Everyone is familiar with cloud-to-ground lightning which occurs during severe thunderstorms and cloud-to-cloud lightning which also is produced during these storms. In rare form, lightning can actually occur high above thunderstorms and give off a rather strange color and shape. These large-scale electrical discharges are called Sprites.
Sprites usually occur 50-90 km (31-56 mi) above cumulonimbus clouds. These cumulonimbus clouds, or simply just thunderstorm clouds, create enough electrical discharge for sprites to form above them. Sprites are usually triggered by the discharge of positive lightning between the underlying cumulonimbus and the ground. As can be seen in the image above, sprites are luminous reddish-orange flashes. They form within clusters above the troposphere, which is what gives them the jumbled or clustered appearance. Cloud-to-ground lightning along with our typical thunderstorms occur in the tropospheric region of the atmosphere. Sprites form within the stratospheric to mesospheric region of the atmosphere and this causes them to have different characteristics compared to lightning. Sprites are technically cold plasma phenomena that lack the hot channel temperatures found in tropospheric lightning. So, these sprites are more akin to fluorescent tube discharges than to lightning discharges. In a fluorescent tube, an electrical current excites a gas present and produces short-wave ultraviolet light which then causes, in the sprites case, light to be produced in the surrounding atmospheric area. As can be seen in the image above and other sprite images, the strands and strokes of sprites are caused by the electrical current. So, the glow of the sprite follows the current from the beginning to the end of its lifespan.
The image above helps visualize the height to which these sprites occur and the wide span of area that they take up. In this example, the sprite is spanning from 40 km to 80 km in the atmosphere. Surely, these electrical phenomena are not measly occurrences in terms of size. In terms of length of time, they are. Through optical imaging, a 10,000 frame-per-second high speed camera conveys that sprites are actually clusters of small, decameter-sized (10-100 m or 33-328 ft) balls of ionization. These are launched high into the atmosphere of about 80 km (50 mi) or so and then move downward at speeds of up to ten percent of the speed of light, which then are followed by separate sets of upward moving balls of ionization milliseconds later. Overall, an entire sprite display only lasts about the same duration of a cloud-to-ground lightning strike which is on average 30 microseconds.
Truly, to see an event like this one will need near-perfect atmospheric conditions along with a watchful eye.
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©2019 Weather Forecaster Alec Kownacki