We now have a better understanding of sprites: Florida Tech researchers

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Sprites – a curious luminous phenomenon that occurs 25 to 50 miles above thunderstorms – have been poorly understood until now and though researchers have theoretically shown that atmospheric gravity waves play a role in initiation of sprites, there have been no studies that could actually support that idea.

Now a team of scientists led by Ningyu Liu, a professor at Florida Institute of Technology has done just that – provided evidence of the atmospheric gravity waves through a novel initiation model.

Through a research published in Nature Communications, the scientists have provided comprehensive computer-simulation results from a novel sprite initiation model and dramatic images of a sprite event, and provides a clearer understanding of the atmospheric mechanisms that lead to sprite formation.

Sprites are fireworks-like electrical discharges seen in upper atmosphere. Sometimes they are preceded by halos of light and appear above thunderstorms. Sprites, which are fascinating, require a better understanding owing to the fact that they hold the potential of interfering with or disrupting long-range communication signals by changing the electrical properties of the lower ionosphere.

Predicted by Nobel laureate C. T. R. Wilson in 1924 but not discovered until 1989, sprites are triggered by intense cloud-to-ground lightning strokes. They typically last a few to tens of milliseconds; they are bright enough to be seen with dark-adapted naked eyes at night; and only the most powerful lightning strokes can cause them.

The team investigated how sprites are initiated and according to Liu, the perturbations in the upper atmosphere created by atmospheric gravity waves can grow in the electric field produced by lightning and eventually lead to sprites.

“Perturbations with small size and large amplitude are best for initiating sprites,” Liu said. “If the size of the perturbation is too large, sprite initiation is impossible; if the magnitude of the perturbation is small, it requires a relatively long time for sprites to be initiated.”

To validate their model, the team analyzed a sprite event captured simultaneously by high-speed, high-sensitivity cameras on two aircraft during an observation mission sponsored by the Japanese broadcasting corporation NHK. The high-speed images show that a relatively long-lasting sprite halo preceded the fast initiation of sprite elements, exactly as predicted by the model.

Hamid Rassoul, an atmospheric physicist and the dean of Florida Tech’s College of Science, said the findings will be critical to future researchers.

“They will allow scientists to study not only sprites but also the mesospheric perturbations, which are difficult, if not impossible, to observe,” he said.

Liu added, “Our findings also suggest that small, dim glows in the upper atmosphere may be frequently caused by intense lightning but elude the detection. There may be many interesting phenomena waiting for discovery with more sensitive imaging systems.”