Pine trees as loudspeakers or how to listen to Electrophonic Meteors

Leonid meteor breakup.

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're hoping that artificial meteors--redundant satellites brought down from orbit to burn up in the atmosphere--will give them the vital data they need to settle it once and for all. At the same time, there's a good chance that they will solve another age-old mystery--the ghostly, rustling songs sometimes heard by observers of the northern and southern lights.

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--anything from "a low moaning" to "an express train travelling at high speed". If these sounds had come directly from the meteorite, people on the ground below shouldn't 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. "When two people reported hearing the sounds before seeing the light of the fireball, I knew it couldn't be psychological," says Keay. "There had to be something to it." 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's 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's 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't hear radio waves. The only way you might hear them is with the help of a suitable "transducer"- an 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's 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 "speakers"--transducers such as pine trees, for example. It would even explain why attempts to record these sounds have always failed. Scientists 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? "I was getting nowhere until I got the idea to look at turbulence," 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's magnetic field and the plasma in a meteor's trail could somehow create VLF waves.

When a meteor crashes into the Earth's 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's magnetic field. "The plasma is swirling so fast that the magnetic field is trapped and scrambled up like magnetic spaghetti," 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.

Full article here at Meteorobs.
Originally from New Scientist magazine, 06 January 2001.

And here for a Leonid Meteor Sounds Witness Accounts.