Sprite (lightning)

High above the massive clouds of a thunderstorm, strange lights flash in the dark night sky. These luminous events are called sprites, or red sprites. They are large-scale electrical discharges that occur in the mesosphere. The mesosphere is a layer of the atmosphere located about 80 km above the Earth.

While they look like lightning, sprites are actually cold plasma phenomena. This means they lack the high temperatures found in typical tropospheric lightning. Instead, they are more similar to the electrical discharge seen in a fluorescent tube. They are often associated with other upper-atmospheric optical phenomena, such as blue jets and ELVES.

The process begins with a powerful thunderstorm far below the mesosphere. When a large bolt of positive lightning strikes from a cloud to the ground, it carries electrical charges away. This leaves the top of the cumulonimbus cloud with a very strong negative net charge. This sudden change creates a strong electric field in the region directly above the storm.

Because the air pressure in the mesosphere is so low, the breakdown voltage is drastically reduced. This allows an electron avalanche, which is a rapid chain reaction of electricity, to occur. The characteristic red-orange color comes from the excitation of nitrogen molecules in this low-pressure environment.

Scientists have categorized sprites into three distinct types based on their visual shapes. In 1999, a researcher named Rodger identified these different forms. Jellyfish sprites are very large and resemble the appearance of sea creatures. Column sprites, or C-sprites, are large-scale electrical discharges that are still not totally understood. Finally, carrot sprites are a specific type of column sprite that features long, hanging tendrils.

Most sprites appear in clusters of two or more. They typically span an altitude range of 50 to 90 km. They often show reddish-orange upper regions with bluish tendrils hanging below.

Humans have reported seeing these mysterious lights for a long time. The earliest known report of a sprite dates back to 1886 by Toynbee and Mackenzie. In 1925, the Nobel laureate C. T. R. Wilson suggested that electrical breakdown could happen in the upper atmosphere. He even believed he witnessed a sprite in 1956. However, sprites were not officially photographed until July 4, 1989.

Scientists from the University of Minnesota accidentally captured the first image using a low-light video camera. They later named the phenomenon "sprites" after mythological air spirits because they are so elusive. Since that discovery, sprites have been captured on video thousands of times.

Research using high-speed technology has revealed incredible details about how sprites move. In 2019, a camera captured video at 100,000 frames per second. This showed that sprites are actually clusters of small balls of ionization. These tiny balls are only about ten meters in scale. They are launched at an altitude of about 80 km and move downward at speeds up to ten percent the speed of light.

A few milliseconds later, a separate set of ionization balls moves upward. These discharges can be horizontally displaced by up to 40 km from the original lightning strike. The time delay between the lightning and the sprite is usually only a few milliseconds.

Sometimes, a sprite is preceded by a unique feature called a sprite halo. A halo is a pancake-shaped region of weak, transient light. It is approximately 400 km wide and quite thin. These halos appear about one millisecond before the main sprite. They are centered at an altitude of about 80 km above the initial lightning strike.

Scientists believe halos are produced by the same physical process as sprites. However, the ionization in a halo is too weak to create the bright vertical structures called streamers. Because they are short and bright, halos are sometimes mistaken for ELVES.

Sprites have been observed in many different parts of the world. They have been recorded over North America, South America, Europe, and Australia. They have also been seen over Central Africa, the Sea of Japan, and Asia.

In 2016, sprites were even observed during the passage of Hurricane Matthew through the Caribbean. While they are associated with large thunderstorms, NASA notes that not all storms exhibit sprite lightning. The exact "root cause" of why some storms produce them remains a subject of ongoing investigation.