(New Scientist) A three-year space survey has produced the first detailed global guide to the location of elves, sprites, halos and jets. These are “transient luminous events” â€“ brief flashes of light â€“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. Sprites, jellyfish-shaped flashes brighter than Venus, had till now received the most attention from researchers, but this new survey relegates them to second place.
Mystech: “Elves like it wet and warm”. Must not snicker at serious science. Must not snicker at serious science.
The real fairy kings of the skies are the elves, it turns out. 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.
Alfred Chen at the National Cheng Kung University in Tainan, Taiwan, a member of the ISUAL team, says 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 says.
Wet and warm
Elves, it seems, like it wet and warm. Wherever the sea surface temperature exceeded 26ËšC, 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 says.
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.
Umran Inan, an elf and sprite expert at the University of Stanford, California, says it is particularly interesting that elves may have an important global effect on the lower ionosphere and the total electron content.
Chen says the models show that elf hotspots may increase the electron density of the ionosphere by up to 5% which “could interfere with ground and space communication as well as navigation,” he says.