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Giant blue jet caught on film

14.03.2002


Flashes this big might explain the 300,000-volt difference between the ionosphere and the ground


A sprite is like a blue jet but travels the other way.
© Uni. Alaska Fairbanks


Blue jets connect Earth’s electric circuit.

Video images captured in Puerto Rico suggest that blue flashes of light, much like lightning, feed energy from thunderstorms up into the Earth’s ionosphere - a blanket of electrically charged air some 70 kilometres above the ground1.

Some researchers suspect that such phenomena may also fix nitrogen for plants to use and interact with the ozone layer2.



The images, taken in September 2001, show the largest blue jet ever to be caught on camera. "It really was a gigantic flash," says Victor Pasko of Pennsylvania State University, who led the observation team. "With the naked eye you could even see it rising," he recalls.

Blue jets are often associated with thunderstorms, but until now were thought to be relatively small. The Puerto Rican jet stretched from the top of a small thunderstorm to the lower edge of the ionosphere, filling an estimated 6,000 cubic kilometres of atmosphere.

Flashes this big might explain the 300,000-volt difference between the charge of the ionosphere and the ground. Physicists have long agreed that something must link the two regions to complete the global electrical circuit (GEC). Until the latest film, nothing had been seen that reached high enough from the cloud tops to do the job.

"We knew the currents were there, but there was no visual evidence" says Davis Sentman, the physicist at the University of Alaska in Fairbanks who discovered blue jets in 1994. The film "really advances the science in this field", he says.

That the sighting was associated with the kind of small, localized storm common worldwide, suggests that large blue jets could also be common. If so, they might influence atmospheric chemistry: their electrical energy could encourage gases to react with one another. "The effect may be there but we don’t know if it’s dramatically important," admits Pasko.

Sprites, elves, trolls and pixies

In the past decade, high-speed, light-sensitive cameras have allowed scientists to describe a menagerie of electrical phenomena, which bear names that would be more at home in a Tolkien novel than a physics textbook. Sprites, blue jets and associated flashes called elves, crawlers, trolls and pixies are all fleeting electrical discharges that accompany thunderstorms.

All these phenomena are hard to spot, as they last for less than a blink of an eye and are obscured from below by cloud. They can be glimpsed along storm fronts and from aeroplanes flying above the clouds.

Sprites, which might also help to maintain the GEC, work a bit like blue jets in reverse. They are pink, or sometimes red, and occur when current from just below the ionosphere moves downwards towards thunderstorms. As with jets, this current excites atoms along the way, causing them to emit light.

References

  1. Pasko, V. P., Stanley, M. A., Mathews, J. D., Inan, U. S. & Wood, T. G. Electrical discharge from a thundercloud top to the lower ionosphere. Nature, 416, 152 - 154, (2002).
  2. Mishin, E. Ozone layer perturbation by a single blue jet. Geophysical Research Letters, 24, 1919 - 1922, (1997).


TOM CLARKE | © Nature News Service

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