Electrophonic sounds:
History

A Mystery of Meteors in the 18^th Century

At the beginning of 18^th century, sir Edmund Halley dismissed the electrophonic sounds by interpreting them as imagination of observers [14,15]. In his letter to the Royal Society of London in 1714 [14], he describes several bright meteors visible over Europe in previous years and derives their heights from witness reports at widely separated locations. He obtained heights of several tens of miles, which suggests that a sound from a meteor can not be heard simultaneously with the meteor's appearance. He also quotes Italian astronomer Geminian Montanari from Bolonia, who derived similar result for a bright meteor in 1676 over Italy:

"..famous Meteor which was seen to pass over Italy, on the 21st. of March O.S.Anno 1676, ... its perpendicular Altitude was at least 38 Miles: That in all Places near this Course, it was heard to make a hissing Noise as it passed, ... it was heard to give a very great Blow, (Tuono di maggior rumore di gross Cannonata), immediately after which, another sort of Sound was heard, like the rattling of a great Cart running over Stones, which continued about the time of a Credo. ... it cannot be wonder'd that so great a Body moving with such an incredible Velocity thro' the Air, tho' so much rarefied as it is in its upper Regions, should occasion so great a hissing Noise, as be heard as such a Distance as this was."

A few years later, Halley had the opportunity to apply the same analysis on a bright meteor over England in March of 1719 [15]. The large height and velocity of the meteor completely excluded the reality of "hissing" sounds which accompanied the meteor:

"Of several Accidents that were reported to have attended its Passage, many were the Effect of Fancy, such as the hearing it hiss as it went along, as if it had been very near at hand: Others imagin'd they felt the Warmth of its Beams, and some there were that thought, at least wrote, that they were scalded by it."

At that time, the origin of meteors was unknown , and Aristotel's view of meteors as atmospheric phenomena was generally accepted.  Intrigued by reports of instantaneous "hissing" sounds from a large meteor on August 18, 1783, Sir Charles Blagden is presenting his arguments for "electric origin of meteors"[16] in 1784. Since the nature of electricity was still not understood, he describes meteors as ``electricity fluid". In this picture, there is a possibility for this fluid to travel faster then sound and produce "hissing" sounds around an observer:

"Various electrical phenomena have been seen attending meteors. Lambent flames are described as settling upon men, horses, and other objects, and sparks coming from them, or the whole meteor itself, it is said, have damaged ships, houses, etc. in the manner of lightning. These facts, I must own, are but obscurely related, yet still they do not seem to be destitute of foundation. If there be realy any hissing noise heard while meteors are passing, it seems explicable on no other supposition then that of streams of electric matter issuing from them, and reaching the earth with a velocity equal to that of the meteor, namely, in two or three seconds. Accordingly, in one of our late meteors, the hissing was compared to that of electricity issuing from a conductor. The sparks flying off so perpetually from the body of fire-balls, may possibly have some connection with these streams. In the same manner the sound of explosions may perhaps be brought to us quicker, then if it were propageted through the whole distance by air alone."

Although his description of meteors was incorrect, he correctly assumed that the instantaneous sounds are of electrical origin. It is interesting that he compares these sounds with the aurora sounds and derives the height of aurora to be even higher than meteors. Thus we can say that this is also the first attempt of explaining both type of sounds with the same theory.

Understanding of the Meteors in the 19^th Century

A real breakthrough in understanding the origin of meteors happened after the great Leonid meteor storm in 1833. From an extensive collection of witness reports, Denison Olmsted derived heights and trajectories of the meteors and rightly concluded that meteors are low density objects from space that burn-out high in the atmosphere [18]. He also gave a very detailed summary of reports from many witnesses of instantaneous sounds, some of whom were scientifically trained, and discusses the results of their electrical and magnetic experiments [17]. These crude experiments did not reveal any certain change in the magnetic field of the Earth, but many electrical anomalies were positively recorded.

"...No change was noticeable in the magnetic dip, variation or intensity. Gold leaf electrometers were excited by a touch, Bennett's, placed on the prime conductor, with the cushion insulated, rose on a slight motion of the machine. The pendulum of <De Luc's dry pile> was accelerated."

"Mr.Palmer, considering the phenomenon as electrical, immediately made some experiments to ascertain the electrical state of the atmosphere. His silk pocket-handkerchief held at one end in the right hand and drawn swiftly through his left hand, emitted a very unusual number of electric sparks. On turning a small machine, he found the sparks which were usually short and feeble, much longer and more intense then he had ever seen before. On presenting silk threads to an iron bar that stood on the ground leaning against the house, they were strongly attracted towards the iron. He next examined his compas; found the needle more unsteady then ordinary, but on adjusting it to the meridian as nearly as he could, judged the declination of the needle to be the same as usual."

"He heard at different times a number of slight explosions, which usually resembled the noise of a child's pop-gun, and was not unlike that of a fire-rocket. They were followed by a particular odor observed by all the company, (four men) which one compared to the smell of sulphur, and another to that of onions. The meteors which afforded these sounds, all passed along in a north-west direction.... They passed below the tops of the trees at the distance of twenty five rods from the place where he stood, giving a "pop" just before they reached the trees. One appeared to strike the barn, and gave a louder pop then any of the others. An auroral light resembling day-break, appeared constantly in the east from the time when his observations commenced."

Since the Olmsted's calculations excluded the electric origin of meteors, these instantaneous, (anomalous), sounds were dismissed also and disappeared from the scientific publications later on, even though Olmsted did not exclude their existence:

"Meteors which were distinguished for their brightness and apparent magnitude, and which would therefore be expected to afford sounds, might stil be too distant for such sounds to be audible; or might be in a region of the atmosphere where the air is too much rarefied for the purpose of sound.
...
The question whether any sounds proceeded from the meteor, must rest, for its decision, on the circumstances of the case; such as the peculiarity of the sounds, their nature as described by different observers, etc."

Revival of the Anomalous Meteor Sounds at the end of the 20^th Century

The term electrophonic sound was introduced by S.S. Stevens in 1937 for sensation of a sound caused by electrical current through the head[19]. A few years later,  in 1940, the name electrophonic bolide was introduced by P. Dravert as a description of a bright meteor accompanied by, (anomalous), sound [20]. Although the existence of electrophonic sounds from meteors was generally ignored or dismissed as the imagination of observers, there were attempts by some authors to rise the problem of the origin of these sounds. For example, in 1917, J.A. Udden [35] proposed dissipation of the ether waves into sound waves "... on objects attached to the earth, such as plants or artificial structures."

The idea that radio, (ether), waves can be produced by meteors was checked for the first time experimentally by A.G. Kalashnikov in 1949 [21]. He reported the detection of ULF pulses, (at frequency of 1 Hz), coincident with meteors, but this remained  a controversial result as other authors performed similar experiments during the 1950's [22,23,24,25].

During that time, the first catalogs of electrophonic sounds appeared [26,27,28]. The most influential was the RAND Corporation report made by M.Romig and D.Lamar [28], which was widely accepted in the West as the last word on this subject.  In Russia some work continued, mostly theoretical, usually inspired by the mystery of Tuguska explosion in 1908.

The situation finally started to change in 1980, when C.S.L. Keay presents the theory that meteors can emit the ELF/VLF radio waves [2]. According to his laboratory experiments, these electromagnetic waves can be transformed into sound on ordinary objects around the observer. The ELF/VLF radiation [4] would be produced by trapping and tangling Earth's magnetic field in the turbulent plasma tail of an ablating meteoroid. Soon after, using this theory, Bronshten gave theoretical prediction that a meteor with minimal brightness of -12^m, (about equal to the brightness of a full Moon) is necessary for production of electrophonic sounds [3].

Since then the interest for electrophonic sounds has been rising. New catalogs were introduced [29,30,31], and extensive statistical analysis of catalogs of electrophonic sounds was presented by Kaznev in 1994 [10]. His results show a variety of electrophonic sound properties, and clearly show that the lover brightness limit of -12^m is not realistic. Keay refined his theory in 1992 and predicted that VLF can be generated in the moment of the explosive disintegration of a bright bolide, and also a little bit earlier [8]. Nevertheless, this still requires slow meteors ablating at low altitudes, while some electrophonic bolides are clearly very fast and ablating at high altitudes, (Leonids, for example).

In 1991, Bronshten gave a very valuable overview of the vital facts and existing theories, (mostly Russian), about electrophonic sounds [32] and/or mechanisms for VLF emission from meteors. His conclusion is that all proposed theories are too simplified to be able to give the right answers and future detailed numerical modeling of the kinetic plasma theory equations may give more precise results.
Recently, a new theory of Electrophonic Bursters was presented by M. Beech and L. Foschini [9] but it also has difficulties like the other theories.

In the meantime, a few reports of detected ELF/VLF radiation from meteors have been presented [6,7][33][34] but a much larger sample is needed for more definite conclusions about the physics of these radio emissions from meteors. Finally, during the 1998 Leonid meteor shower, the first instrumental detection of electrophonic sound was obtained [34].

Nevertheless, the phenomenon of electrophonic sounds from meteors is still mysterious. It is not clear how the meteors can produce such a strong ELF/VLF emission and what kind of observational conditions are necessary for detection of these sounds. Thus the witness reports are very valuable source of information.
 

Read about more recent developments in the ILWCRO press release regarding the first instrumental detection of electrophonic meteors: fizika.org/ilwcro/results

 read more: Electrophonic sounds - introduction
 read more: Electrophonic sounds - existing catalogs

Other sources of information about the electrophonic sound:
Colin Keay's Home Page
Phil Bagnall's Home Page