LISTENING TO LEONIDS
From NASA Science News, 26 November 2001
November 26, 2001: All at once there was a eye-squinting flash of light and a strange crackling noise. Puzzled sky watchers looked at one another ... and confessed: "Yes, I heard it, too."
Hearing meteors? It could happen -- and indeed it did, plenty of times during this month's Leonid meteor storm.
"I am sure I could hear several of the meteors," recalled Karen Newcombe, a Leonid watcher from San Francisco -- one of many who reported meteor sounds to Science@NASA on Nov. 18th. "Several times when a Leonid with a persistent debris train flew directly overhead, I heard a faint fizzing noise [instantly]." There was no delay between the sight and the sound.
"How is that possible when the meteor was so many miles above my head?" she wondered.
The same question has bedeviled some of history's greatest scientists. For example, in 1719 astronomer Edmund Halley collected accounts of a widely-observed fireball over England. Many witnesses, wrote Halley, "[heard] it hiss as it went along, as if it had been very near at hand." Yet his own research proved the meteor was at least "60 English miles" high. Sound takes about five minutes to travel such a distance, while light can do it in a fraction of a millisecond. Halley could think of no way for sky watchers to simultaneously hear and see the meteor.
Baffled, he finally dismissed the reports as "pure fantasy" -- a view that held sway for centuries.
Yet just last weekend scores of people little inclined to fantasy heard the Leonids. The sounds weren't rumbling sonic booms or the loud crack of a distant explosion arriving long after the meteor's flash had come and gone. Rather, these were exotic, delicate noises, heard while the meteor was in full view. Scientists call them "electrophonic meteor sounds."
Meteor listeners have long been reluctant to report their experiences -- a result of Halley-esque skepticism. But hearing a meteor doesn't mean you're crazy. Indeed, modern researchers are increasingly convinced that the electrophonic sounds are real.
Colin Keay, a physicist at the University of Newcastle in Australia, not only believes in electrophonic meteors, he's also figured out what causes them. According to Keay, glowing meteor trails give off not only visible light, but also very low frequency (VLF) radio signals. Such radio waves, which oscillate at audio frequencies between a few kHz and 30 kHz, travel to the ground at the speed of light -- solving the vexing problem of simultaneity.
Of course, human ears can't directly sense radio signals. If Keay is right, something on the ground -- a "transducer" -- must be converting radio waves into sound waves. In laboratory tests, Keay finds that suitable transducers are surprisingly common. Simple materials like aluminum foil, thin wires, pine needles -- even dry or frizzy hair -- can intercept and respond to a VLF field.
Here's how it works: Radio waves induce currents in electrical conductors. "Strong, low-frequency currents can literally shake ordinary objects," explains Dennis Gallagher, a space physicist at the NASA Marshall Space Flight Center. "When things shake, they launch vibrations into the air, which is what we hear."
Higher-frequency radio waves, like TV transmissions or FM radio broadcasts, oscillate much too fast (hundreds of millions of times per second) to substantially shake conductors. Even if they did, we couldn't hear the resulting MHz-frequency sound waves, which are far above the frequency range of a human ear.
But VLF waves can do the job. Keay discovered that even a pair of glasses could be made to vibrate slightly. Perhaps that explains the experience of Erich in Troy, New York: "When I was out [viewing the Leonids on Nov. 18th]," he reported, "I had my head back on the ground and heard a sizzling sound. My head was close to grass and leaves and I wear wire frame glasses as well. The sound was definitely simultaneous with the observation of a rather large streak."
But how do meteors generate VLF radio signals?
"It was a knotty problem," recalls Keay. When he began his work on electrophonic meteors in the 1970's, physicists had no idea how VLF waves might emerge from a meteor's ionized trail. "Some new mechanism had to be found."
"[I was inspired by] Fred Hoyle's sunspot theory in which energy is trapped in twisted magnetic fields," he says. Magnetic fields that suddenly untangle -- snapping back like stretched rubber bands -- can trigger solar flares: violent blasts of electromagnetic radiation and energetic particles.
Perhaps, thought Keay, magnetic fields in the glowing trail of a meteor might do something similar.... only on a much less energetic scale.
Left: Nuclear explosions release VLF radio waves, which are reportedly "heard" by soldiers in nearby bunkers. This VLF spectrum from such an explosion peaks at 12 kHz. [more]
When a meteoroid races through Earth's atmosphere, the air around it becomes a plasma -- that is, a cloud of ionized gas. Plasmas have a curious property: Lines of magnetic force that permeate them become trapped. Wherever the plasma goes, the magnetic field follows. If a magnetized plasma becomes turbulent, the magnetic fields inside it become twisted and tangled as well.
The plasma tails of certain meteors do become turbulent, says Keay, and they are permeated by a magnetic field: Earth's. "The plasma is swirling so fast that the magnetic field can be scrambled up like spaghetti." And therein lies a source of energy for VLF waves.
Keay continues: Eventually the plasma cools. Electrons return to the atoms from which they were earlier ripped, and the gas becomes neutral again. Magnetic fields find themselves suddenly free to straighten out. That abrupt rebound is what produces the low frequency radiation.
It's a plausible theory, says Gallagher: "It's easy to understand and is supported by Keay's laboratory work."
Gallagher added, "I think what makes this exciting is that we're talking about a phenomenon that has been experienced by people for perhaps thousands of years. Even in modern times folks who reported hearing such sounds were ridiculed. It was only about 25 years ago that Keay was able to do the research and legitimize the experiences of all those generations of people."
"It shows there are still wonders in nature yet to be recognized and understood. We should take this experience with meteors as reason to open our minds to what may yet be learned."
Last Revised: 2001 December 3rd
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