A weakly luminous, but extensive optical phenomenon that occurs in the mesosphere, high above the decaying region of a thunderstorm. Lasting just a few milliseconds, sprites are barely detectable by the naked eye. Anecdotally reported for many years, they were first confirmed by low-light video techniques in 1989. There are at least three common forms, the most frequent shaped like a jellyfish, with the brightest blood-red region at about 65–70 km, a fainter red glow or wisps that may reach 90 km, and blue tendrils that may extend down to 40 km. The other types resemble a carrot, and also a simple column of light.
Sprites frequently occur in clusters up to 50 km across, and often appear at the same time as the higher elves. They are associated with positive cloud-to-ground lightning flashes, not the more frequent negative type. After powerful positive discharges there is an abrupt change in potential between the cloud top and the ionosphere. This accelerates electrons upwards to levels at which they interact with air molecules, which then radiate the energy as visible light. See also blue jet; gamma-ray and X-ray events.
Subjects: Meteorology and Climatology.