Abnormal sounds produced by meteors
"... a phenomenon called electrophonics. What this is about is when you see a fireball of course the sound doesn't travel as fast as light! So normally you hear the sound two or three minutes or... ten minutes later depending on how far you are. But for many years, all the way back to ancient times, people have reported seeing fireballs and hearing sounds like crackling sounds or whistling sounds or swishing sounds; sounds like a jet airplane and so on at the same time they see the fireball. And for many years right up into the 1980's no one believed that it was a true phenomenon. They thought it was in the observer's head, it was their imagination and so on. And now there is a growing body of knowledge that suggests that the fireball emits a type of electromagnetic radiation called "Very Low Frequency radiation" in the tens of kilohertz range and what then happens is this: The electromagnetic radiation is given off by the fireball, and it is so intense that objects around the person like hair or steel objects like glasses or wire or wire-meshes of fences vibrate, actually physically vibrate, as a result of this electromagnetic field. And as they vibrate, if it is intense enough, this can actually produce the sound at a few tens of kilohertz at exactly the same frequency in the VLF as people hear normal sounds. Thus you end up with a fireball passing nearby and at the same time they see the object, they hear this whistling or crackling sound from the transduction of the VLF waves. And they're not actually hearing sound from the fireball but rather hearing sound from local objects vibrating in response to the intense VLF emission of the fireball; this is called transduction. And so people have heard these sounds for years, but without having been able to convincingly detect the accompanying VLF emission; but we have done that. In 1994 we went to France and we put up an antenna system that has a lot of gain in the VLF (plus filtering) and actually registered a bright fireball that was giving off Very Low Frequency radiation. This is strong proof that it is VLF emission which is producing the simultaneous sounds which people have been hearing for years. The theory behind the VLF emission is due to Colin Keay at the University of Newcastle in Australia; it is not what I consider a complete theory, more heuristic in nature, (even Colin would admit this) but it is the best we have at the moment. It goes something like this: the plasma train behind the fireball entrains the Earth's magnetic field and "distorts" this field and as the field relaxes after fireball passage, like a few milliseconds afterwards, the field unwinds and as it unwinds it emits the VLF waves. But the precise mechanism is not really well understood. More work needs to be done in this area of investigation. Electrophonic sounds are covered in a good article by Colin Keay in 1991 in "Meteoritics" where he talks all about the specific details: entry angles and what you need to produce it such as the turbulence that you need in a big fireball to produce VLF emission. So all meteorite falls don't produce it but BIG fireballs do." Our article on the detection in France appeared in Earth, Moon and Planets, v.68, (1995), pp. 181-188.
PETER BROWN (1994)
Comments to:Denis Pagé: Astrolv@cam.org