-speed cameras show they are made of clusters of balls of ionisation, tens of metres across, that race downwards at up to a tenth of the speed of light, followed by a set moving up. A sprite can appear up to 50 kilometres to the side of the lightning that set it off.
Their history is one of near-misses. In 1925 the Nobel-winning physicist C. T. R. Wilson predicted on theoretical grounds that electrical breakdown could happen high above storms, and in 1956 he may have seen it. Then, in July 1989, University of Minnesota scientists using a low-light video camera captured one by accident. The flashes were named after the air spirits of folklore several years later, and have since been filmed from the ground, aircraft and space. They have been seen over every inhabited continent and are thought to accompany most large storm systems.
Much remains unknown. NASA notes that, beyond the link with positive lightning, the root cause of sprites is still unclear, and not every storm produces them. They were seen above Hurricane Matthew in 2016, but their role in tropical cyclones is a mystery. Since 2022, a NASA citizen-science project called Spritacular has gathered images from the public; with a dark sky, a clear view of a distant storm and a red-sensitive camera, amateurs can help.
Source: Wikipedia — Sprite (lightning) · Text summarised from Wikipedia (CC BY-SA 4.0)
