Above: Sprites are lightning
flashes that rise from storms and apparently connect with the
upper atmosphere. Their exact origin and nature are mystery.
These images were captured by a University of Alaska aircraft
studying storm systems over the Midwest in 1994.
Ironically, the team may have to pack quickly and evacuate,
with no time for lengthy farewells (the group photos were already
done Saturday) because Georges is threatening Patrick Air Force
Base on Florida's east coast, where the CAMEX-3 team is based.
The evacuation of the Florida Keys has already been ordered.
The hurricane's farewell gift - the lightning and sprites
- are a recently discovered phenomenon that NASA's Global Hydrology
and Climate Center, coordinator of CAMEX-3, has been studying.
Red sprites and blue jets, as they are known, were seen by high-altitude
pilots and Space Shuttle crews in the 1980s, perhaps earlier,
but most observers were unsure of what they saw and did not report
the events. Sprites and jets are very faint and can be recorded
only with cameras using image intensifiers (see sidebar).
Observations from the Shuttle in 1989-91 and aircraft videotapes
of sprites in 1993 confirmed their existence. The exact cause
remains a mystery, although they appear to be a part of the global
electrical circuit.
Tuesday's Georges synoptic/water
vapor inflow flight was a success. The hurricane had already
made landfall over Hispaniola and was impacting the mountains
there. Although Tuesday was the DC-8 Airborne Laboratory's last
CAMEX-3 flight - the team sent special thanks to the flight crew
for its excellent support.
Monday's Georges eye wall flight was a success. Georges showed
many unique characteristics including large cloud turrets, variable
eye wall winds, and wind speeds up to 148 km/h (80 knots; 92
mph) at 10 km (33,000 ft) altitude. The eye wall pattern was
also significantly displaced from the radar eye wall location
unlike those seen with Bonnie, Danielle, or Earl. |
Sprites and jets
Reports of strange bursts of colored light coming out of the
tops of powerful thunderstorms date back to the 1800s. And even
though aircraft pilots reported them in the 1950s and '60s, they
remained unconfirmed until recently.
These weird flashes were first observed from the ground, when,
quite by accident, they were captured on video on July 5, 1989
by University of Minnesota scientists John Winckler, Robert Franz
and Robert Nemzek. The scientists were actually performing a
calibration test for a low light level monochrome camera, and
weren't particularly looking at the thunderstorm to the east
of their observing site at all. The next morning, while viewing
the test video, they saw giant twin pillars of light extending
upward more than 30 kilometers above the thunderstorm.
The flashes were first recorded from space by the Space Shuttle
(STS-34), as it passed over a highly active thunderstorm in northern
Australia on Oct. 21, 1989. The shuttle's monochrome TV cameras
filmed what are now called sprites and jets. The observations
were being conducted as part of the NASA/Marshall
Mesoscale Lightning Observation Experiment. Otha H. Vaughan,
Jr., of NASA's Global Hydrology Center was the principal investigator.
In 1994, while flying an extremely sensitive color camera
normally used for auroral photography in a high altitude aircraft,
University of Alaska scientists confirmed that the flashes have
a generally reddish color which often fades to purple or blue
in the downward extending tendrils. Dr. Davis Sentman of UAF
named these "sprites" after the creatures in Shakespeare's
"The Tempest," in part because of their transient,
ephemeral nature. The UAF team also discovered and named blue
jets.
The sprites appear high above the thunderstorm while the jets
shoot out from the top of the thunderstorm. Sprites appear to
cascade as high as 96 km (60 mi) above the Earth. Sprites can
look like giant red blobs, picket fences, upward branching carrots,
or tentacled octopi, and can occur singly or in clusters. The
jets appear to be ejected from the storm top with velocities
as high as 100 km per sec and move up as high as 32 kilometers.
Stratospheric lightning events could generate strong electric
fields and electromagnetic pulses which may interact with the
Earth's ionosphere and magnetosphere. Strong fields at high altitudes
may generate runaway electrons which could then produce high
energy x-rays and even gamma rays. Thus, it is possible that
lightning may generate a broad spectrum of electromagnetic radiation,
ranging from extremely low energy to extremely high-energy gamma
radiation. This theory is supported by the Burst and Transient
Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory
which has detected gamma rays coming up from the Earth - not
deep space - when the spacecraft was over thunderstorms.
Researchers want to know what effect upward lightning may
have on future commercial aviation operations and high altitude
balloon research flights in the stratosphere.
What's
up there? by Vaughan reviews the history and science
of sprites and jets.
Gamma-ray flashes from
Earth and sprites and jets - searching for a connection. |
|
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