Scientists catch lucky break with 'upward lightning' photo
By Jeff Salton
August 24, 2009
Scientists have scored a lucky break by capturing a one-second image and the electrical fingerprint of a rarely-seen ‘gigantic jet’ - a huge lightning that flowed 40 miles upward from the top of a storm. Images of highly charged meteorological events like this have only been recorded on five occasions since 2001. The team from Duke University team captured a one-second view and magnetic field measurements that scientists hope will give them a much clearer understanding about these occurrences.
In the past it has been difficult to photograph or record gigantic jets because they happen so quickly that cameras have to be trained on them at the precise moment they occur. This one was caught almost by accident. The Duke University equipment had been set to capture another phenomenon known as sprites, which were first photographed in 1989. Sprites are electrical discharges that occur above storm clouds and are colored red or blue, with jellyfish-like tendrils hanging down.
Gigantic jets don’t happen every time there is lightning, but they are substantially larger than their regular, downward-striking cousins.
"Despite poor viewing conditions as a result of a full moon and a hazy atmosphere, we were able to clearly capture the gigantic jet," said study leader Steven Cummer, an electrical and computer engineer at Duke University in North Carolina.
"This confirmation of visible electric discharges extending from the top of a storm to the edge of the ionosphere provides an important new window on processes in Earth's global electrical circuit," said Brad Smull, program director in NSF's Division of Atmospheric Sciences, which funded the research.
"Our measurements show that gigantic jets are capable of transferring a substantial electrical charge to the lower ionosphere," Cummer said.
"They are essentially upward lightning from thunderclouds that deliver charge just like conventional cloud-to-ground lightning. What struck us was the size of this event."
While the amount of electricity discharged by conventional lightning and gigantic jets is comparable, Cummer said it appears that the gigantic jets cover much greater distances and are faster than conventional lightning because thinner air between the clouds and ionosphere provides less resistance.
Whereas a conventional lightning bolt follows a six-inch channel and travels about 4.5 miles down to earth, the gigantic jet recorded by the scientists contained multiple channels and traveled almost ten times the distance upward - about 40 miles.
"Given that reservoirs of electric charge in thunderstorms are the sources for both lightning and gigantic jets, and that both events involve contact between these reservoirs and a very large conducting surface, it is not surprising that their charge transfers are comparable," Cummer said.
At this stage scientists aren’t sure what conditions or what types of storms are conducive to gigantic jet formation.
A watching brief
Cummer keeps a low-light video camera trained to the sky and programmed to start recording when specific meteorological conditions occur while, at the same time, other equipment constantly measures radio emissions in the identical sector to capture electrical events. A GPS system helps synchronize all the equipment’s readings.
Cummer is planning to install a low-light, high-speed camera to capture gigantic jet images in color, which could provide additional information about chemical processes and temperatures inside the phenomenon.
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