for National Geographic News
Cloud-to-Earth lightning bolts are a well-known natural electrical occurrence. Now scientists have discovered and photographed for the first time enormous, 90-kilometer-high (55-mile), luminous, electric discharges that deliver large quantities of current from thunderstorms to the edges of the Earth's atmosphere.
The flashes, described in a letter published in the current issue of the journal Nature, is believed to be a type of phenomenon known by atmospheric physicists as a "transient luminous event."
Though other transient luminous events, known as sprites and blue jets, have been the subject of intense study since the first was photographed accidentally about 14 years ago, nothing quite so large has been observed before. "It is likely that these gigantic jets are a special feature of oceanic thunderstorms," says the letter that describes the flashes detected above a thunderstorm in the South China Sea last year.
The enormous jets may play an important role in the global electric circuit that surrounds the Earth, helping reduce the huge differences in charges that build up between the surface and the ionosphere, the part of the atmosphere 70 to 80 kilometers (44 to 50 miles) above the ground in which ionization of atmospheric gases affects the propagation of radio waves.
Scientists have known that the global electric circuit is powered by thunderstorms, two thousand of which rage across the Earth's surface at any given moment. These storms act as a generator, driving electric charge upwards, said co-author of the letter, atmospheric physicist Han-Tzong Su, of the National Cheng Kung University in Tainan, Taiwan.
The electric current travels through the ionosphere and back towards the Earth's surface through regions of fair weather, slowly and continuously discharging through the weakly conducting atmosphere. The circuit is completed when the current travels through the Earth back to regions with thunderstorms, said Su.
Su and his team observed the jets in the evening of July 22 last year. After they discovered a thunderstorm raging over the South China Sea near the Philippines' Luzon Island, they set up low-light-level cameras at the southern tip of Taiwan, in the hope of catching a glimpse of a sprite or other known transient luminous event. Many transient luminous events appear and disappear so quickly that they are not visible to the human eye, necessitating the use of low-light-level cameras to capture them.
Totally Unexpected Find
Instead of finding sprites, the team caught something totally unexpected. They observed five jets spreading outwards and upwards from the thundercloud. Three were tree-shaped and two were carrot-shaped, said Su (see images).
Su and his team have named these events "gigantic jets." The jets covered 40 kilometers (25 miles) at their widest point, and are estimated to have a volume of 30,000 cubic kilometers (about 7,200 cubic miles). "That's equivalent to ten billion Olympic-size swimming pools," said Su. Despite their scale, each of the jets had come and gone within less than half a second.
This new observation provides compelling evidence that transient luminous events can work against the battery-charging effects of thunderstorms, and offload some of the charge from the ionosphere, said Su. It's the first time that luminous events spanning the entire distance from the cloud toparound 16 kilometers (10 miles) upto the ionosphere have been recorded, he said.
In addition, these jets appear to have emitted radio waves that were detected in Japan, and as far away as Antarctica. Radio waves would be produced if the jets were working against the charging effect of the thunderstorms and discharging the ionosphere, said Su. Though a similar jet reaching 70 kilometers (44 miles) in height was discovered in 2001 by an American group, no radio signals were detected in conjunction with it.
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