NINETY MINUTES before sunrise on 7 April 1978, an extraterrestrial guest
arrived over Eastern Australia. For about 20 seconds it streaked across the sky
leaving a bright trail that turned night into day, before finally exploding into
glowing fragments that vanished into the sea. This meteor was just one of
thousands that enter our atmosphere every year, yet dozens of witnesses in
Newcastle and Sydney reported something particularly strange about this visitor.
Just before it blew apart, it produced an unearthly soundtrack of hisses,
crackles and pops.
Reports of noisy meteors appear in the Bible, yet the cause of their bizarre
sounds has always been a mystery. One person might hear the popping and
whooshing clearly while another, standing just a few metres away, hears nothing.
Explaining this oddity is especially tricky since there is almost no hard
scientific data to go on: even if you spent two hours every night looking for
them, you might have to wait fifty years to hear one.
Yet researchers believe they are finally closing in on the origins of these
strange sounds. All they need now are some meteors on which to test their
theories. But rather than waiting around for one to show up, they鈥檙e hoping that
artificial meteors鈥攔edundant satellites brought down from orbit to burn up
in the atmosphere鈥攚ill give them the vital data they need to settle it
once and for all. At the same time, there鈥檚 a good chance that they will solve
another age-old mystery鈥攖he ghostly, rustling songs sometimes heard by
observers of the northern and southern lights.
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One of the pioneers of these studies is Colin Keay, a physicist at the
University of Newcastle in Australia. The day after the New South Wales fireball
fell to Earth, Keay was phoned by a colleague at the Australia Museum in Sydney
who asked him if he would search for any fragments of the meteorite that might
have landed on dry ground. During this hunt, he discovered something about the
fireball that would change the course of his work forever.
The meteorite, Keay calculated, had streaked across the sky at almost 20
kilometres per second, 30 kilometres up, yet he met dozens of reliable witnesses
who claimed to have heard it produce strange noises as it flew
overhead鈥攁nything from 鈥渁 low moaning鈥� to 鈥渁n express train travelling at
high speed鈥�. If these sounds had come directly from the meteorite, people on the
ground below shouldn鈥檛 have heard them until almost a minute after it exploded.
It would be like seeing a distant flash of lightning and hearing the thunderclap
at the same instant.
What finally clinched it for Keay was meeting two witnesses who claimed the
sounds first alerted them to the meteorite trail. 鈥淲hen two people reported
hearing the sounds before seeing the light of the fireball, I knew it couldn鈥檛
be psychological,鈥� says Keay. 鈥淭here had to be something to it.鈥� Intrigued, he
set to work to uncover the mechanism behind these noises. He spent months
creating and discarding one physical model after another. Finally, he settled on
one that he suspected was the only way to explain how an observer could hear a
meteor鈥檚 fiery entry at the same time as seeing it. It all comes down to
electromagnetic radiation.
Keay suspected that the light given off by a meteor鈥檚 trail must be
accompanied by invisible electromagnetic radiation in the form of very low
frequency (VLF) radio waves at frequencies from 10 hertz to 30 kilohertz.
Travelling at exactly the same speed as visible light, these waves would reach
the observer as soon as the meteorite itself came into view. The problem is that
you can鈥檛 hear radio waves. The only way you might hear them is with the help of
a suitable 鈥渢ransducer鈥濃�攁n object that acts rather like a loudspeaker,
converting electromagnetic signals into audible vibrations.
After some experiments in a soundproof chamber, Keay found that all kinds of
things can act as transducers. Aluminium foil, thin wires, pine needles or dry,
frizzy hair all respond to a VLF field. The radio waves induce small charges in
such objects, and these charges force the object to vibrate in time with the
oscillating waves, effectively making them act like the diaphragm in a
loudspeaker. Even a pair of glasses, he discovered, will vibrate slightly. And
since they rest against the bones of the skull, glasses could increase an
observer鈥檚 chances of hearing VLF waves.
Pine speakers
The transducer effect would explain why some people heard noises from the
Australian meteor while others close by heard nothing. Those who heard sounds
were simply nearer to the 鈥渟peakers鈥濃�攖ransducers such as pine trees, for
example. It would even explain why attempts to record these sounds have always
failed. 杏吧原创s go out of their way to place their microphones well away from
any possible sources of interference such as trees or electric cables. But
without any transducers nearby, the meteors would appear silent.
So the transducer effect seems a plausible source of the strange noises, but
how do meteors generate VLF waves? 鈥淚 was getting nowhere until I got the idea
to look at turbulence,鈥� Keay says. He remembered a theory put forward by
physicist Fred Hoyle which used turbulent plasmas to explain sunspots. Perhaps,
thought Keay, interactions between the Earth鈥檚 magnetic field and the plasma in
a meteor鈥檚 trail could somehow create VLF waves.
When a meteor crashes into the Earth鈥檚 dense atmosphere, it ionises the air
around it, leaving a blazing trail of plasma. For a few metres behind the
meteor, this trail flows smoothly, but a little further back it becomes
turbulent. Since a plasma is a mixture of ions and electrons, it can trap and
hold the Earth鈥檚 magnetic field. 鈥淭he plasma is swirling so fast that the
magnetic field is trapped and scrambled up like magnetic spaghetti,鈥� explains
Keay. But as the meteor races across the sky, the plasma left behind cools, and
the electrons and ions in it recombine almost immediately. Without the
electrical charges to keep the magnetic field lines tangled, they suddenly pop
free and vibrate like a plucked violin string. It is these vibrations, Keay
believes, that broadcast VLF electromagnetic waves over a range of several
hundred kilometres (see Diagram).
Keay has named the sounds generated by these radio waves 鈥渆lectrophonic鈥�
noise. He even believes that VLF waves are responsible for another eerie effect:
the rustling and sighing sounds of the northern and southern lights.
For centuries strange noises have been said to accompany the exquisite
curtains of colour seen in the sky near the Earth鈥檚 magnetic poles. These sounds
are heard often enough to be known as the 鈥渨hisper of souls of the dead鈥� in
Eskimo folklore. Yet just as with the burps and whistles of meteors, some people
hear the swish of the aurora while others nearby are left in silence鈥攐ne
reason the sounds were often written off as a psychological illusion.
Auroras are created as the Earth鈥檚 magnetic field captures charged particles
from the solar wind. These particles stream along the field lines and down
towards the magnetic poles. Here they strike the upper atmosphere and ionise
nitrogen and oxygen molecules to produce the characteristic red and green glow
of the auroras. During these electrical 鈥渟torms鈥�, scientists have recorded
abnormally high electric fields and many believe these fields are responsible
for the noises auroras emit. They suggest that they cause 鈥渂rush discharge鈥�,
which occurs when electric fields induce an electric potential gradient in
objects on the ground. If these objects have points or spikes鈥攕uch as
those on leaves or pine needles, for instance鈥攖here can be an electric
discharge at their tips that creates an audible crackling.
But Keay believes that the electric fields are rarely strong enough to create
brush discharge. The whispering of the auroras must have another cause, he says.
He believes that just as with meteor noises, auroral sounds are generated by VLF
waves acting on transducers such as hair. These waves seem to be produced by
ions and electrons from the solar wind that are reflected back and forth in the
Earth鈥檚 magnetic field.
Keay鈥檚 model might explain sounds from large meteors and auroras, but it
doesn鈥檛 seem to explain the noises that very small meteors make. In November
1998, astronomers from all over the world flocked to Mongolia for the biggest
Leonid meteor display in decades. Over two nights, they witnessed more meteors
than they could hope to see in four years of normal observations. There were
even seven reports of electrophonic sounds鈥攊ncluding the first brief
meteor 鈥減op鈥� ever captured on tape, recorded by the Croatian-based group,
International Leonid Watch.
Previous recordings of meteors had produced a time delay between the visual
observation and the sound, allowing the possibility of interference or even the
odd sonic boom to slip in. But the Croatian researchers showed that the VLF
signal picked up by radio receivers coincided with the sounds picked up by
microphones and an image recorded on video to within one-hundredth of a second:
enough to convince all but the most sceptical that this wasn鈥檛 a statistical
freak.
Yet according to Keay鈥檚 theory, there shouldn鈥檛 have been any noise at all.
Leonids are small objects made of porous, fragile material. Weighing no more
than a dried pea, the average Leonid burns up long before it reaches the lower
atmosphere, where turbulence in its plasma tail can generate VLF waves.
According to Keay鈥檚 model, only a giant Leonid, upwards of one metre across
would stand any chance of producing electrophonics. 鈥淲hen you calculate how
bright a meteor of that size would be, the number becomes enormous and would
violate the observations,鈥� says Dejan Vinkovic, an astrophysicist from the
University of Kentucky who attended the Mongolian display. Also, the sounds from
Leonids are short pops or clicks, quite different from the prolonged hisses
accounted for by Keay鈥檚 theory.
Martin Beech, an astronomer at the University of Regina, Canada, believes he
can resolve the problem. He has studied noisy Leonids on and off for the past
decade and has just written a paper that expands his theory to explain these
strange pops. 鈥淲e produced the name `burster鈥� to distinguish them from the
longer-duration sounds that Keay researched,鈥� says Beech.
In a model developed with colleague Luigi Foschini, the electromagnetic
signal is formed suddenly when a fast, light meteor breaks up. When this
happens, says Beech, a shock wave explodes out into the plasma trail just behind
it. Since the electrons and ions in the plasma have different masses, the
lighter electrons tend to ride the front of the shock and are separated out from
the slower-moving ions. 鈥淭hat sets up something called the space charge,鈥� says
Beech, 鈥渨here you鈥檝e got a separation of the negative charge of the electrons
from the positively charged ions.鈥� This separation is unstable and the charges
recombine almost immediately, but not before the short-lived electric field
generates a sudden pulse of VLF waves. When this burst reaches the ground it
creates audible sound in the same way as the radio waves from larger meteors.
Violent explosion
Keay likens these electrophonic pops to the audible 鈥渃lick鈥� that occurs at
the moment a nuclear bomb detonates. 鈥淎 nuclear bomb is a violently exploding
plasma that causes such a shock to the Earth鈥檚 magnetic field that it generates
a pulse of electromagnetic radiation,鈥� says Keay. Beech agrees that the physics
may be similar. 鈥淏ut to do that you need something that is literally like a
nuclear explosion, and in the case of bursters they just don鈥檛 have that kind of
energy,鈥� he says. Despite the progress, it seems that there is still no single
theory that can explain all the effects.
The real problem is that Beech and Keay simply don鈥檛 have enough data to go
on. 鈥淲ith bursters, it is not entirely clear yet what sort of signal you鈥檇
expect to see, and it鈥檚 hard to look for something when you don鈥檛 know what it
looks like,鈥� says Beech. To collect more information, he has set up an all-sky
video camera and microphone at the University of Regina. 鈥淧rogress in the future
is going to depend upon getting reliable data,鈥� he says.
Vinkovic is also busy hunting for noisy meteors. Last year he set up the
Global Electrophonic Fireball Survey to gather reports of meteor noises. So far
it has 20 separate incidents on its database, and Vinkovic plans to collect
further electrophonic information by persuading other international meteor
surveys to start listening for sounds.
He is also looking to artificial meteors for help. 鈥淓ven when you observe
electrophonic sounds from a meteor, you don鈥檛 know what properties that body had
when it entered the atmosphere. You don鈥檛 know the physical parameters,鈥� he
says. The answer, he has realised, is to listen to satellites as they burn up in
the atmosphere. They will behave just like natural meteors, but you know their
size and exactly what material they鈥檙e made from. If you can find out when and
where they鈥檙e coming down, he says, you should be able to get a good idea of
what鈥檚 going on.
Recently, when Motorola drew up plans to dispose of its 66 Iridium
satellites, Vinkovic thought that he had hit the electrophonic jackpot. Now a
rescue package means the Iridium network looks set to stay up there for the time
being, but Vinkovic is not too despondent. Other artificial meteors, such as
failed communications satellites, are regularly brought burning down to Earth.
The Russian space station Mir is coming down in February. And there are even
unconfirmed reports that the space shuttle returns to Earth with an
electrophonic crackle. Vinkovic has a busy time ahead, but he knows that only
hard evidence will silence the sceptics.
Colin Keay, on the other hand, feels that electrophonics and the theory he
has pioneered are on a firm enough footing to put the ball back into the cynics鈥�
court. 鈥淚 believe that I鈥檝e solved the problem and started a new science,鈥� he
says. 鈥淚t is healthy for people to doubt, but the onus is on them to prove their
doubts.鈥� The challenge to physicists is clear鈥攜ou may not subscribe to
these theories, but do you have any better ideas?
THE researchers admit that their efforts to account for electrophonic sound
do not provide anything like the whole picture. Colin Keay鈥檚 plasma-turbulence
theory works well for long-duration sounds from large fireballs, and Martin
Beech鈥檚 burster model may work for lightweight meteors, but there are still a
number of reports that neither can explain on its own. The real answer may lie
in a mixture of both. If a Leonid disintegrates gradually on entry rather than
its more typical catastrophic break up, for instance, a repeated burster effect
could resemble the longer-duration sound modelled by Keay. There may well be
other mechanisms at work that scientists just haven鈥檛 considered yet.
鈥淧ersonally, I don鈥檛 think there is one single theory that can explain
everything going on out there,鈥� says Dejan Vinkovic of the Global Electrophonic
Fireball Survey. He thinks that meteors must be able to distort the Earth鈥檚
magnetic field, even at heights where the air is too thin to create turbulence.
In preliminary calculations, Vinkovic has found that this distortion could start
at the edge of the ionosphere, some 100 kilometres above the ground. But the
question remains, how?