In
the blink of an eye, an enormous bright red light flashes above a
thundercloud, spreading energetic branches that extend five times taller
than Mount Everest and look like jellyfish tendrils and angel's wings.
These mysterious phenomena are known as Transient Luminous Events
(TLEs), and are usually invisible to the naked eye because they happen
on millisecond timescales, too fast to be seen. They occur between 50 to
100 kilometers above the ground, a long-ignored area of the atmosphere
that is too high for aircraft but too low for satellites to investigate.
There, the thin air interacts with strong electrical fields to ionize
molecules and create arcing plasmas.
These spectacles are relatively new to science. Pilots had reported
enigmatic bright flashes throughout the 20th century, but their
anecdotal evidence didn't amount to proof. The first image of a TLE was
captured accidentally in 1989 when a University of Minnesota professor
aimed a low-light TV camera at the sky to film a rocket launch.
Replaying the tape later on, Professor John R.
Winckler saw brilliant columns of light extending from the tops of storm
clouds. Hearing of the finding, NASA officials
immediately ordered a review of video tapes taken from the space shuttle
that looked at lightning events on Earth. They found dozens more
examples of TLEs, and later scientists have been recording them ever
since.
"One of the neatest things about TLEs is that first image in 1989 was
just a serendipitous capture," said amateur radio astronomer
Thomas Ashcraft, who has been
photographing the events for several years.
Using a relatively simple camera and radio dish, Ashcraft has seen
a whole bestiary
of odd TLE phenomena. The most common are sprites, tall and highly
structured bursts of light that appear above thunderstorms. They ionize
the nitrogen in our atmosphere, causing a red glow. Often, they happen
in conjunction with “Emissions of Light and Very Low Frequency
Perturbations due to Electromagnetic Pulse Sources,” also known as
ELVES, which are enormous halos of light that shoot outward to cover up
to 500 kilometers in a millisecond. Though they are too short-lived to
see, ELVES can produce bright afterglows that some people have mistaken
for UFOs. Other TLEs have names like blue jets and trolls.
To deliver great TLE shots, Ashcraft first checks radar maps of the
local area around his observatory in Santa Fe, New Mexico. Red spots on
such maps indicate strong lightning cells, which increases the
probability of sprite activity. Because the phenomena are mostly visible
in near infrared wavelengths, he uses a modified off-the-shelf DLSR
camera from which he removed the clear glass filter covering the CCD
that blocks infrared light.
By taking continuous three-second exposures, Ashcraft records
thousands of pictures each night. He then goes through the catalog
looking for a sprite to appear. If he spots something, he can check a
video camera that he has running during the night to see if captured
more detail there. He shares his most interesting findings with other
sprite observers, who may chime in with their own pictures from other
positions.
From Santa Fe, Ashcraft says he can usually catch sprites up to 1,000
kilometers away. “I can see big storms out over the Great Plains,
usually beyond Oklahoma City and into Nebraska,” he said. “After that,
the curvature of the Earth gets in the way.”
Using a radio dish, Ashcraft also captures extremely low frequency
emissions that the TLEs give off. He converts these into sound files,
which can be heard in his videos, and can help researchers pick out
details they might otherwise miss.
A lot of
research regarding TLEs is still cutting-edge science, said Ashcraft. Only in recent years have
scientists aimed high-speed cameras
capable of capturing thousands of frames per second to study the
spectacles in detail. While researchers had originally hypothesized that
the phenomena were starting at the tops of thunderclouds, fast-motion
videos prove that TLEs start as luminous spheres and then shoot upwards
and downwards at the same time.
In this gallery, we take a look at some of Ashcraft’s most
spectacular TLE recordings to get a better appreciation of these weird
and wonderful phenomena.
Above:
Spectacular Sprites
Ashcraft captures a large sprite hanging over West Kansas. Play the
video below to listen to the radio sounds given off by the sprites and
to see some slowed-down black and white video.
(para ver video ir ao site:)
http://www.wired.com/wiredscience/2013/07/transient-luminous-events/
2013.07.20 - nuvem cumulonimbus sobre Denver
(não é photoshop como podem verificar na net... mas tem rosto;))
