[meteorite-list] Meteors Go Pop In The Night

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:54:01 2004
Message-ID: <200202061658.IAA12394_at_zagami.jpl.nasa.gov>

http://www.nature.com/nsu/020204/020204-3.html

Meteors go pop in the night

Recordings of sounds from shooting stars defy explanation.

Nature Science Update
Philip Ball
February 6, 2002

Scientists camping out in the Mongolian snow at minus 30 degrees C have made the
first recordings of an elusive sound: the crackle and pop of a meteor
shower[1]. Their observations defy all current explanations of what happens
when debris burns up on entry to the Earth's atmosphere.

"Basically, we are back to the drawing-board," says expedition member Dejan
Vinkovic of the University of Kentucky.

Some meteor booms are simply acoustic waves like those from supersonic
aircraft. But for centuries there have been rumours of more baffling
'electrophonic' noises occurring at the same time as meteors become visible.

Because light travels much faster than sound, there should be a delay
between the appearance of a meteor and its sound - just as thunder generally
comes seconds after a lightning flash. In fact, meteors burn up so high in
the atmosphere that this time delay ought to be a few minutes.

Yet many observers report hearing pops, buzzes and whistles while sighting a
meteor for the first time. The Italian Geminiano Montanari was the first to
note this discrepancy in 1676. Reviewing such reports in 1714, the
astronomer Edmond Halley concluded that they were merely figments of the
imagination. Until now, many other scientists thought so too.

Fireball survey

Vinkovic was not so sure. He has collected about 700 accounts of meteor
noise, and is pursuing dozens of recent claims. The phenomenon was
particularly well documented during the spectacular Leonid shower of 1833,
when people described sounds resembling "the noise of a child's popgun".
Similar reports accompanied two more recent Leonid storms.

In 2000, Vinkovic and others established the Global Electrophonic Fireball
Survey, to collate such reports worldwide.

Next Vinkovic and researchers from the Croatian Physical Society journeyed
to a "remote desert-like plain" 20 kilometres from Ulaanbaatar in Mongolia
to observe the 1998 Leonid shower. As this area is virtually free from
background noise and radio interference, the researchers measured audio and
low radio-frequency signals that the meteors triggered.

On the radio

Why radio signals? Because one of the leading candidate theories is that
electrically charged particles streaming behind meteors interact with the
Earth's magnetic field and produce radio waves, which cause the
electrophonic noises. These radio waves are broadcast to an observer at the
speed of light. They could be converted to sound by exciting vibrations in
objects at ground level.

Very-long-frequency (VLF) radio signals have occasionally been detected
alongside meteors. The Croatian team recorded two electrophonic pops but no
simultaneous VLF signals.

If radio waves do create the pops (as the team still believes), these waves
must be of lower frequency than anticipated. No explanation so far proposed
explains the measurements made in the Mongolian plains.

References

  1. Zgrablic, G. et al. Instrumental recording of electrophonic sounds from
     Leonid fireballs. Journal of Geophysical Research, (in press), (2002).
Received on Wed 06 Feb 2002 11:58:21 AM PST