Newsgroups: sci.astro Path: aplcen.apl.jhu.edu!saimiri.primate.wisc.edu!sdd.hp.com!spool.mu.edu!howland.reston.ans.net!europa.eng.gtefsd.com!uunet!munnari.oz.au!metro!seagoon.newcastle.edu.au!cc.newcastle.edu.au!phcslk From: phcslk@cc.newcastle.edu.au (COLIN KEAY) Subject: Audible Meteors Message-ID: <1993Aug20.110311.1@cc.newcastle.edu.au> Lines: 100 Sender: news@seagoon.newcastle.edu.au Organization: University of Newcastle, AUSTRALIA Date: Fri, 20 Aug 1993 01:03:11 GMT Newsgroup: sci.astro Topic: The instantaneous sound of meteors Yes, It's real. But rather rare. The average person may hear one in a lifetime. But if a person spent all night every night outdoors, the number could rise to about ten. A meteor fireball needs to be brighter than about -9 mag. to give rise to sustained electrophonic sounds (as the sounds simultaneous with the sight of meteor fireballs are now known). Although brief electrophonic sounds (like "pops" or "phuts") can be generated by fainter fireballs, down to maybe -6 mag, if they explode. It is a purely physical effect not to be confused with electrophonic hearing, which is physiological. Back in the time of Edmund Halley, such meteor fireball sounds were thought to be purely psychological, a view held by some meteor scientists until quite recently. And there is also Ben Zellner's theory about the bones crackling in his neck when he looks up (must be painful to use telescopes as much as Ben does!). The mechanism of sustained (longer than a second or so) electrophonic sounds is fairly straightforward (see Keay, SCIENCE, vol 210, pp 11-15, 1980): 1. If a meteor fireball penetrates deeply enough into the atmosphere to enter the continuum flow regime, and its plasma trail becomes turbulent, it can trap, scramble, and later release the geomagnetic field, producing megawatts of ELF/VLF (audio-frequency) electro- magnetic radiation in the Earth-ionosphere cavity. 2. The electric vector of the e-m radiation can cause mundane objects (hair, pine needles, whatever) to vibrate at twice the e-m frequency, that is, act as transducers to produce audible sound. Nature's little loudspeakers. This has been verified by laboratory experiments in anechoic chambers, and published. 3. Under reasonably quiet conditions the hissing, swishing or crackling noises are audible to humans, often serving to draw attention to the fireball before it is detected visually. And that is essentially the explanation. The model satisfies Occam's Razor in its economy of principles. But for a decade it did have me a little bit apprehensive about its validity because I could find no records of the ELF/VLF signals from fireballs. OK, so electrophonic meteor fireballs are very rare and no ELF/VLF records had ever revealed them. So I didn't push my model too hard until one fine day in Japan in 1990. There Dr T Watanabe showed me a chart recording from a VLF receiver with timing matching the photometry of a fireball photographed by K Suzuki and his pupils, one of whom heard the fireball electrophonically. A colleague, Dr T Okada, obtained the signal spectrum - 500 Hz to about 8 kHz. These records blew my mind. It was my most exciting experience in 40 years work in science. I believe the Japanese feat has recently been repeated by Dr Peter Brown of Canada. I'm dying to see his results. Anyway, meteor fireballs are not the only progenitors of electrophonic sounds. Having interviewed Canadians who have heard sounds produced during extremely intense auroral displays, I'm fairly sure they are similar. Reports of people hearing "clicks" and "vits" coincident with lightning flashes are probably due to the same process, since it is well known that lightning produces strong e-m transients. Another geophysical mystery is the existence of many anecdotes about the alarm exhibited by animals immediately before an earthquake: there are records of seismically generated ELF/VLF signals, which, by propagating much faster than seismic shock-waves, may alert the animals. The transducers may be the loose hair which many animals have in and around their ears. Maybe there are other effects which can be similarly explained by my model. If the above four examples of electrophonic sound production are all verified I believe we could say that a new branch of science has been born: Geophysical Electrophonics. Who knows what it may lead to. It is clear from the sci.astro news contributions that there are lot of folk out there who don't know about the progress that has been made in this field over the past fourteen years. An invited review article "Progress in Explaining the Mysterious Sounds Produced by Very Large Meteor Fireballs" is about to appear in the Journal of Scientific Exploration (with 70 refs). I wrote another article on the subject for Scientific American, but they wrote back saying they were not interested in the topic. Any suggestions for a respectable, widely-read journal that might be interested in an up-to-date presentation of the subject? Lastly, I would appreciate any first-hand witness accounts of any of the four electrophonic phenomena mentioned above. They can be e- mailed to me, or sent by ordinary mail (especially if other supporting material is available). ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + Colin Keay ::::::: Shame physics is not a + + Physics Dept ::::: pseudo-science. Would + + Newcastle Univ :::\ | / be ahelluva lot easier + + NSW, AUSTRALIA 2308 - o - to raise public money. + + PHCSLK@cc.Newcastle.edu.au / | \ - Cynicus Maximus + ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++