THERE鈥橲 good news and bad news for sky watchers who insist that meteors can
be heard from the ground. For the first time, researchers have recorded sounds
picked up as meteors blaze through the atmosphere. But their recordings could
undermine the leading theory of why they make a noise.
For centuries, people claim to have heard pops, crackles, whooshes and
sizzles coinciding with meteor falls. One mystery is why there is no time delay,
as it should take several minutes for the sound to travel many kilometres to the
ground.
In an attempt to nail down the phenomenon, a team from the Croatian Physical
Society and the University of Kentucky set out in November 1998 to observe that
year鈥檚 rich Leonid meteor shower. In the quiet of a frozen, lifeless plain in
central Mongolia, the team set up a sensitive digital video camera to watch the
sky, along with two microphones to capture any sounds, and radio receivers to
monitor electromagnetic signals.
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In an article accepted for publication by the Journal of Geophysical
Research鈥擲pace Physics, they report that at least two deep, popping
sounds coincided with bright fireballs tearing across the sky. But the radio
equipment, which was tuned to very low frequencies (VLFs), failed to pick up any
signals.
It鈥檚 the very first time anyone has picked up these sounds under controlled
conditions, says Dejan Vinkovi鈥檆 of the University of Kentucky. 鈥淲e proved it is
辫辞蝉蝉颈产濒别.鈥�
But an explanation of the phenomenon seems as far away as ever. According to
Australian meteor researcher Colin Keay, very low frequency radio waves produced
when the turbulent ionised wakes of meteors tangle with the Earth鈥檚 magnetic
field could generate sounds on the ground by making plants or other objects
vibrate. The lack of such radio signals puts this theory in doubt.
The work has got people taking notice. 鈥淚t鈥檚 certainly a step in the right
direction,鈥� says meteor specialist Martin Beech of the University of Regina in
Canada. But he cautions that with so many fireballs in the Leonid shower, the
timing of the sounds and the meteors could have matched by chance.
Nor is he yet convinced that Keay鈥檚 theory is wrong. Unlike the slow, deeply
penetrating, large meteors Keay was describing, the Leonids are swift sand
grains that do not make it far into the atmosphere. That鈥檚 why you get no VLF
transmissions, says Beech. 鈥淵ou wouldn鈥檛 expect Keay鈥檚 theory to apply.鈥�