THE curious red flashes of light nicknamed 鈥渆lves鈥� that flicker above
thunderstorms are powered by lightning, say researchers in California. They have
shown that the flashes, which appear at altitudes of about 90 kilometres, are
rings of light that move outwards like ripples on a pond.
Elves are the cousins of the red 鈥渟prites鈥� that occur lower in the
stratosphere (see 鈥淏olts from the blue鈥�, 快猫短视频, 19 August 1995,
p 32). 快猫短视频s captured the first images of elves more than a year ago, but
because they last for less than a thousandth of a second it has been impossible
to see how they develop. 鈥淎lthough elves are bright, they are extremely quick,
so the detectors have to be very sensitive,鈥� says Umran Inan of Stanford
University in California. 鈥淚t would be a wonderful light show if our eyes were
sensitive enough.鈥�
So to record elves flitting across the sky, Inan and his colleagues from
Stanford and Lockheed-Martin Research Laboratory in Palo Alto, California, built
a sensitive instrument they dubbed the Fly鈥檚 Eye. The detector is made up of 10
sensitive photomultipliers, each one looking at a different portion of the sky.
The Fly鈥檚 Eye is able to catch flashes of light as short as 30 millionths of a
second.
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Last summer, the researchers took the Fly鈥檚 Eye to Yucca Ridge in Colorado,
where the team was able to follow thunderstorms more than 600 kilometres away.
The instrument revealed that an elf often formed around 150 microseconds after a
lightning flash. Then, after a short delay, a second longer-lived but dimmer elf
would appear. The Fly鈥檚 Eye showed that the elf expanded rapidly across the sky
for around 220 microseconds, by which time the ring of light was more than 230
kilometres wide. The findings are reported in Geophysical Research
Letters (vol 24, p 583).
Inan says that this fits with his theoretical predictions that the elves are
caused by a burst of radio waves generated by the lightning. 鈥淲e wouldn鈥檛 expect
any other mechanism to produce the same detailed signature,鈥� he says. As the
radio pulse passes though the ionosphere, he says, its intense electric field
accelerates electrons which then smash into nitrogen molecules, causing them to
emit red light.
Elf expert Robert Roussel-Dupre of Los Alamos National Laboratory in New
Mexico agrees that heating by radio waves is the likely cause of elves. 鈥淏ut the
time delay between the lightning and the elves鈥� formation is wrong,鈥� he says. He
suspects that radio pulses from sprites could be the real power source. The
sprites themselves may be triggered when high-energy cosmic rays strike the
atmosphere, he says.
The next challenge, according to Yuri Taranenko from Los Alamos, will be to
pinpoint where the radio pulse comes from. 鈥淚t is definitely coming from lower
in the atmosphere,鈥� he says. Whatever the origins, these phenomena are
important, says Roussel-Dupre. They may affect ozone production in the upper
atmosphere.