Sprites: The Mesospheric Light Show

Red sprite lightning
Ksenija Brakočević
Ksenija Brakočević
4 min

Sprites, also known as red lightning, are electrical discharges that occur in the mesosphere above thunderclouds. Although they are often referred to as upper atmospheric lightning, this term is somewhat misleading, as we will explain below.

Sprites are triggered by positive lightning discharges between thunderclouds and the ground. They are red or orange, which is why they are also referred to as red lightning. Visual reports of sprites date back to 1886, and they were first photographed in 1989. To observe red sprites from the ground, you need a clear view within a radius of 150-500 km, a dark sky and red-sensitive equipment.

Sprites were first theoretically described in 1925 by renowned scientist and Nobel Prize winner C.T.R. Wilson, known for his work on the electricity separation and the mechanisms of lightning. He suggested that electrical breakdowns could occur in the upper atmosphere. Decades later, sprites were caught on camera, and subsequent high-resolution NASA videos have greatly expanded our understanding of this phenomenon.

Despite numerous recordings, the exact mechanisms of sprite formation remain unclear. What is certain, however, is that they are closely linked to positive lightning. In contrast to tropospheric lightning, sprites consist of cold plasma and have no hot ion channels, so they are more comparable to fluorescent tube discharges.

Sprites have also been observed during hurricanes, although the reasons for this are not fully understood. The best-known classification of sprites, established by Rogers in 1999, divides them into jellyfish, columnar and carrot shapes.

High-speed camera images have shown that sprites consist of clusters of small ion balls that rise and fall at about 10% of the speed of light. This movement can extend horizontally for up to 50 km from the first lightning strike, with a time delay of between a few milliseconds and 100 ms.

It is assumed that the sprites are caused by the electric field of the underlying thunderstorm. A large positive lightning strike leaves a net negative charge on the cloud cover, creating a quasi-static electric dipole. This induces a strong electric field above the thundercloud for about 10 ms, which leads to an electric avalanche in the mesosphere due to the reduced interruption voltage.

The red color of the sprites is due to nitrogen excitation in the low-pressure environment of the mesosphere, where nitrogen emissions dominate. These discharges can damage equipment at high altitudes, highlighting the need for future protective measures.

This article has attempted to shed light on the fascinating phenomenon of sprites. Further research will undoubtedly deepen our understanding of their electromagnetic properties and lead to the development of protective devices.

References

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  • Malagón Romero, A., Teunissen, J., Stenbaek‐Nielsen, H. C., McHarg, M. G., Ebert, U., & Luque, A. (2019). On the emergence mechanism of carrot sprites. Geophysical Research Letters. doi:10.1029/2019gl085776
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  • Spritacular: NASA’s New Citizen Science Project to Capture Elusive Upper Atmospheric Electrical Phenomena on Camera - NASA