St Elmo's fire
Sailors out at sea would sometimes see a bluish glow seeming
to shoot out of the ends of the masts of ships at night. The
light wasn't hot and wouldn't burn anything on board. They
took it to be a good omen and called it St Elmo's fire.
Atmospheric scientist Steve Ackerman at the University of
Wisconsin-Madison in the US has been fascinated by St
Elmo's fire ever since his brother encountered it.
I've still not seen it first-hand, but I'll keep looking for it
Ackerman's brother was working on some copper piping in
his basement during some bad weather. "A thunderstorm
moved into the area, and at one point many of the pipes
had a blue glow over them," says Ackerman. "That's when I
start looking into what caused it."
Thunderclouds create a strong electric field, because there
is a strong difference in electrical charge between the
cloud and the ground, which you can sometimes feel as
static. The field can be intensified by pointed objects, like
a metal pipe or the mast of a ship.
If this electrical field becomes strong enough, it can break
air molecules down into electrically-charged particles. The
gases become "plasma", and give off a glowing light.
A similar plasma glow can be created in the laboratory,
using sharp or pointed objects to intensify an electrical
field. Even so, Ackerman wants to see St Elmo's fire
naturally. "I've still not seen it first-hand, but I'll keep
looking for it."
Will-o'-the-wisp
Like St Elmo's fire, the will-o'-the-wisp is a faint light
that has been reported for centuries. But unlike St Elmo's
fire, in recent times people have reported it less and less.
As you might expect of a phenomenon whose name has come
to mean something elusive, it has never been created in the
laboratory.
It could be that the reports are fictitious
The will-o'-the-wisp is normally described as a light,
flickering or constant, lying close to the ground, mostly in
marshy areas of the countryside. It supposedly disappears
after a couple of minutes.
Luigi Garlaschelli of the University of Pavia in Italy – best-
known for recreating the Turin shroud with a few
laboratory tricks – would like to study the will-o'-the-wisp
in nature. But it is not clear there is anything to study.
"The risk is indeed that we are looking for something that
does not even exist," Garlaschelli says. "We must trust or
hope that all the sightings of will-o'-the wisps are those of
a real phenomenon."
If will-o'-the-wisp really did represent a natural process,
there are some possible explanations that Garlaschelli could
test. For example, the association with marshy areas
suggests that the light comes from burning marsh gas, which
is mainly methane. However it is not clear what would set
the gas alight.
Alternatively, it could be that the reports are fictitious;
that the lights are imagined or hallucinated; or that the
lights were reflections of the Moon or other lights that
observers misinterpreted.
Earthquake lights
"You could be standing there in the middle of the ball of
light," says Friedemann Freund of NASA's SETI Institute in
Mountain View, California, US. "Maybe your hair would be
electrified, you might have a halo like a saint. But it
doesn't burn anything. You might feel a bit funny, but you
wouldn't be harmed."
This is what it would feel like to find yourself in the middle
of an earthquake light.
The shock waves were racing through, and as the later waves
arrived there was an explosion of lights
These lights are a plasma discharge that happens when a
particular type of rock is under stress and builds up an
electric charge, Freund says. "We think that when we push
the rocks together very fast, the charge is relieved through
a plasma discharge up from the rock."
They can come in many different shapes, forms, and
colours.
Coseismic earthquake lights, which happen during an
earthquake, are bursts of light coming out of the ground
over a space of a few kilometres. They rise 200-300m into
the night sky in a fraction of a second, one after another.
In recent years, the abundancy of security cameras has led
to beautiful videos of earthquake lights.
"Some of the best records are in Peru," Freund says. "A
friend at a local university secured footage when a
magnitude 8 earthquake hit the south of Lima. The shock
waves were racing through, and as the later waves arrived
there was an explosion of lights."
Ball lightning
Often dismissed as a myth, ball lightning turns out to be
quite real.
When an ordinary cloud-to-ground lightning bolt strikes
the ground, it can vaporise certain minerals in the soil
In 2012 a team of researchers were measuring ordinary
lightning in a storm-prone region of the Qinghai Plateau in
China. Suddenly a ball of light about 5m across appeared in
front of them . It burned white and then red for a few
seconds before vanishing.
This was the first natural ball lightning to be studied. The
researchers recorded the spectrum of light that the ball
gave off, and analysed it to see if they could determine
what this unusual lightning was made of.
It turned out to have a very earthly origin: soil. When an
ordinary cloud-to-ground lightning bolt strikes the
ground, it can vaporise certain minerals in the soil. Some of
these contain silicon compounds, and under the extreme
conditions they undergo chemical reactions to form silicon
filaments.
These filaments are highly reactive, and burn with the
oxygen in the air to create the orange glow that the
researchers measured.
Green flash
In the very last seconds before the Sun sets, its light can
turn bright green. But the Sun has not changed colour: the
flash is caused by a mirage.
These are the mirages that can make the Sun seem to move
in shimmering waves
The atmosphere splits the Sun's white light into its separate
colours, just like a prism: it bends red more than orange,
orange more than yellow, and so on. Because red undergoes
the strongest bending effect, it appears to fall past the
horizon first, followed by orange, yellow and green.
The colours beyond green – blue, indigo, and violet – are
strongly scattered by the gases in the atmosphere. That's
why the sky appears blue. But as a result, the last coloured
light that can be seen as the Sun falls below the horizon is
green.
Normally this effect is very slight. To make the last green
rays visible, there also has to be a mirage that makes the
Sun appear much larger than usual. These are the mirages
that can make the Sun seem to move in shimmering waves,
and almost seem liquid as it pours past the horizon.
Ocean horizons often produce the best mirages for spotting
a green flash.
Upward lightning
Perching cameras on top of the Empire State Building in
New York, US in 1935, Karl McEachron of the General
Electric Company recorded something strange. Lightning
was travelling not from cloud to ground, but shooting up
from buildings into storm clouds.
Meteorologists now know around one in a thousand lightning
bolts strike upward. But despite decades of research on
upward lightning, its exact mechanism is still a puzzle.
Every time I flew through a thunderstorm, I reaffirmed
that it was no place for a plane
Storm photographer Tom Warner is now researching how
upward lightning is triggered, at the South Dakota School of
Mines and Technology in Rapid City, US.
His and others' research has shown that there are two
distinct forms of upward lightning. Both of them require a
tall structure, such as a skyscraper or wind turbine, to
happen.
The first kind requires a nearby ordinary cloud-to-ground
strike first. The sudden disruption to the electric field
causes a "lightning leader", a channel of positive or
negative charge, to travel up to an area of thundercloud
with the opposite charge.
The second kind doesn't require a downward lightning
strike nearby, and can travel upward spontaneously.
Warner has studied and photographed these rare events
since becoming fascinated by an upward lightning bolt in
2004. To get his data and images, he has piloted an
armour-plated plane into the hearts of storms.
"Being able to experience storms up close and even from
the inside was absolutely amazing," Warner says. "It was
challenging and required intense concentration. Every time
I flew through a thunderstorm, I reaffirmed that it was no
place for a plane."
Sprites
High above a thundercloud and its exchange of lightning
with the ground, you might find a sudden reddish glow
stretching for tens to hundreds of kilometres. It looks a bit
like the straggling tendrils of a jellyfish.
Very large thunderstorms can produce these phenomena,
which are known as sprites. "They're very intense," says
Martin Fullekrug of the University of Bath in the UK. "The
thunderstorm needs to produce a special kind of flash,
and they're rare. Maybe only one in a thousand flashes
produces a sprite."
You can get a low-quality picture of a sprite with a camera
of a couple of hundred pounds
These flashes need to remove a lot of electrons from the
thundercloud. A long, slow current is needed to make a
sprite, and such currents can form in big storm systems
reaching 100km across.
The elusiveness of these deep red flashes earned them
their ethereal name, adopted from Shakespeare's A
Midsummer Night's Dream . But as the price of powerful
cameras has dropped, sprites are being caught on camera
increasingly often.
An ordinary CCTV camera with good night vision can snap a
low quality image. Amateur meteor observers are also
collecting extensive data on sprites.
"You can get a low-quality picture of a sprite with a
camera of a couple of hundred pounds," says Fullekrug.
"With a little bit of guidance anyone can do it."
ELVES
The term ELVES is a clunky acronym chosen to complement
their cousins the sprites. It is short for "Emissions of Light
and Very low frequency perturbations due to
Electromagnetic pulse Sources", but that is something
"hardly any scientist can spell out for you", according to
Fullekrug.
It is very difficult to see an ELVE with the naked eye
Appearing around 80-100km above the ground, they look
very different to sprites.
"They're expanding rings of light," says Fullekrug. "They
look like a doughnut from space, with a dark hole in the
middle, and they spread out for 1,000km or so."
ELVES are fleeting, lasting for less than a millisecond. The
storm conditions necessary to make an ELVE (pronounced
"elf") include a particular type of lightning, with a very
sharp rise in current. Unlike for sprites, to get an ELVE the
discharge has to be very sharp, so the two rarely occur at
the same time.
ELVES occur more often than sprites, with about 1 in 100
lightning flashes producing one. Small storms are just as
likely to make them as big storms, as a really fast current
can happen in any storm.
ELVES are mainly white because they're so intense.
"They're really, really fast," Fullekrug says. "It is very
difficult to see an ELVE with the naked eye. I've not seen
one myself, though I've been looking quite a bit."
Blue jets and gigantic jets
"Blue jets are a little bit of a mystery," says Fullekrug.
The first problem is that they're blue. Blue atmospheric
phenomena are hard to study from the ground, because the
atmosphere is so good at scattering blue light. They're also
very narrow and rare.
There are marvellous examples of gigantic jets developing
off the coast of Africa
"We don't know the ideal conditions for a blue jet to
form," says Fullekrug. "One idea is that when
thunderstorms get really tall, they pierce into the thinner
layers of the atmosphere above." Storms have powerful
updrafts, which can push them above normal altitudes.
"When this happens it could generate a blue jet, but we
really don't know for sure."
Researchers do know that there is another phenomenon
called a gigantic jet, which seems to be a hybrid of a blue
jet and a sprite. They are broader, wedge-shaped jets of
light and easier to see. They can last 10-100ms, so they are
relatively slow compared to other storm events.
"There are marvellous examples of gigantic jets developing
off the coast of Africa," says Fullekrug. "But gigantic jets
are rare. Perhaps only one in ten or one in a hundred
sprites will combine with a blue jet to make a gigantic jet."
The auroras
The green, blue and red shapes of the auroras, swirling over
the two poles of the Earth, are a visible map of events that
happened thousands of kilometres away. When the solar
wind – charged particles from the Sun that brush past our
planet – meets the Earth's magnetic field, the two interact.
The particles from the Sun slide along the contours of the
magnetic field towards the poles. When they reach the
upper atmosphere, they interact with gases. The particles
can give the air molecules enough energy to release
electrons, causing them to glow in a range of colours.
Earlier in 2015, Swenson launched a rocket into the aurora
"Auroras can have many shapes and structures, depending
on what the magnetosphere is doing," says Charles Swenson
of Utah State University in Logan, US. "There are arcs,
westward surges, beading, all kinds of names for different
visible shapes. You can imagine it like a sheet flapping in
the wind, and every once in a while it will get really messed
up and that's when these dramatic events happen."
Earth is not the only planet with auroras. "All you need is a
solar wind blowing past a planet that has gases on it and a
magnetic field," says Swenson. Jupiter and Saturn both
have unique auroras, as the gases of their atmospheres are
very different.
The aurora also has an invisible component, which is the
subject of Swenson's studies. Charged particles from the
solar wind cause electrical currents in the aurora, which
are hard to study from the ground. Earlier in 2015,
Swenson launched a rocket into the aurora to measure
these invisible elements.
"The question is, are the invisible parts of the aurora
dancing and moving as rapidly as the visible parts?" he
says. "It's very early days, but we think the answer will be
yes."
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