7 AUGUST, 10 am. Everyone in Ottumwa, south-east Iowa, hopes for a
day of sunshine. Everyone except Edgar Bering, that is. He is praying for a
serious downpour.
At this time of year, America’s Great Plains experience some of the most
violent storms on the planet. And above the rolling storms dance bewitching and
mysterious light shows. Bering, a physicist at the University of Houston in
Texas, and a team from NASA’s National Scientific Balloon Facility are hoping to
get a closer look at these lights. So they’re preparing to send a giant balloon
above a storm. It’s in for a rough ride.
The lights that Bering hopes to catch are called sprites—colossal
towers of red and blue light up to 10 kilometres across and up to 30 kilometres
tall that flash in and out of existence so quickly that our eyes can barely
register them. They light up in the mesosphere, an icy region of the atmosphere
starting 50 kilometres above the ground, and their fleeting appearances seem to
be triggered by lightning flashes in storms far below.
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Sprites were first photographed in 1989, but we still don’t know much about
them. We do know that they are fairly common and that they are involved in the
creation of nitrogen oxides, which can damage the ozone layer. There is evidence
that they can endanger airliners and spacecraft, and even reach right down into
the storm clouds below, keeping the chemistry of the atmosphere right above our
heads in balance.
After more than a decade of disagreement, atmospheric physicists think they
are finally close to agreeing on how sprites form. Bering’s balloon offers one
of the first real chances to confirm their theories. But it seems a storm is
brewing that threatens to set Bering against his colleagues.
7 August, 6.30 pm. Bering and his team ready their instruments for lift-off.
They may be chasing a storm, but to get the balloon away safely requires calm
conditions at ground level. And in order to get close to their target the balloon needs
to be launched while the storm is still at least 150 kilometres away. Such precise
take-off conditions require accurate weather forecasts, and in 35 days of
waiting, this is only the third time that Bering’s meteorologists have given him
the go-ahead.
Helium is pumped into the balloon, which is only partially inflated—it
will expand to its full size in the ultra-low pressures of the stratosphere. A
previous launch has already failed, when a sudden leak left one $25,000
balloon a sagging wreck. The mood is tense as the team waits for the
release.
Bering has spent years planning and organising this unusual “sprite flight”.
Very few direct measurements of sprites have been made. The mesosphere is too
thin to support research aircraft, yet too low for orbiting spacecraft to peer
into, so most of what’s known comes from cameras and other sensors based in
mountain-top labs.
What little information we have has led to a model of sprite formation that
many in this close-knit community of investigators now agree on. It depends on
intense but short-lived electric fields created in the atmosphere by lightning
discharge.
When a thunderstorm forms, dust and droplets of moisture rub against one
another, creating electric charges that build up into
horizontal layers within the thunderstorm
(see Diagram).
Negative charges collect at the bottom of the
cloud and eventually the intensity of the electric field between the cloud and
the ground reaches a level at which molecules in the air begin to ionise. This
is known as the critical breakdown limit. Electrons are released and form a
conducting channel through the air—which usually behaves as an insulator.
Suddenly the accumulated electric charge can flow through this conducting
channel. We see this short-lived discharge as a bolt of lightning.
Although most lightning originates in the negative charges at the bottom of
storm clouds, roughly 1 in every 5 lightning strikes originate in the positive
charges near the cloud tops. This results in an energetic positive
cloud-to-ground discharge, in which the positive charge is neutralised by an
upwards flow of electrons from the ground. The negative charges left in the
lower part of the cloud set up what physicists call a “quasi-electrostatic
field”—an intense electric field that extends high into the
atmosphere.
The critical breakdown limit for air depends on its density. At very high
altitudes—about 75 kilometres up—where air density is low, the QE
field now exceeds the critical breakdown limit for air. Electrical breakdown
occurs and molecules such as nitrogen and oxygen are ionised, releasing
electrons. Under the influence of the QE field, free electrons are accelerated
upwards, while positive ions accelerate down towards the ground.
These ions and electrons hit other atoms and molecules, creating an avalanche
of charged particles. When the high-energy electrons smash into molecules of
nitrogen in the air, they transfer much of their energy to electrons in the
nitrogen. These are excited mainly into two energy states, one of which emits
its excess energy as red light, and the other as blue. It is this glow that is
seen as a sprite. “There is absolutely no question in my mind,” says Umran Inan,
director of the Space, Telecommunications and Radioscience Lab at Stanford
University. “Sprites are caused by QE fields.”
Sprites are confined to the thinner layers of the atmosphere between 40 and
90 kilometres altitude. At lower altitudes where the air is denser, charged
particles can’t move far before they knock into other atoms and molecules in the
air. But in the mesosphere, where air density is low, the QE field can
accelerate electrons to higher speeds. This means that when they collide with
nitrogen molecules, they have sufficient energy to excite the electrons.
Almost as quickly as it appears, the sprite fades away, disappearing
completely in just a couple of milliseconds. However, the QE field lasts much
longer. Researchers on the ground can monitor its presence using radio receivers
since the field produces a continuous electromagnetic signal at frequencies from
a few hertz to tens of kilohertz. The signal often persists long after the
sprite has disappeared, slowly fading as charges in the cloud disperse. This
electromagnetic signal, Inan and his colleagues argue, is the signature of the
QE field.
But what scientists need now are direct measurements of the electric
field—and what could be better than information gathered by a balloon
flying high above the clouds?
7 August, 7.30 pm. As it inflates, the balloon behaves perfectly. The
instruments to record the electric and magnetic fields of the atmosphere are
placed directly underneath, and the balloon starts its 30-kilometre journey into
the stratosphere. The ascent takes two hours. Darkness settles and as the night
goes on the winds carry the balloon into the sprites’ playground in the lee of
the Rocky Mountains. The show is about to begin.
Sprite research is fertile ground for new discoveries. In 1999, scientists
from the Geophysical Institute of the University of Alaska Fairbanks recorded
some startling new aspects of sprite behaviour. With the help of high-speed
cameras that shoot up to 1000 frames every second, Hans Stenbaek-Nielsen and his
colleagues saw that even after a sprite has faded away, it can be easily
reactivated.
This implies that each sprite leaves some kind of long-lived chemical or
electrical fingerprint in the atmosphere that reduces the threshold for
subsequent sprite formation. According to Stenbaek-Nielsen, this fingerprint
most likely takes the form of ionised atoms or molecules, some of which are
almost certainly compounds such as ionised nitrogen molecules or nitrogen
oxides.
Long-lived species may also be present at lower altitudes—in the long
tendrils that stretch down below the sprite’s body to the cloud tops like the
tentacles of an octopus. These tendrils light up with bright, spherical “beads”
which on some occasions outlive the main sprite, lasting up to a hundred
milliseconds in some cases, and can even momentarily flare up long after the
sprite body has faded. “Such bright spots give the impression of embers in a
dying fire,” says Stenbaek-Nielsen.
While the identity of the molecular fingerprints has not been measured
directly, the Alaskan scientists believe that worldwide sprite activity could
affect the atmosphere’s protective ozone layer, since many nitrogen compounds
are known to destroy ozone. Sprites have been spotted above storms in Japan,
China, the Middle East and Europe. Researchers spotted 97 sprites in just three
hours of observations in Wyoming, and estimate that these sprites affected an
area of 23,000 square kilometres. Scale this up and you have a recipe for some
serious atmospheric havoc. “If each [sprite] event produces nitric oxide, we’re
talking about a large chemical change in the mesosphere,” says Stenbaek-Nielsen.
And if global warming creates a more stormy atmosphere, then more sprites, and
more nitrogen oxides, will form.
Walter Lyons agrees. His home at the Yucca Ridge Field Station, Colorado,
commands panoramic views over the Great Plains and has served as an experimental
base, think tank and social centre for the world’s sprite researchers. Lyons
thinks these results become even more important in the light of observations
that vast smoke palls from forest fires appear to enhance the percentage of
positively charged lightning strikes, and thus sprite formation. Large-scale
forest fires could indirectly be creating even more ozone-eating NOx than
climatologists had believed was possible.
8 August, 3 am. The balloon has long disappeared into the night, but an
onboard Global Positioning System unit tells the scientists exactly where it is.
Flying at an altitude of about 32 kilometres, it is now high enough to begin
listening for sprites.
Sprites dance on the horizon as high-altitude ground stations in Wyoming,
South Dakota and Colorado watch the skies around the storm. Equipped with
cameras designed to work in low light conditions, these observatories provide
the visual footage to match the electrical signals received by the balloon.
It has been a good night for Bering. Forests of sprites have flashed across
the skies. The team scours the results for the signature of a QE field—the
low-frequency radio hum. But they are in for a surprise. The balloon’s
instruments did not record it. As dawn breaks, the researchers realise the
favoured model of sprite formation doesn’t measure up.
The results from the ground stations suggest that once a positive lightning
strike occurs, the intensity of the electric field in the mesosphere builds up
over two or three milliseconds until breakdown occurs, and the sprite lights up.
This delay may be related to the flow of currents created by the lightning which
bring the high-altitude electric field to the level required for breakdown, says
Victor Pasko, an atmospheric physicist at Pennsylvania State University in
University Park. Then, once the sprite has faded, charges in the clouds begin to
disperse or flow away, and the electric field observed from the ground decays
slowly over tens of milliseconds.
However, the balloon data paints a very different picture. It implies that
sprites are produced by a sudden burst of current (see Danger: gnomes at work),
and there is no slow build-up of the electric field. Several milliseconds after
the positive lightning strike, sensors recorded a sudden upward-flowing current
pulse. Just 300 microseconds later, the sprite lit up in the sky. To add to the
mystery, the electric field disappears far more rapidly than ground observations
suggest, in just a few milliseconds.
Bering’s results—some of which he presented at the recent American
Geophysical Union meeting in San Francisco—turns sprite theory on its
head. “The charge that produces sprites is not below in the cloud, it’s in the
mesosphere itself,” suggests Bering. So now there are new puzzles: where could
this charge be coming from, and if there’s no QE field, what causes the
delay between lightning and sprite? “We have a problem understanding why the
sprite takes so long to form,” admits James Benbrook, a colleague of Bering’s in
the physics department at the University of Houston.
And what of the low-frequency hum picked up by labs on the ground? Bering
thinks the signal may be caused by the lightning strike itself rather than the
mechanism that lights up a sprite. Researchers on the ground face an additional
problem, they are close to one electrical contact of the global electric
circuit—the Earth itself. The low-frequency hum could be an artefact and
we hear it if we are on the ground when the charges in the clouds flow to earth,
Bering suggests.
Benbrook agrees. The signal received on the ground is more likely due to the
rearrangement of charge in the cloud tops, he says, or the flow of current in
the lightning channel. “But I don’t see what that has to do necessarily with an
excitation mechanism in the mesosphere.”
Other researchers urge caution in interpreting Bering’s results. “At high
altitudes the field can be very small,” says Pasko. Inan suggests that more
sensitive instruments on the balloon may have picked up the hum of the QE field.
“Whether or not there is a continuing field signature is a matter of how
sensitive your measurements are. It could be there but below the noise level of
your instrument.”
Most sprite investigators agree that Bering should have been able to detect
the low-frequency hum, and blame his instruments for failing to do so. Bering
defends the quality of his experiment and insists his instruments were working.
“We wouldn’t have seen the electric signal of the sprite if they weren’t.”
Can the QE field theory recover from this blow? “My personal guess is no,”
says Bering. “None of the existing models will survive when people finally pay
attention to what our data actually says.”
8 August, 11 am. While the balloon’s results reveal a whole new puzzle, the
changing weather means this summer’s sprite hunting season is drawing to a
close. A pick-up truck is sent to meet the balloon as it comes to rest.
Scientists in the ground observatories are packing up too. It has been a long
and exhilarating night, but for now the complete story of sprites remains
tantalisingly out of reach.
Benbrook estimates that the sprite riddle could be solved by another 10
balloon flights, but Bering insists that his results are enough. “I’m trading on
my reputation as a quality experimenter. These results are right and you’re just
going to have to believe me.” Other sprite experts are standing by the
established theoretical model. “There’s going to be considerable debate whether
it’s instrument failure or whether it’s something about propagation we don’t
know,” says Lyons. “But when you only have a sample of one that leaves a lot of
room for discussion.”
Edgar Bering’s “sprite flight” also suggests that the currents responsible
for sprites may carry far more oomph than anyone had suspected. Previous
estimates suggested that the sprite-inducing current carries about 3000 amperes.
Bering’s data, on the other hand, puts the figure nearer 12,000 amperes.
Whether this huge current could pose any direct physical danger to anyone is
unknown. Airliners don’t fly in the mesosphere, but sprites can reach down into
the cloud tops. And it is certainly possible that sprites could affect
spacecraft, Bering suggests. In particular, sprites are the prime suspect in the
unexplained downing of a high-altitude balloon a few years ago.
And it appears that sprites aren’t the only creatures dancing on top of
thunderstorms. “We’re seeing things we’ve never seen before on top of active
storms—electrical discharges coming out the top of clouds that could be a
new form of lightning,” says Lyons. They have tentatively been christened
gnomes. “They look like fingers of light going straight up out of the cloud but
at rather slow speed. It looks like lightning in pictures but takes over a
second or two to happen.”
Could gnomes be more energetic than sprites? “I wouldn’t volunteer to sit in
one,” says Lyons. “Sprites have tremendous amounts of energy spread over a great
volume. We’ve got no idea how much energy is in a gnome, but it’s compressed
into a smaller area.”
Danger: gnomes at work
-
Further reading: for more on sprite hunting see
www.uh.edu/research/spg/Sprites99.html - sprite.gi.alaska.edu
- www-star.stanford.edu/~vlf/optical/
- www.fma-research.com/
- lightning.nmt.edu/sprites/sprites.html
