Electrophonic Sounds from Bolides
On 19th March 1719 a very bright meteor, usually known as a fireball or bolide, was seen across England and Scotland. A broadside was issued about this and other "Strange and Wonderfull Apparitions", reporting that in the air over Glasgow the bolide was seen as "a great and surpriseing Light of several Coulers, and immediatly thereafter, there appeared as it were a large Flaming Sword to the amasement of all Spectators, and immediatly vanished".
Edmond Halley, who would replace John Flamsteed as Astronomer Royal the next year, collected a series of accounts of this bolide and published them in the Philosophical Transactions of the Royal Society. The sightings enabled him to derive the bolide's course by triangulation, and as Halley concluded, "they abundantly evince the height thereof to have exceeded sixty English miles." Halley also wrote about the effects of fancy that people under the bolide's path reported, including "hearing it hiss as it went along", feeling "the Warmth of its Beams", and such like, which were ridiculous.
At that time the origin of meteors was unknown. The Apparitions broadside says that people thought that the meteor "touched the Earth, and Came down with a great Noise, which made many of the beholders run into their Houses". People continued to report sounds associated with meteors, and after another remarkable bolide in 1783 the physician Sir Charles Blagden published a study into the subsequent hissing reports, linking the noises with electricity but noting it as a point to be cleared up by future observers.
The debate carried on into the twentieth century. In 1933 noted meteor researcher C.C. Wylie dismissed noises accompanying meteors as a purely psychological phenomenon, claiming that people who know the mechanisms behind meteors never report associated noises. In 1937 Stanley Smith Stevens coined the term "electrophonic noise" for sound heard by electrical stimulation, which Peter Dravert adapted in 1940 as "electrophonic bolides" for meteors which cause electrophonic noise.
The first major turn towards the acceptance of electrophonic bolides came with C.S.L. Keay's 1980 theory of Geophysical Electrophonics whereby radio waves emitted by meteors could be transduced into audible noise by objects on the ground. It was another remarkable bolide that had prompted his studies, one which passed over Sydney, Australia, on 7th April 1978. First published in the journal Science, Keay's theory was such that meteors can emit radio waves in the VLF range, which is an equivalent frequency to audible sound. It also proposed a mechanism by which these radio waves could be produced, called the "magnetic spaghetti" effect, though it wasn't until 1990 that they were first detected by a Japanese team at Nagoya University.
The first recorded evidence of aural electrophonic noise associated with a meteor came in November 1998 when Dejan Vinkovic, Slaven Garaj, and others recorded several such noises on the Mongolian steppes a dozen miles south of Ulan Bator. The team also took video footage of the events, and their findings were subsequently published in the Journal of Geophysical Research. Vinkovic went on to coordinate the GEFS or Global Electrophonic Fireball Survey, which collects reports of electrophonic noise and conducts further research into the phenomenon.
The question is still not settled. Keay notes that there may be other natural sources of electrophonic noise, such as very bright auroræ, nearby lightning, and earthquakes. It would also seem that even if electrophonic noises are a genuine yet rare phenomenon, the psychological theory may still account for a huge portion of the reports. Nor is Keay's theory accepted as a universal explanation yet. For example Andrei Ol'khovatov, an electrophonic noise researcher, writes of "how little we know still", believing that the level of VLF generated by some audible meteors is insufficient for the perceived effect, and that people would otherwise hear man-made VLF transmitters.
In 1999, astronomer Martin Beech discerned and wrote a paper on a separate category of the fizzing, hissing, swooshing noises reported from the sky: he theorises that there are shorter more concentrated electrophonic pops called bursters. So although Keay has said he believes he's "solved the problem and started a new science", the situation looks set to get increasingly more complex as substantial reliable data is accumulated. Twenty-five years into the fledgling field of electrophonogy, there is no doubt plenty more to be discovered.
Cite: Palmer, S.B. (2005). "Electrophonic Sounds from Bolides", in: What Planet is This?
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