[ RadSafe ] " X-rays from lightning photographed "

[Jaro Franta]

http://www.physorg.com/news/2010-12-x-rays-lightning.html 
X-rays from lightning photographed
December 14, 2010 
By Carrie Peyton Dahlberg

Using a custom-built camera the size of a refrigerator, Florida researchers
have made the world's first crude pictures of X-rays streaming from a stroke
of lightning. 

Seeing X-rays in relation to the lightning "leader," the initial spark and
the channel it makes through the air, should help researchers build better
models of the twisty and still unexplained ways that lightning behaves.

The images are beyond blurry. They look like near-abstract blotches of white
and green, better deciphered when displayed in a series with a rough sketch
of the lightning tip superimposed.

"You can see the X-ray source descending," Joseph Dwyer, a physics professor
at the Florida Institute of Technology, told a group of lightning
specialists gathered Monday in San Francisco. "You start to see the air glow
in X-rays."

Dwyer took the pictures in July and August in collaboration with a team from
the University of Florida at the International Center for Lightning Research
and Testing. He turned 25 of them into video clip that shows 2.5 millionths
of a second in the life of one lightning strike. It is a painstaking look at
an elusive subject.

"We understand how a star explodes halfway across the universe better than
we understand the lightning right above our heads," Dwyer said in an
interview before his talk at the American Geophysical Union conference, a
gathering of 18,000 scientists in specialties ranging from glaciers to
volcanoes.

The basics of lightning taught in grade school are correct as far as they
go: a negative charge builds within a cloud and then is released downward,
connecting with a positive charge. Once that connection is made, we see the
lightning discharge, a glowing streak of the electrically altered, ionized
air.

Yet how that spark begins is still unknown. Electrical fields that have been
measured in clouds seem too small account for it, Dwyer said. We also don't
know what happens in the millionth of a second when lightning attaches to
the ground, or why the lightning leader sometimes darts and sometimes,
relatively, dawdles.

Scientists have long speculated that lightning might produce X-rays, and in
the past decade Dwyer has been one of the key researchers who helped confirm
the presence of high energy photons. 

His group is among a handful worldwide that triggers lightning, generating
it by shooting a rocket trailing copper wire up into a thunderstorm.

This past summer the group made a series of pictures of four different
lightning strikes, and recorded both X-rays and higher energy gamma rays
along with them.

"It actually worked the very first time. It was unbelievable," Dwyer said.

The hulking camera, towed on a trailer, relies on 30 radiation detectors
pointed above the horizon and software written by Dwyer that turns its
voltage measurements into images. Each detector contributes its bit of data
to the picture, so the result is looks like a photo with only 30 pixels. 

Having even such low resolution pictures "is a big step forward," Steven A.
Cummer, a Duke University professor who was not involved in the work, said
in an interview. "Showing in the images exactly where the X-rays are coming
from and when is significant," he said. 

While Dwyer's group has taken other measurements of X-rays in lightning,
nothing replaces seeing it.

"This is actually sort of what Superman would see," Dwyer said at a Tuesday
morning press conference on his work. "Superman has X-ray vision."

Eventually, Dwyer hopes to build a bigger machine with 150 detectors, but in
the meantime and he and Meagan Schaal, who helped build the smaller version
as part of her doctoral work, plan to improve its focus and take many more
pictures this coming summer.

"There's a whole zoo of different kinds of lightning that we'd like to see,"
Dwyer said. 

There are dart leaders, which can hurtle at one-sixth the speed of light,
and step leaders, which zip forward, hesitate for perhaps 10 microseconds,
then zip again. There are chaotic leaders, which hesitate too but lack a
regular pattern. And there's upward, positive lightning, which moves in the
opposite direction of the most common, downward lightning strike.

One of lightning's many mysteries is whether the X-rays are essentially just
along for the ride, or actually play a role in its steps, darts and other
dynamics.

It's possible that the work might eventually lead to reducing the damage
lightning can do, but that's not what sends Dwyer out to the testing site
during hot summer thunderstorms.

"It's just great fun," he said. "I love the idea that this is something so
familiar, and we don't understand the simplest things about it, how it gets
started, how it moves."