A guide to the atmospheric home of elves, sprites, halos and jets revealed

London, August 22 : A three-year space survey has produced the first detailed global guide to the location of flashes of light in the Earth's upper atmosphere, which are referred to as elves, sprites, halos and jets.

According to a report in New Scientist, these are "transient luminous events" - brief flashes of light -in the upper atmosphere during thunderstorms.

Their precise origins are largely a mystery, but an analysis of satellite photographs has revealed surprising details about the spooky lights' preferred habitats.

Though sprites, jellyfish-shaped flashes brighter than Venus, had till now received the most attention from researchers, this new survey relegates them to second place.

It turns out that the real fairy kings of the skies are the elves.

This is the name given to doughnut-shaped bursts of light that appear higher but more briefly than sprites, decorating the ionosphere skies at a height of 90 km for less than a millisecond.

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL), a dedicated instrument on board the FORMOSAT-2 satellite, counted 5434 elves and only 633 sprites over 3 years.

Halos, luminous disks that often occur directly before sprites, closely matched sprite numbers. In contrast, blue jets, which are bigger than sprites and have a prominent trumpet shape, are rare.

A mere 13 were detected by the imager over 3 years.

According to Alfred Chen at the National Cheng Kung University in Tainan, Taiwan, a member of the ISUAL team, he is excited by his unexpected discovery of the large global elf population.

"Unlike the sprites, which mainly congregate over continents, elves are found mostly over oceans," he said.

Elves, it seems, like it wet and warm.

Wherever the sea surface temperature exceeded 26 degree Celsius, Chen found elves.

"The warm ocean surface can provide the heat source needed to drive intense oceanic lightning with very high peak currents, which generate elves in the ionosphere," he said.

Sprites, however, are thought to require lightning with an overall higher charge. This means they simply accompany the most powerful category of flashes, and since most flashes happen over land, sprites cluster there, too.

According to Umran Inan, an elf and sprite expert at the University of Stanford, California, it is particularly interesting that elves may have an important global effect on the lower ionosphere and the total electron content.

Chen said that the models show that elf hotspots may increase the electron density of the ionosphere by up to 5 per cent, which "could interfere with ground and space communication as well as navigation."

ANI