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Monday, March 20, 2006

Strange fire: the lightning chasers

Meteorologists have been hunting down several new kinds of lightning that they still don't understand well. They seem to be related in part to the earth's planet-wide magnetic field. For example, a "sprite", shown at right, occurs far above normal lightning and can reach from the ground all the way into the edge of space, where there is nothing besides the magnetic field to exchange such a large discharge of electrons.

Livescience.com has a lightning slide show, as well as a series of articles on the subject. From the first article:

Back in the 1980s, airline pilots were told they must have been seeing things when they reported flashes of light shooting toward space atop thunderstorms.

But in recent years, scientists have photographed the mysterious flashes and come up with interesting names for them: elves, blue jets, tigers and sprites. The flashes are associated with thunderstorms, and each type is incredibly brief and behaves differently.
...
The observations show that sprites normally start nearly 50 miles high. Streamers rain down from the bottom of an initial, diffuse halo. The streamers branch out on the way down. As all this is unfolding in the blink of an eye, a bright column of light expands vertically from the starting point, reaching toward both Earth and space.

A blue jet, photographed at high speed
These new kinds of lightning, if that is what they are, are being grouped together under the term "transient luminous events", or TLEs, though it doesn't seem clear how related to each other they really are.

The "tiger" type mentioned in the excerpt above has only been recorded once, by the crew of the space shuttle Columbia shortly before it was destroyed. "TIGER" stands for "Transient Ionospheric Glow Emission in Red", an acronym that seems cleverly worded to fit the fact that it was observed over the Indian Ocean.

More on chasing sprites:

To date, the fastest images ever taken of sprites have been at a speed of 1,000 frames per second. From those and previous studies, scientists had some insights into these so-called transient luminous events that burst and fade out faster than the blink of a human eye.

For example, they knew that sprites occurred 20 to 50 miles above powerful thunderstorms; that they happened after unusually strong lightning strikes; that they had a branching structure; and that they were often characterized by intense bright spots that persisted seconds after the sprite disappeared.

[Duke professor Steven Cummer] and his team rented a camera typically used to film such high-speed action as crash tests, rocket launches, and explosions. They turned its lens on several thunderstorms in Fort Collins, Colo., over a six-week period in the summer of 2005.
...
According to Cummer, the tips of some of the branches of some of the sprites are attracted by electromagnetic forces to the channels left behind by branching that occurred just milliseconds earlier.

The intense brightness indicates some kind of interesting chemistry that is still unknown.

Off-topic pic: lightning striking near the space shuttle at Cape Canaveral, Florida

Livescience.com also reports that scientists recently succeeded in inducing artificial aurora borealis, which probably did not look much like these two amazing aurora borealis pics:

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