Thunderstorms Have Flickering, High-Altitude Halos

December 16, 1996

Thunder clouds may not always have a silver lining - but they definitely can sport high-altitude halos of flickering red light.

That phenomenon has been confirmed by researchers at Stanford's Very Low Frequency Research Group who for the first time have measured the horizontal structure and dynamics of a new kind of stratospheric lightning that scientists have named "elves." The new measurements, obtained with a specially constructed device called the Fly's Eye, confirm the prediction that these flashes take the highly unusual form of luminous rings that spread across the sky at speeds faster than light.

In February 1996 the Stanford scientists predicted that elves would have such a rapidly expanding ring-like structure. They based their prediction on the assumption that the newly discovered phenomenon is produced by powerful electromagnetic pulses generated by large lightning strokes.

The new observations will be reported by Umran Inan, professor of electrical engineering, on Sunday morning, Dec. 15, at the American Geophysical Union meeting in San Francisco. The results are also contained in a paper - currently under review by the journal Geophysical Research Letters - written by Inan, applied physics graduate student Christopher Barrington-Leigh, electrical engineering senior Sean Hansen, electrical engineering graduate student Vyacheslav S. Glukhov, senior research engineer Tim F. Bell, and Richard Rairden from Lockheed-Martin Palo Alto Research Laboratory.

"For a long time we have known that certain events in the upper reaches of the atmosphere, like solar storms, can affect the lower atmosphere resulting in significant consequences like power blackouts," Inan says. "Now we are learning that certain events in the lower atmosphere can affect the upper atmosphere. Because about 1,000 lightning strokes occur each minute around the world, it is not unlikely these effects may have a global impact on the atmosphere." Pilots have reported strange flashes of light above thunderstorms for some time. It wasn't until the late 1980s, however, that scientists began taking these reports seriously. Once they began studying these lights, researchers found a number of new phenomena. So far they have detected:
-- Blue jets, streamers of blue light that stretch horizontally as far as 30 miles just above the cloud tops.
-- Red sprites, diffuse blobs of red light that stretch from 25 miles to 55 miles in altitude.
-- "Elves" that occur at altitudes ranging from 55 to 60 miles, and can expand horizontally to sizes more than 160 miles across.

These new forms of lightning can be quite bright, but they are subliminally brief, existing for only a few thousandths of a second. Elves are the shortest lived, lasting significantly less than a millisecond. They are so quick that scientists aren't certain that they can be seen with the naked eye. But if they can be seen, they should look red, Inan says.

Elves are also too brief to register on most video cameras, which use electronic light detectors with a time resolution of 16 milliseconds. They can only detect elves that are unusually bright, the scientist says.

In 1995 a group of Japanese scientists produced the first direct evidence that elves exist. They used an instrument called a photometer to detect distinctive sub-millisecond light flashes originating higher in the sky than those from sprites, which last several milliseconds.

Inan and his colleagues, however, needed more information about the shape and the dynamics of the elves to test their model. So they built a special device, christened the Fly's Eye, to study the elusive lights. The instrument has a dozen 18-inch barrels. Each barrel points to a different part of the sky and is connected to electronics that amplify the incoming light to detectable levels. Because the Fly's Eye has a time resolution of 30 microseconds, it can measure the way elves change over their brief lifetimes.

Last July the scientists set the Fly's Eye up at the Yucca Ridge Field Station in Colorado and were able to record the flickering life cycle of 10 elves. They all started in a small region centered above the position of a lightning stroke and rapidly expanded outward until reaching sizes as large as 200 miles across.

This picture fits remarkably well with theoretical predictions made earlier by Inan and former students Wesley A. Sampson, now at Qualcomm Inc. in San Diego, and Yuri Taranenko, now at Los Alamos National Laboratory.

When they heard the first reports of the new kinds of lightning above thunderstorms, the Stanford researchers realized that lightning strokes would generate electromagnetic pulses. Electromagnetic pulses first came to public attention in the 1960s when scientists realized that nuclear explosions could generate waves of electromagnetic energy strong enough to destroy unshielded electronic equipment. Although not nearly as intense as those produced by a bomb blast, the pulses generated by lightning should carry enough energy to create optical effects, the scientists calculated.

The researchers modeled these pulses and proposed that they travel radially outward and upward from the lightning stroke and generate light when they intersect the bottom of the ionosphere, the region above the stratosphere that contains electrically charged atoms. The pulse travels at the speed of light. The first part of the wave front to hit the ionosphere is a small ring above the lightning stroke. This expands outward as portions of the pulse that travel longer and longer distances strike the ionosphere.

"The ring expands faster than the speed of light for the same reason that waves, when striking the beach at an angle, travel along the shore at a faster speed than the waves move through the water," Inan says.

Because of the ring's superluminal expansion rate, light from its newer parts actually reached the instrument before the light from the older parts. The researchers had to take this into account when interpreting the data from the Fly's Eye.

Stanford University

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