scientists know little about lightning, as most will admit, they know even
less about a hard-to-spot electrical cousin that shoots up above a thunderstorm.
But a new study sheds precious light on the mysterious high-altitude discharges,
showing us they may be far less rare than suspected.
them red sprites. They may or may not be related to blue jets and elves.
And, yes, we're completely serious here.
The first images
of the enigmatic spurts of energy were made by accident, just 10 years
ago. Before that, pilots had reported seeing mysterious red flashes above
thunderstorms, but their claims were not always taken seriously. Since
then, researchers have learned a great deal about the high-altitude discharges,
but still they exist in, or rather above, a cloud of mystery.
A new study,
to be published in the Dec. 15 issue of Geophysical Research Letters,
explains the observation of two red sprites above a 1998 New Mexico storm.
The sprites were associated with negative cloud-to-ground lightning. Previously,
sprites were thought to be related only to positive cloud-to-ground lightning,
a far less common variety.
which frequently occur in clusters of three or more, soar up to 95 kilometers
(59 miles) into the atmosphere, above thunderstorms. Their flash is incredibly
brief -- just 3 to 10 thousandths of a second. The flashes expand to cover
a wide area but are weak in electrical energy. Their brightest portions
exist 65 to 75 kilometers up, above which wisps and glowing regions often
(The black &
white animation of multiple sprites at the top of this page was captured
in October 1997. Credit: New Mexico Tech/NASA.)
have been spotted extending below the sprites. These tendrils are not to
be confused with blue jets, a separate phenomenon that shoots more slowly
upward, and not as high. Red sprites have on occasion been preceded by
elves, lower-altitude flashes produced by the widespread heating of the
atmosphere, caused by lightning.
extent of red sprites, and their physics, could bear on the understanding
of upper atmospheric chemistry and even on high-energy particles known
to affect satellites. Researchers also say there may be similar phenomena
that occur above other planets, though interplanetary comparisons have
not yet begun.
cloud-to-ground (CG) lightning strikes, electrons move from the clouds
to the Earth (and, by definition, current moves upward). In a positive
CG, electrons move upward. While all of this might be interesting to devoted
lightning researchers, red sprite aficionados are most curious about the
physics happening above the clouds. And the new research, produced by Stanford
University's Christopher Barrington-Leigh and colleagues, changes the perception
of what's going on up there.
of negative CG sprites suggests that the normal mechanism for sprites does
not depend greatly on the sign of the charge moved by the lightning, but
only on how much charge is moved by the lightning," Barrington-Leigh told
space.com. "The scarcity of -CG sprites must then lie in the fact that
most negative CG lightning just doesn't often move enough charge around."
of every day, on average, lightning hits Earth about 50 times. Barrington-Leigh
points out that the majority of that activity involves negative CG strikes,
of the type he's just observed to be associated with red sprites. This
raises the possibility that sprites may be far more prevalent than previously
higher frequency could affect our understanding of numerous upper-atmosphere
phenomena. Some researchers have speculated that sprites might create nitric
oxide, which destroys the protective ozone layer. And there are other possible
"We have seen
evidence that sprites, and the electromagnetic pulses from lightning, may
be able to both increase and decrease [respectively] the populations of
high-energy particles in the Earth's radiation belts," said Barrington-Leigh.
"These remarkable effects could thus affect the rate at which satellites
in low and middle earth orbit are being bombarded by electrons."
It's also possible
that similar discharges might occur on other planets -- Jupiter and Venus
being likely candidates. "Differences in how the analogies to sprites and
lightning appear in other planetary atmospheres could serve as a diagnostic
for the nature of extraterrestrial ionospheres, atmospheres, and electrification
processes," Barrington-Leigh said.
"The study is
quite significant," said Matt Heavner, an atmospheric electricity researcher
at the University of Alaska, Fairbanks. Heavner, noting that we still understand
very little about the lightning that comes out of the bottom of clouds,
said knowledge of red sprites is growing rapidly, considering researchers
have only been on the trail for a decade.
How the discovery
are so fleeting, the trick to recording them is to use very fast equipment.
Barrington-Leigh and his colleagues set up high-speed photometers (which
can detect flashes lasting only a few millionths of a second) atop a mountain
in New Mexico, then monitored satellite weather images for storm activity.
"If the weather
is clear overhead, we uncover our photometric equipment, start up its software,
and point the cameras over distant storms," he said. Each evening, all
night for two months, they watched and waited, adjusting the equipment
based on what they could see. "Sprites often occur in a repeatable location,
so if you don't get one right where you want it, you can make an adjustment
for the next one."
imaging has only been used for sprite hunting in the past couple of years,
Heavner said, adding that results of similar studies are expected to be
released in the coming weeks.
space.com there are several lines of additional research that need pursuing.
For one, scientists need to arrive at an estimate of the global rate of
occurrence. All sprite observations to date have been in North America,
he said, and the heaviest concentrations of lightning around the world
have not been investigated.
Armed with more
knowledge, researchers might one day include red sprites in other atmospheric
of sprites in models of the middle atmosphere, and the improved determination
of the total energy that a sprite deposits in the middle atmosphere, will
be an exciting next step for sprites research," Heavner said.
A red sprite
shooting above a thunderstorm with blue tendrils extending downward. Credit:
University of Alaska, Fairbanks. Click to enlarge.
observing and reporting sprites from the Universi
research at Stanford
images from NASA