Blue jet streaks high

The lights move into the upper atmosphere at a rapid rate

By Dr David Whitehouse BBC News Online science editor

Lightning-like flashes called blue jets may link the electrical energy of thunderstorms and the charged layer of the upper atmosphere.

Writing in the journal Nature, Victor Pasko at Pennsylvania State University, US, and colleagues used high-speed low-light cameras to capture an image of a fleeting blue jet that extended from the top of a thunderstorm 70 kilometres up into the ionosphere

Blue jets are caused by an electric current shooting upwards out of thunderclouds, ionising molecules and making them glow. Previous measurements suggested that blue jets could only reach about 40 km high.

The finding boosts a long-held belief that something must provide a link between the electrical charge of the ground and the oppositely charged ionosphere - a difference of some 300,000 volts.

Two types

The blue jet seen was associated with a small thunderstorm, suggesting that large blue jets are common. Their ionising power could even make important contributions to atmospheric chemistry, such as fixing nitrogen and manufacturing ozone.

For over a century there have been numerous undocumented reports of strange lights high above thunderclouds. Some scientists suggested that the light was from an electrical discharge between the thunderclouds and the upper atmosphere.

The situation hardly changed until about 10 years ago when observations from the space shuttle and high-altitude aircraft revealed white and blue flashing lights shooting up and down above thunderstorms.

The lights came in two types. Sprites were mainly whitish and began near the base of the ionosphere, shooting rapidly downwards at speeds in excess of 10 million metres a second.

Advanced camera

In contrast, blue jets, shoot upwards from the cloud tops at somewhat slower speeds, about 100,000 metres a second. They typically were conical in shape.

The new observations, made from the Arecibo Observatory in Puerto Rico, provide the first evidence that sprites and blue jets are linked and provide a direct electrical connection between the lower and the upper atmosphere.

The crucial observations were obtained on 14 September, 2001, when a cluster of thunderstorms was seen approximately 200 km to the north of Arecibo. They were recorded using an advanced night-sensitive digital camera.

During the thunderstorm, which lasted several hours, an event occurred that was detected on 24 video frames, each lasting 33 milliseconds each. It ended in an intense lightning flash.

Lights were seen shooting upward at 50,000 metres a second, a typical conventional lightning speed.

Global circuit

Suddenly the speed of the upwardly moving lights increased fivefold and split into two branches that continued upwards to an altitude of about 70 km, whereupon the lights brightened considerably before petering out.

Researchers say the initial phase of the event closely resembles the blue jet phenomenon and could be the first time such an event has been recorded from the ground.

They say that as the event ascends the atmosphere, it comes more and more to broadly resemble what have been called sprites, although the comparison is not exact.

It proves, say the researchers, that there exists a direct electrical connection between thunderclouds and the upper atmosphere.

They say that since the electrical link was seen in what was a relatively small commonplace thunderstorm, it could be a widespread phenomenon and a hitherto unappreciated arm of a global electrical circuit.