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A Glimpse of a TIGER

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TIGER image

The first TIGER, captured high in the sky south of Madagascar in infrared on 20 January 2003

Courtesy MEIDEX/ISA/NASA
Research continues, as it has for more than 10 years, into red sprites and the other strange colored lights—transient luminous events or TLEs—that occur high in the upper atmosphere above large lightning discharges. (See the Sprites category for background.) In December 2004 the small band of sprite researchers gathered in San Francisco at the American Geophysical Union fall meeting to share notes, stories and pictures, including a new kind of TLE dubbed a TIGER.

Taiwan's second-ever satellite ROCSAT-2, launched in 2004, is dedicated to tracking sprites and elves systematically for the whole world. (Elves are huge dim disks of light, a hundred kilometers high and hundreds of kilometers across, that last for mere milliseconds.) In its first few weeks of operation, the Imager of Sprites and Upper Atmosphere Lightning (ISUAL) instrument nearly 150 sprites and their precursor haloes, plus 606 elves. Clearly the instrument is doing a good job, although it has had a tendency to shut down into "safe mode."

A European team reported on its summer 2003 campaign of hunting sprites from the top of the Pyrenees. They coordinated with an infrasound station in northern France and, for the first time, conclusively documented subsonic signals created by TLEs: sprite thunder. (This research subsequently appeared in Geophysical Research Letters (GRL) on 14 January 2005.) Notably, the infrasound continued past dawn, when sprites disappear in the sun's glare. This gives us a new tool for monitoring sprites, especially during the daytime hours when ISUAL is blind. With the completion of the worldwide infrasound network being set up as part of the Comprehensive Test Ban Treaty, the nuclear watchdogs will produce other useful data while they wait for nuclear explosions that may never occur.

Part of the European project involved looking for conjugate TLEs, which are suspected to occur when accelerated electrons from sprite/elve events travel along geomagnetic field lines to the other side of the equator and cause light signals there, something like an aurora. The magnetic conjugate site for central Europe is in South Africa, where detectors were set up. Calculations suggest that an electron beam created in Europe would produce a purple sprite-like flash and other subtle disturbances in South Africa, but no "conjugate sprite" has yet been observed.

A conjugate sprite was what an Israeli-led team hoped they had found in recordings made on the last flight of the space shuttle Columbia in January 2003. Astronauts studying atmospheric dust during the day turned their sensitive cameras toward likely sprite localities at night—this is why the TLE search fell under the Mediterranean Israeli Dust Experiment (MEIDEX). The TLEs they found were easily classified, except for one flash detected south of Madagascar on 20 January 2003. The team prepared to present their evidence at the 2004 meeting as a possible conjugate sprite, but in their final publication, in GRL on 20 January 2005, they had rejected that explanation and declared a new class of TLE called Transient Ionospheric Glow Emission in Red, or TIGER.

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