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AUDIBLE METEORS

  • Subject: AUDIBLE METEORS

The ability of a meteor or fireball to produce sound has never been seriously doubted. When taken logically, any solid body entering the atmosphere at hypervelocity is almost bound to make a noise because of its collision with air molecules and the build-up of a bow shock wave in much the same way that an aircraft will produce a sonic boom or a whip will “crack” – but the situation is not as simple as it may at first seem.

Fireballs – officially, meteors brighter than mag -4.0 – are often associated with sound and, indeed, have been for centuries. The great fireball of 1676 March 21, for example, was accompanied by “cannon-like” and “rattling” sounds together with a “hissing” noise. If a fireball plunges low into the atmosphere and takes several seconds to cross the observer’s sky then conditions are favorable to both see and hear the fireball simultaneously. The sounds may be detected in the wrong order because of the location of the observer, and the way sound is refracted and reflected by both the atmosphere and the terrain, and some observers may not be able to hear the event at all for the same reasons. But it was for many years generally accepted that for to produce simultaneous sound and light phenomena the fireball must enter the atmosphere at a low angle, have a diameter in excess of 10 cm, plunge deep into the air and, therefore, be of the robust asteroidal-, rather than the fragile cometary-type. Magnitudes under such circumstances are brighter than -8.

If all audible fireballs fulfilled these conditions there would be few problems. But many reports suggest high, swift objects, not as bright as -8 and some considerably fainter and classed as meteors. On numerous occasions it was the sound that caused the observer to look up. An obstacle immediately becomes clear: light from a meteor at an altitude of, say, 80 km (50 miles), would take only 0.0003 second to reach the ground-based observer whereas sound waves would take in excess of four minutes! Simultaneous observations of these two phenomena are not, apparently, possible. The solution to this problem may lie in the “hissing” sound that is often reported.

Havey H. Nininger in his classic book¬†Out of the Sky¬†recalls that in 1934 E.R.Weaver of the US Bureau of Standards suggested that electromagnetic waves – or “ether waves” as Weaver called them – may be produced by meteors and fireballs. These would then be transformed into audible sound by objects such as buildings and cars. Though ignored at the time, the proposal later received much attention, most notably by Colin S.L. Keay of the University of Newcastle in New South Wales, Australia.

Keay has investigated reports of “electrophonic” sound emitted by a number of bright fireballs. In 1980 he showed that the plasma trail of a large fireball could generate Extra Low and Very Low Frequency (ELF/VLF) radio emissions in the range 1 to 10 kHz. His theory was eventually verified in 1988 by three groups of Japanese observers who, working together, managed to obtain simultaneous photographic and radio observations of a bright fireball together with an electrophonic sound report of the event.

While bright fireballs may be capable of producing electrophonic sound there are still problems with the fainter meteors. It seems unlikely that such a small object could produce enough energy to generate ELF/VLF radio emissions, but reports do exist and need to be satisfactorily explained.

The main objection to audible meteors is that humans cannot normally hear electrophonic sound. But various theories have been proposed in which sharp objects – such as aerials, wires and even blades of grass – could convert any ELF/VLF emissions present into audible sound. In effect, the meteor discharges its sound to the ground in the same way that a charged cloud will discharge lightning – a common enough occurrence. Another, though less likely explanation came from A.Paine who speculated that sound may be quite common at certain wavelengths that are outside the normal range of the human ear. He went on to propose “bat-like” sensitivity under certain conditions.

It is interesting to note that the Inuit have a belief that aurorae – which are caused by solar atomic particles interacting with the Earth’s magnetic field and atmosphere – can also be heard, despite occurring at altitudes of 100 – 700 km where the air is too thin for the transmission of sound. In Inuit mythology, aurorae are to souls of the dead who, if you listen very carefully, will whisper to you. Clearly, the same processes that make meteors audible may also account for whispering aurorae.

Copyright 1996 Philip M. Bagnall

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