Electrophonic Meteors

Abstract

This article was originally written as “Hiss, clicks and pops, The enigmatic sounds of meteors" by J.A.Finnegan. Armagh Observatory.

The perceived improbability of `meteor sounds` has facilitated scientific incredulity, allowing it to remain on the margins of scientific interest and research. This is unjustified, since these audibly perceived electric field effects indicate complex, inconsistent and still unresolved electric-magnetic coupling and charge dynamics; interacting between the meteor; the ionosphere; mesosphere; stratosphere and the surface of the earth.

Foreword

"Famous Meteor which was seen to pass over Italy, on the 21st of March 1676 Anno Domini... 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.” Italian Astronomer Geminian Montanari.

Meteor mythology dates back at least three thousand years. Meteor comes from the Greek word `meteoros`, meaning `suspended in the air`. Aristotle, the most influential of the ancient Greek philosophers, erroneously grouped meteors, comets, aurorae and the Milky Way into the same category as clouds, wind, lightning, thunder and rainbows. The terrestrial explanation of meteors expounded in Aristotle`s 340 BC treatise Meteorology persisted, until Prof Denison Olmsted and Alexander C. Twining`s pioneering analysis and naming of the great Leonid meteor shower of 13 November 1833 established modern meteor science. Olmsted also reported noises from the meteors, “The sounds supposed to been heard by a few observers are represented either as a hissing noise, like the rushing of a skyrocket, or as slight explosions, like the bursting of the same bodies. These comparisons occur too uniformly and in too many instances to permit us to suppose that they are either imaginary or derived from extraneous sources”

“For a length of time the fact was altogether denied by the highest authorities in science, and the strongest evidence resisted, when adduced in support of an event which was conceived repugnant to the laws of nature. Philosophic incredulity, though generally useful, was carried too far, and proved injurious to the progress of science; for while doubts were entertained concerning the reality of stony showers, the sources of the aeroliths and their nature was not, of course, likely to be made objects of investigation.” Dr James Apjohn, Professor of Chemistry. 23 May 1836.

Surprisingly, even in recent times scientific incredulity and the `doubts entertained` concerning the reality of `meteor sounds` also made it unlikely that they would be made objects of investigation. In 2000, Dr. Donald Yeomans, presently manager of NASA`s Near Earth Object Program office, commented, "It's coming out of the realm of myth and into the realm of possibility, but there are some serious doubters."

Introduction

“The stars rushed across the heavens...like grasshoppers in a field. This continued until dawn. The inhabitants cried out with terror and fervently implored the mercy of the Most High.” Arab account of the 1202 Leonid meteors.

On the evening of the 16 August 1783, just six years before the foundation of Armagh Observatory, a great ball of flaming light appeared over The Shetland Islands and in only half a minute passed across Britain, continuing onwards to France and northern Italy. After about ten minutes a rumbling noise, "as it were of thunder at a great distance", was heard. Sir Charles Blagdon, secretary of the Royal Society of London, collected the reports of this startling event and prepared a paper for the society. He was puzzled by reports from some observers that a kind of hissing sound “attended the meteor” as it passed across the sky. He was sceptical that sound from an object 50 miles high “should be conveyed in an instance” but added, “testimony in support of it is, however, so considerable, on the occasion of this as well as former meteors, that I cannot venture to reject it, but would leave it as a point to be cleared up by future observers.”

Future observers reported similar effects to Armagh Observatory on the 3 March 2012, 21 September 2012 and 15 January 2014. Before correct identification, the 15 January 2014 meteor, with its delayed sonic boom, initiated an air-sea rescue alert over the Irish Sea.

This paper endeavours to better explain these effects and presents some illustrating reports, including a summary of similar and additional phenomena observed during the 15 February 2013 asteroid fragment disintegration above the Russian district of Chelyabinsk. Links to the Armagh Observatory Allsky meteor cameras, electrophonic meteor research and full construction plans for an extremely low frequency (ELF) detector, are also included.

Meteor colours

“It was the colour of a green traffic light - amazing! Seen shooting stars before, but never anything like this.” Fireball report to Armagh Observatory.

When a meteoroid enters the Earth's atmosphere it collides with air molecules, rapidly creating very high temperatures which incinerates its outer surface, creating a luminous plasma envelope of charged particles. The meteor`s colour depends on its altitude. At higher altitudes meteor metal atom emissions dominate; at lower altitudes the shock front air plasma emissions can become dominant.

The light emitted by atmospheric nitrogen molecules (N2) and oxygen atoms (O) appears red, and light emitted from the metal atoms composing the meteoroid can appear blue, green or yellow. These metal atoms emit light by the same process as an electric discharge lamp. For example, Sodium (Na) atoms give an orange-yellow light, Iron (Fe) atoms a yellow light, Magnesium (Mg) a blue-green light, ionised Calcium (Ca+) atoms cause a violet hue, Nickel (Ni) emits a green light, and silicates a red light. With fainter objects, slow meteors often appear red or orange, fast meteors frequently have a blue colour.

The velocity of the meteor also plays an important role, since greater kinetic energy will intensify certain colours compared to others. The collision velocities of meteoroids can range from as low as 15 km/s up to 72 km/s. Distinct colours can be associated with particular meteor showers, since these meteoroids will commonly be made of similar material and enter our atmosphere at similar velocities.

Here there is a link with the colours of the aurora borealis, or `Northern Lights'. These comparatively rare, rapidly changing patterns of light in the sky are caused by solar flares or coronal mass ejections, which disturb the Earth's magnetic field and produce high-energy particles that penetrate deep into the Earth's mesosphere, where their interactions with the different types of molecule in the Earth's upper atmosphere produce the characteristic green, pink and sometimes red colours associated with aurorae.

Although scientific knowledge of the aurora explains the phenomenon, myths still persist among the inhabitants of the Arctic. One says that you can communicate with the aurora by whistling. The Inuit believe that whistling to the aurora will affect its motion and that in response to a whistle you will hear a “rustling and sighing sound”, known as “the whisper of souls of the dead”.

Meteor related sounds

Armagh Observatory receives reports of sounds directly associated with some bright fireballs. They comprise two types: delayed sonic booms; and simultaneous `electrophonic' sounds. The investigation of meteor electrophonic effects at extremely low frequencies up to 24 kHz is an ongoing line of enquiry and observation at the Observatory.

Sonic Booms

“We had never seen anything like it and we both commented if it would eventually crash. About two minutes later we heard a boom and were astonished as to what we had just witnessed.” Fireball report to Armagh Observatory.

Meteors with magnitudes exceeding −3 are called fireballs. Fireballs brighter than −14 are usually called bolides and when brighter than −17, super bolides. If a very bright fireball, usually greater than magnitude -8, penetrates the stratosphere to below an altitude of about 40km and explodes, it is possible that time-delayed `sonic booms' may be heard on the ground. This is more likely if the disintegration occurs at an altitude angle of about 45 degrees to the observer and less likely if it occurs overhead or near the horizon. As sound travels at ~330 m/s (~20km per minute at sea level), it will generally be several minutes after the visual event before a sonic boom can be heard. After more than 5 minutes, only inaudible infrasound affects can propagate to the ground from bolides at ~70+ km.

The world`s first confirmed recording of a fireball sonic boom occurred at 9:22pm on 25 April 1969 at Bangor, N.Ireland, when the fall of the Bovedy meteorite was recorded by an amateur ornithologist recording bird song in her garden. Fireballs which produce meteorites are often associated with sounds, both sonic and electrophonic.

Electrophonic sounds

“It was very bright and very orange and I heard bangs and crackling, which is what alerted me to it in the first place.”

“It disappeared about 10 seconds later in the west. I heard pops almost like fireworks sounds coming from the same direction I first saw the orange lights.” Fireball Reports to Armagh Observatory.

`Electrophonic sound' is audibly perceived effects attributable to electricity and magnetism. The term was introduced in 1937 by Prof. Stanley Smith Stevens, to describe the sensation of sound caused by an oscillating electric current flowing through the head. One cause was then thought to be electro-thermal elastic expansion and contraction of portions of the auditory system. In 1940 Prof. Peter Dravert of Omsk University used the term `electrophonic fireball' to describe a bright meteor simultaneously accompanied by non time delayed, therefore anomalous, `crackling' sounds.

“The witness recalls hearing a cracking sound during the whole light of the meteor, in duration of 5 seconds. The sound did not have direction. It was in the middle of his head like a sound in a stereo headphone." Eisse Pieter Bus. Visual and photographic meteor observations, 01:12:47 UT, 23 April 1972 at The Observatory of the University of Groningen at Roden. Global Electrophonic Fireball Survey (GEFS) catalogue.

Electrophonic fireballs might simultaneously cause an intense, fluctuating, electric field effect around an observer. An electric field is generated by electric charge, as well as by a time-varying magnetic field. The fluctuation rate of this intense field could coincide with the ~10Hz to ~15kHz adult audio hearing range and might be demodulated and perceived under acoustically quiet circumstances; possibly by a direct electro-static response of portions of the auditory system causing the sensation of sound in the head with no corresponding wave motion of the air.

“The surrounding air is often mentioned as the direction or source of the meteor sound.” Global Electrophonic Fireball Survey (GEFS) catalogue.

"The [fizzing] sound was clearly very distant, from above, only with meteors that had sparkly persistent tails, and only when they were nearly directly overhead. Well, some people hear auroras, so I hope I'm not going crazy!” Karen Newcombe, San Francisco.

It is also possible that an associated increase in the natural vertical electric field gradient at the observer's location could temporarily stress, polarise and disassociate atmospheric Oxygen molecules. The resulting electric field reactions [O2 → 2O] with the free radicals of oxygen recombining to create ozone [O3] with nitrogen oxides as by products, could also produce incoherent, wideband audio frequency air pressure modulations; causing hissing sounds to simultaneously appear `out of thin air' up to several hundred feet above an observer.

Photoelectric production of Ozone, with nitrogen oxides as by products, verified during the Chelyabinsk super bolide event (see sections 4.2 and 4.3) could also cause audio frequency air pressure modulations, with associated noises, to occur around an observer during phases of an exceptionally large meteoroid`s disintegration and before the arrival of acoustically propagated sound.

“Proposed dissipation of the meteor `ether waves` into sound waves on objects attached to the earth, such as plants or artificial structures.” Prof. J.A. Udden. 1917. “There are also many individuals who report that the sounds seemed to have originated from surrounding objects rather than the fireball.” Lamar and Romig. 1964.

Experiments by Prof Colin Keay at The University of Western Ontario, Canada, and at the University of Newcastle, New South Wales, have shown that a localised passive transducer effect can cause a field conversion process; creating around the observer coinciding airborne sound waves from suitably stimulated `emitters'.

An electric field which fluctuates with time, such as due to the motion of charged particles producing the field, affects the local magnetic field. Magnetically stimulated into faint audible vibration, sound emitters could include nearby metallic objects such as spectacle frames and electrostatically stimulated non-conductive materials such as hair and vegetation. In tests, three volunteers with fine hair were able to hear faint sound vibrations from their hair, passively transducing a 4 kHz oscillating electric field with a lower limit of 160 V peak to peak per metre. Most volunteers required Kilovolt per metre electric field levels before [cranial?] sound detection occurred. “Under [intense] electric fields of 400 kV peak to peak per metre varying at 0.5, 1, 2 and 4 kHz, samples including aluminium cooking foil and typing paper vibrated, producing [low] sound levels in the 40 to 60 dB range” Stimulating the self resonant frequencies of the transducing emitters will increase their radiated acoustic energy. Magnetic fields of up to 0.1mTesla (~1.5 times the geomagnetic field) varying at audio frequencies were not audibly transduced.

Between a fireball and an observer, within the Earth's intervening magnetic, electric and charged particle fields, other electromagnetic processes can occur. Caused by turbulent head and trailing plasma interaction with the Geo-magnetic field, meteor-stimulated extremely low frequency (ELF) `radio wave' generation may be one of these. Sustained radio frequency radiation is associated with turbulence in the continuum flow regime as the bolide ablates, decelerates and descends in altitude. Continuum flow usually occurs within a second of maximum brightness.

A bolide lasting less than 3 seconds is unlikely to enter this continuum flow regime. Bolides which produce sustained electrophonic sounds remain in the continuum flow regime to lower altitudes, due to their smaller inclinations and correspondingly longer trajectories.

A radio wave propagates because of the self sustaining energy exchange between the oscillating electric and magnetic fields of which it is composed, even when its source ceases. If of sufficient intensity, and the frequency of oscillation coincided with the ~10Hz to ~15kHz frequency range of adult hearing, the passive direct conversion processes described previously might cause the sensation of simultaneous `sound` to be experienced by an observer.

Electromagnetic ELF phenomena attributable to meteors are an ongoing field of research at Armagh Observatory. Detectors covering from ~5 Hz to 24 kHz have been constructed and utilised simultaneously with our meteor cameras. The ELF recordings are subject to detailed spectrum analysis. Since the Earth's geo-electric-magnetic ELF background is intensely active involving natural phenomena such as Sferics, Tweeks, Whistlers and Sprites, and also strong man-made emissions, distinct effects or disturbances directly attributable to individual fireballs are elusive.

Further information examining these topics can be found at:

• https://www.dropbox.com/sh/ep2f86udmy0q3s8/GQqBYxslY6.
• http://vlf.stanford.edu/research/transient-luminous-events
• http://www.heliotown.com/Sprites_March_30_2012_0502UT.html
• http://star.arm.ac.uk/movies/2012/AllSky/
• http://www.allskycam.co.uk
• http://www.gefsproject.org/

Full instructions to build an ELF detector, can be found at:

https://www.dropbox.com/sh/ep2f86udmy0q3s8/AACQcdebeA9qt0aMJ7gtgE3Ia/02a%20ArmObs%20ELF%20Rx%20Construction%20manual.doc

Meteor electrophonic sound reports

“By an optimistic prediction, a person who could spend every night outdoors may expect to hear an electrophonic meteor once in a lifetime.” Prof Colin Keay.

Reported fireball sounds include sustained hissing, swishing and sizzling. Also clicks and pops, believed to be associated with fragmentation or pre-luminous phases. Review of the Armagh Observatory fireball records and previous international surveys suggests that many of these electrophonic sound reports originate within a ~250km wide fan shaped area ahead of and below some fireballs.

Historic reports

817. Strange noises heard simultaneously with the passage of a bright fireball have a long history. Some are hidden in very old Sumerian, Arab and Chinese chronicles. For example, a Chinese record from the year 817 describes a meteor ``which made a noise like a flock of cranes in flight."

The following is a link to a similar meteor, recorded by an acoustic microphone in Mongolia during the 1998 Leonids.

1026. ``A loud sound and intense light.” An Arab record of a meteor shower, probably the Perseids.

7 November 1472. “A great stone fell out of the sky...accompanied by crashing thunder and lightning”. “Gruesome thunderbolt and long lasting roar". The Ensisheim meteorite. France.

21 March 1676. "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 at such a distance as this was." Italian Astronomer Geminian Montanari

19 March 1719. Electrophonic effects of a large bolide seen over England. Sir Edmund Halley reported some eye-witnesses as "hearing it hiss as it went along, as if it had been very near at hand" but dismissed such claims as "the effect of pure fantasy."

18 August 1783. "Intrigued by reports of instantaneous hissing sounds from a large meteor, Sir Charles Blagden is presenting his arguments for the "electric origin of meteors" Since the nature of electricity was still not understood in 1784 he describes meteors as ``electricity fluid". It was thought possible for this fluid to travel faster than sound and produce "hissing" sounds around an observer."

10 September 1813. The Great Limerick meteor. The `Brasky mass`, on loan from The National Museum, Dublin.

The largest piece, weighing more than 27kg, is currently on display in the Ulster Museum, Belfast, with a cast on display in Armagh Planetarium. It is the biggest meteorite to fall in Ireland and the UK in historic times (exceeding the largest of the 45kg, total weight, 1965 Barwell meteorite fragments). There were numerous eyewitness accounts in newspapers showing that it was a spectacular morning event, similar to the 2013 Russian Chelyabinsk meteor, with `bright lights`, loud bangs, `hissing noise` and `extraordinary smoke`.

13 November 1833. “One or two instances were reported of persons who died with terror; many others thought the last great day had come” Prof Denison Olmsted, writing about the great Leonid meteor shower of 1833.

13 August 1930. “a multiple hissing noise is heard...the hissing noise comes closer and becomes more and more frightening...fishermen saw large balls of fire which fell from the sky like thunderbolts...they landed in the centre of the forest with a triple shock...causing tremors like those of an earthquake.” River Curuça. Brazilian Amazon.

Recent reports

This selection of electrophonic reports also includes high altitude trails, fragmentation, sonic booms and meteorite recovery; suggesting that fireballs which result in meteorites can generate sounds, both sonic and electrophonic.

1965 December 24. 16h 12m GMT.

Az. 20° over Coventry area England.

Multiple (at least three) fireballs. Magnitude very bright. Slope 20°.Few visual observations due to extensive cloud. Fireball 2 exploded into 4 major fragments. No such explosion for fireball 1 or 3. Fireball 3 developed a tail, 1 and 2 possibly did. Fireballs 1 and 2 left white trails. Many chondrites recovered from near Barwell village.

Numerous reports of sonic phenomena, including a few reliable reports of electrophonic noises. “One observer, for example, reported noises of this type before he saw the bolide break through the cloud cover towards him. Witnesses in Barwell itself have given consistent accounts of the actual sounds heard at impact. These are typically described as starting with a “swishing“ noise and ending with a succession of dull thuds.” Contact: Prof. P.C. Sylvester-Bradley, University of Leicester, England. [Miles, H.G. and Meadows, A.J., “Fireballs Associated with the Barwell Meteorite,” Nature, June 4, 1966, pp. 983-986]

1969 April 25. 21h 20m- 21h 25m GMT.

Az. 322°, 1km W of Lisburn, Northern Ireland.

Two meteorites recovered, one near Lisburn and another at Bovedy, near Kilrea, 45 km further along the path.

Many observers in SW England, Wales and Ireland. From various areas as far away as Dublin a swishing or rushing sound was reported as the fireball passed over.

[Meghan, I.G and P.S. Doughty, “Recent Fall of the Bovedy Meteorite,” Nature 223, 1969, pp. 24-29.] [Andrews A.D., T. W. Rackham, and P. A. Wayman, Nature 222, 1969, pp. 727.] [Annual Report for the 1969, Smithsonian Institution Center for Short-lived Phenomena, Cambridge MA, 1970, pp. 162-165.]

1978 April 7. New South Wales. Australia.

Edgecliff, Sydney, 20 km from the ground track, A. Hayes "Heard a noise like an express train or bus travelling at high speed. Next an electrical crackling sound, then our backyard was as light as day."

Vales Point, 40 km from the ground track, J. Ireland "Heard a sound like an approaching vehicle and saw a flash of light, from behind my right shoulder, as everything was lit up like daylight."

Kotara, Newcastle, 40 km from the ground track, N. Jones heard a noise like a "phut" when the bolide flared, but "It was not loud enough to wake anyone."

Other descriptions of sounds simultaneous with the above sightings were "a loud swishing noise"; "a humming sound like a transformer or distant siren"; "like steam hissing out of a railway engine for a count of about ten"; "a swishing sound like the onset of an unexpected high wind" and "a low moaning, whooshing." Prof Colin Keay.

2001 November 18. San Francisco.

“The weirdest thing was that I am sure I could hear several of the meteors. Several times when a meteor with a persistent streamer seemed directly overhead, I heard a faint fizzing noise. How is that possible when the thing is hitting the edge of the atmosphere a couple of miles above my head? Even if there were some sort of meteor thunder, I wouldn't think it could reach my ears through the air until after the meteor was no longer visible. The first time it happened I thought I was making up my own sound effects, but after five or six repetitions, the sound was clearly very distant, from above, only with meteors that had sparkly persistent tails, and only when they were nearly directly overhead. Well, some people hear auroras, so I hope I'm not going crazy!” Karen Newcombe, San Francisco.

I just walked outside at 4:46 a.m. EST [on Nov. 18th] ..and it's actually loud. There's a solid stream of hissing.. Is it possible to hear meteors? Chris Hann. Lawrence, MA. Reports on NASA TV Live Leonids show. 2001 November 18.

2012 March 03. 21:38 Cullen, Scotland.

“I noticed a slow moving bright light over North Sea in North direction, about 60 deg from horizon, moving towards me in southerly direction, a bit brighter than Venus, slowly getting brighter as it got directly overhead. It had a long fainter tail. There was a slight Shhh sound and it kept going South at the same brightness until it was obscured by the houses behind me. Probably took about 10 seconds from noticing it, until it was obscured, to traverse the sky above - a lot slower moving than I've ever seen before!” Fireball report to Armagh Observatory.

2012 September 21. 22:55 Upton, Wirral, Merseyside.

“I saw a huge fireball travelling across the sky from the East heading West. It had a huge tail and parts of it were visibly breaking off. It was going very fast and it was fairly low in the sky, so would be hitting the ground shortly after I saw it disappear out of my view. It was very bright and very orange and I heard bangs and crackling, which is what alerted me to it in the first place.” Fireball report to Armagh Observatory.

"What makes this exciting is that we're talking about a phenomenon that has been experienced by people for perhaps thousands of years, even in modern times people who reported hearing such sounds were ridiculed.

It was only about 25 years ago that Professor Colin Keay was able to do the research and legitimise the experiences of all those generations of people." Dr Dennis Gallagher. NASA Marshall Space Flight Centre.

Electrophonic meteor theories

Keay investigated reports of electrophonic sounds associated with a number of bright fireballs. He classified the sounds into three groups: smooth (71%), staccato (18%) and sharp (11%). In 1980 he calculated that meteor plasma interaction with the Geo-magnetic field could generate Extremely Low and Very Low Frequency (ELF/VLF) `radio emissions` in the range ~500 Hz to 10 kHz. In 1983 Dr V. A. Bronshten “confirmed that through this mechanism bright fireballs may produce radiated power levels of the order of Kilowatts” and “for such a fireball the kinetic energy dissipation rate exceeds ten Gigawatts” In 1988, this `magnetic spaghetti` theory was reinforced when groups of Japanese observers from Nagoya University obtained simultaneous photographic and radio observations of a bright fireball, together with an audible “phut” sound report of the event.

However, the `magnetic spaghetti` theory was not universally accepted as the definitive explanation.

Electrophonic noise researcher Dr Andrei Ol'khovatov, The Radio Instrument Research Group, Moscow, reasoned that since the extremely active Geo-electric-magnetic and anthropic background environment up to 15 kHz is not routinely transduced to sound by Keay`s passive field conversion processes, then the magnitudes, at an observers location, of the magnetic and electric field components of the postulated meteor ELF `radio emission` would have to exceed this background level to be passively detected. Keay stated that in his experiments, magnetic fields up to ~1.5 times the background geomagnetic field, varying at audio frequencies, were not actually audibly transduced.

Ol'khovatov wrote, "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.” and “I think that a bolide can trigger some [other] geophysical processes resulting in various geo-electric field disturbances.”

"Personally, I don't think there is one single theory that can explain everything going on out there," Dr. Dejan Vinkovic Global Electrophonic Fireball Survey.2001.

As a phenomenon distinct from the longer duration hissing sounds, the sounds associated with electrophonic `burster` meteors are characteristically described as staccato-like `clicks` and short duration `pops`. They are similar in their sound characteristics to instrumentally detected Sferics and distorted Tweeks but remain difficult to explain. Research by Dr Martin Beech and Dr Luigi Foschini suggested that unlike the longer duration electrophonic sounds, the electrophonic bursters are not generated as a consequence of interactions between the meteoroid ablation plasma and the Earth's geo-magnetic field but appear as short-duration pulses in the observer's local electrostatic field. This is believed to be due to the generation of a strong electric field across a meteor shock wave propagating in a plasma. Calculations for the description of the electric field strength, in terms of the electron temperature and the electron volume density, can link the electron line density to a meteor's absolute visual magnitude. This suggests a lower limit to the visual magnitude of electrophonic burster meteors as ~Mv-6.

"Ironically Leonid meteors are least suitable devices for production of the VLF radiation via the Keay-Bronshten mechanism, which demands the Reynolds number in the meteor plasma flow to exceed 10x6. In the case of Leonids, which are mostly dust grains, this leads to unreasonably large initial size, D0 > 3 m and mass 3000 kg (Zgrabli´c et al. 2002). Nevertheless two clear electrophonic signals were instrumentally recorded during the 1998 Mongolian Leonid expedition. The first originated from the meteor at the altitude of 110 km and the second at an altitude of 85 to 115 km. In both cases the `sounds` preceded the meteors’ light maximum”

“These features are hard to explain, also in other models suggested for electrophonic bursters. No ELF/VLF signal was detected in these two events. But the receiver apparatus was insensitive for frequencies below 500 Hz, while the frequency range of the observed electrophonic sounds was 37 to 44 Hz. If one assumes that these sounds originated from the transduction of a ELF/VLF transient, the observed sound intensities will imply unreasonably high ELF/VLF radiation power, impossible to explain by any theoretical mechanism starting from [the] meteor alone” (Zgrabli´c et al. 2002)

“Therefore these remarkable observations show that the existing theories are at least incomplete and the electrophonic meteor mystery remains still largely unsolved. Zgrabli´c et al. (2002) suggested that the Leonids acquire large enough space charge and can trigger a yet unidentified geophysical phenomenon upon entering the E-layer of the ionosphere at ~110 km. It is assumed that such phenomena in its turn will generate a powerful EM radiation burst. Note that this possibility was advocated much earlier by Ol’khovatov (1993)"

Almost 300 years since Sir Edmund Halley`s erroneous scientific dismissal, the phenomenon of sounds perceived simultaneously with meteors is still without a robust physical explanation or unified model. Current ideas and associated models usually present the following process:

Disturbance/excitation-¬¬--Coupling/transmission—Observer perception.

Simple, selective, models can fail to indicate which of their physical factors changed or were absent between `sound` perception and non perception. For example, they do not unambiguously explain why some periodic meteors appear to enable their proposed `electromagnetic` field characteristics at an observer`s location, while subsequent apparently identical meteors or most others do not.

According to accepted knowledge, audible meteor `sounds` are simultaneously created by meteor `radio emissions` in the audio frequency range reaching ground level. How the field fluctuations at audio frequencies then emit acoustically or are perceived or heard passively is mostly speculative. It also remains undetermined whether reported `audible sounds` should coincide with specific instrumental ELF electric and magnetic field spectra, attributed unambiguously to the observed meteor. It is also necessary to clarify what is meant by `simultaneously`, since sounds perceived during several continuous seconds of visual observation may not coincide exactly with observed events, i.e. flaring, fragmentation: perceptible delays strongly suggesting additional, almost immediate, intermediary processes.

An augmenting theory, involving electric field production of Ozone as an associated “unidentified geophysical phenomenon” (see above) to explain some hissing `electrophonic sounds` is suggested in section 5.2. Unlike previous theories, a field fluctuation rate is not required to occur in the audio frequency range for this process to acoustically emit hissing and intermittent impulsive sounds; removing the requirements of direct conversion, passive human transduction or excited localised `emitters`.

The extreme rareness of the phenomenon [electrophonic meteors] has prevented substantial experimental work so far; consequently it remains on the margins of scientific interest. Vinković, D et al. WGN, vol. 30, no. 6, p. 244-257.

This disinterest is unjustified, since these audibly perceived electric field affects indicates complex, inconsistent and still unresolved electric-magnetic coupling and charge dynamics; interacting between the meteor; the ionosphere; mesosphere; stratosphere and the surface of the earth.

Meteor Interaction with the Ionosphere/neutral atmosphere in the complex 80km to 100km Mesopause transition region might create other processes which facilitate `electrophonic sound` production. There is possibly a dominant and currently unrecognised diurnal, seasonal and yearly variance in the occurrence of electrophonic meteors, caused by Sun dependent changes in ionosphere composition. For example, the 2001 Leonid activity occurred during the 1999-2002 peak of solar cycle 23. This may have contributed to the reports of high altitude meteors and prominent hissing sounds, since the enhanced atmospheric and ionospheric densities could extend the meteoroid interaction region within the atmosphere. These mostly unexplored aspects merit further observation, research and discussion.

At Armagh Observatory, sensors with continuous coverage from ~5 Hz to 24 kHz have been utilised simultaneously with our meteor cameras. The ELF recordings are subject to detailed spectrum analysis. Since the Earth's geo-electric-magnetic ELF background is intensely active involving natural phenomena such as Sferics, Tweeks, Whistlers and Sprites, and also strong man-made emissions, distinct effects or disturbances directly attributable to individual meteors are elusive. An exceptionally powerful disintegration, such as the Chelyabinsk super bolide airburst, causes all of the historically reported `electrophonic` affects to occur.

The Chelyabinsk Airburst

"This object never got bright enough to be detected by a ground-based survey. Because it came at Earth from the direction of the Sun, It was basically undetectable before it hit Earth.” Prof M.Campbell-Brown.

“Such large scale invaders may be far more common than we previously suspected, the Earth may be subjected to three or four such events a century." Prof Mark Bailey. Armagh Observatory.

The Chelyabinsk super bolide was 100 times more energetic than the 4kT of TNT equivalent* Sutter’s Mill meteorite fall in the USA on 22 April 2012. Possibly originating from the 2km asteroid 86039, the 10,000 tonne,19m diameter fragment that disintegrated with the equivalent energy of 500kT of TNT over the Chelyabinsk Oblast on 15 February 2013, was one of the largest airbursts since the ~10 to 15MT TNT equivalent Tunguska event of 30 June 1908. *1kT TNT equivalent = 4.185 x 10x12 Joules.

The Chelyabinsk meteorite is classified by its low Iron, low metal, composition as a rare LL5 Chrondite. It is impregnated with cracks that had filled in with metal rich glass, suggesting that its parent body had survived an impact which had compressed and fissured it. This may have facilitated disintegration during its previous orbital history, preventing even larger fragments impacting the Earth. There is one other LL Chondrite parent body whose orbit is known: the asteroid Itokawa, visited by the Japanese Hayabusha spacecraft in 2005.

Two large parts of the asteroid fragment survived the 30 km airburst. One broke up at an altitude of about 18 km while the other fell into Chebarkul Lake. This 1.5m, 570 kg fragment was later recovered by divers. Thousands of much smaller meteorites fell 40 km south of Chelyabinsk, around the villages of Pervomaiskoe, Deputatsky and Yemanzhelinka. The total mass of all the recovered fragments are estimated to account for only about 0.04% of the original body, suggesting that most of the material ablated during the 30 second fireball. Its size and velocity suggests that a shock wave first developed at 90 km. Observations show that dust formation and fragmentation started at around 83 km, increasing at 54 km. Peak radiation occurred at an altitude of 30 km at 03:20:32.2 UTC, at which time orbiting sensors measured a meteoroid speed of 18.6 km/s. Disintegration left a thermally emitting debris cloud, with the final burst occurring at an altitude of 27 km. Dust and gas settled at 26 km with the dust cloud splitting, creating two billowing cylindrical vortices due to the buoyancy of the hot gases.

Chelyabinsk. Electrophonic sound reports

During the Russian Academy of Sciences sponsored field study some detailed reports of electrophonic sounds were obtained. None of the following observers wore glasses.

1. While in his office in Yemanzhelinsk, Evgeny Svetlov, an electrical engineer, heard a noise like the buzz of an electrical transformer during the main bolide flash.
2. While standing on a street in Yemanzhelinsk, Alexander Polonsky, heard a noise like the roar of two fighter planes even before he saw the bolide.
3. In an open area near the Chelyabinsk regional hospital, Vladimir Bychkov, a police programmer and physicist by training, heard a noise like the sizzle of oil in a frying pan during the bright stage of the bolide. The noise appeared to be from the direction of the bolide. The noise stopped at the main bolide flash, accompanied by a sound like a clap.

Of the 1,674 people interviewed during the internet survey 198 reported hearing sounds (Table1.)

The sound effects were described as “hissing”, “fireworks noise” “interference”, “the sound of Bengal light”, “crackle”, “sparking”, “crackling”, “rustle”, “rustling”, “like a whistle”, “squeaking”, “rumble” and the “sound of a passing plane”. The term onomatopoeic refers to the formation or use of words such as “Hiss” or “Swish” that imitate the sounds associated with the objects or actions they refer to.

Table 1. Summary of electrophonic sounds; eye witness reports. Compiled by Sergey N. Zamozdra.

Hiss or hissing, fireworks noise, interference	76
Like a passing plane	31
Whistle sound	26
Crackle, sparking or crackling	25
Rumble	19
Like sound of Bengal light	13
Rustle or rustling	6
Squeak	2

Chelyabinsk. Meteor Smells

A group of four observers of the Leonid meteor shower of 1833 reported a peculiar odour, “like sulphur or onions.”

It was thought that “This apparent transmission of smells at the speed of light could be explained if they were due to nitrous oxide or ozone produced by an electric discharge.” (Ozone [O3] a gas. From the Greek, ozein, for smell). Observers of the Texas fireball of 1 October 1917 also reported the odour of sulphur and burning powder as it passed.

A possible explanation is suggested by the following Chelyabinsk observer reports.

Field survey reports of smells were concentrated in the area surrounding the fireball trajectory. After an initial strong burst, the smells continued for a few hours. The eastern edge of this area coincides with the eastern edge of the glass damaged area. Arkhangel’skoe is the most western village where smells were reported. It is situated near the western edge of the glass damaged area. Fourteen villages reported similar smells, with nearly all described as a sulphur smell, a burning smell, or a smell similar to that of gunpowder.

These smells may have originated from the decomposition of Troilite (FeS), an iron sulphide mineral named after Domenico Troili, who first noted it in a meteorite that fell at Albareto, Modena, Italy in 1766. Troilite is one of the main components of the Chelyabinsk meteorite. Some burning smells may also have been caused locally when the shockwave dispersed soot from flues and stoves.

Respondents in Emanzhelinka, immediately under the fireball trajectory, also reported an ozone smell, similar to the smell after a thunderstorm. Ozone, with nitrogen oxides as by products, may have been produced in the immediate surroundings of the fireball by Ultra-Violet (UV-B λ= 290-320 nm wavelength) radiation from the meteor. This reinforces reports about sunburn caused by UV radiation from the fireball.

Chelyabinsk. Eyestrain, Heat and Sunburn

Compiled by: A. Kartashova, P. Jenniskens, O. P. Popova, S. Khaibrakhmanov, S. Korotkiy, I. Serdyuk.

“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." An account by Sir Edmund Halley of the extraordinary meteor seen all over England on the 19th of March 1718.

People who looked directly at the Chelyabinsk fireball had painful eyes. 180 people said their eyes hurt and 70 were temporarily blinded. All of them closed their eyes or turned in the opposite direction. Many mentioned feeling heat in the neck when the fireball was behind them. There were no reports of lasting eye damage to the lens or retina from watching the fireball, estimated as reaching magnitude -27.3, approximately the same brightness as the midday Sun.

The total radiated energy of the fireball was estimated by NASA to have been ~3.75 x 10x14 Joules.

Throughout the survey area, there were reports of mild sunburns following the fireball sighting. Of 1,113 respondents in the internet survey who were outside at the time of the fireball, 25 were sunburned (2.2%), 315 felt hot (28%), and 415 (37%) felt warm. In Kokino, approximately 33 km below the point in the trajectory where peak luminous radiation occurred, Vladimir Petrov reported sunburn so severe that his skin peeled off sometime after the event.

"We calculated how much UV light came down and we think it is possible, but he was also in a snowed-in landscape and snow is very efficient at scattering UV light. This may have helped." Dr P.Jenniskens.

Potentially hazardous near earth objects

“The entire planet is vulnerable to meteors and a system is needed to protect the planet from similar events in the future.” Dmitry Medvedev. Russian Prime Minister.

A similarly powerful airburst occurred on 13 August 1930, in the neighbourhood of the River Curuça in the Brazilian Amazon. The associated energy is uncertain but may have been in the range 0.2 to 2 Mt of TNT.

Another happened on 11 December 1935 in Guyana. Even less is known of the energy of this impact, but an aircraft pilot reported seeing an elongated area of destroyed forest more than 30 km across.

Detailed analysis of the Chelyabinsk event provided an opportunity to better assess these effects, with implications for the study of near-earth objects (NEOs) and to develop hazard mitigation strategies for future planetary protection.

NEOs are asteroids and comets with paths that approach within ~195 million km of the Sun. See Table 2.

In 2014 there were 642,348 known asteroids, 10960 known near earth asteroids (NEAs) and 1473 Potentially Hazardous Asteroids. PHAs are near earth asteroids with a Minimum Earth Orbital Intersection Distance (MOID) of less than 7.5 million km and an absolute magnitude brighter than +22, corresponding to objects > ~150 m diameter. This represents an asteroid large enough to cause a global climatic catastrophe.

A United Nations action team has been set up to detect and counter potentially hazardous NEOs. A warning network and a planning advisory group to coordinate an international response to collision hazards have been established. A global group of experts on NEOs met in Vienna on 10 to 11 February 2014 for the 51st session of the United Nations' Scientific and Technical Subcommittee of the Committee on the Peaceful Uses of Space. The meeting came just a few days before the first anniversary of the Chelyabinsk meteor impact, highlighting the reality of the asteroid threat. The experts’ plans had been developed over a decade by the UN Action Team on Near Earth Objects, known as Action Team 14. AT-14 was established in 2001 and has formulated recommendations for an international response to the asteroid impact threat. Establishing an International Asteroid Warning Network (IAWN) is a crucial step in collecting and sharing information about potentially hazardous NEOs. In the event that an Earth threatening object is detected, the UN Committee on the Peaceful Uses of Space could help to facilitate a spacecraft mission intended to deflect it from a collision course with Earth.

The primary purpose of the Space Mission Planning Advisory Group (SMPAG) is to prepare for a worldwide response to a NEO threat “through the exchange of information, development of options for collaborative research and mission opportunities and to conduct NEO threat mitigation planning activities”. Thirty representatives from 13 agencies, seven government ministries and the UN will share knowledge and the latest research related to impact case studies to develop a work plan for the next two years. The next SMPAG meeting will be held in Vienna in June 2014. Participants will focus on the exchange of information on relevant activities in the field of NEO hazard mitigation and progress on the future work plan.

• http://cosmos.esa.int/web/smpag
• http://www.minorplanetcenter.net/IAWN/
• http://minorplanetcenter.net/blog/
• http://www.esa.int/Our_Activities/Operations/Space_Situational_Awareness/Getting_ready_for_asteroids

NEO Research at Armagh Observatory

“If a 30-metre asteroid were to hit, it could wipe out an entire city” - Dr Alan Fitzsimmons. Queen's University, Belfast.

Armagh Observatory continues to be actively engaged in researching NEOs. Internationally, there are about 100 researchers who regularly observe asteroids to better determine their shape and orbital parameters. To assist this research, Armagh Observatory uses its onsite robotic telescope to record and measure star occultation by asteroids. Direct observation with larger telescopes such as the Faulkes telescopes in Australia and Hawaii is also undertaken. These telescopes are used remotely from Armagh, including extensive use by students on short term projects. http://star.arm.ac.uk/~dja/faulkes/

The Observatory is also a collaborating partner in EURONEAR, The European Near Earth Asteroid Research project. This is a project, including professionals, students and amateur astronomers, to establish a coordinated network to follow-up, recover and discover Near Earth Asteroids (NEAs), Potentially Hazardous Asteroids (PHAs) and Virtual Impactors (VIs).

• http://www.ing.iac.es/astronomy/science/ING_seminar_20121211_Vaduvescu.pdf
• http://euronear.imcce.fr/tiki-index.php
• http://euronear.imcce.fr/tiki-index.php?page=Consortium

Table 2.

NEOs are asteroids and comets with perihelion distance q less than 1.3 AU, ~195 million km. Near-Earth Comets (NECs) are further restricted to include only short-period comets (orbital period P less than 200 years). The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). NEAs are divided into groups, Aten, Apollo and Amor according to their perihelion distance (q), aphelion distance (Q) and their semi-major axes (a).

Group	Description	Definition
NECs	Near-Earth Comets	q<1.3 AU, P<200 years
NEAs	Near-Earth Asteroids	q<1.3 AU
Atiras	NEAs whose orbits are contained entirely with the orbit of the Earth (named after asteroid 163693 Atira)	a<1.0 AU, Q<0.983 AU
Atens	Earth-crossing NEAs with semi-major axes smaller than Earth's (named after asteroid 2062 Aten)	a<1.0 AU, Q>0.983 AU
Apollos	Earth-crossing NEAs with semi-major axes larger than Earth's (named after asteroid 1862 Apollo)	a>1.0 AU, q<1.017 AU
Amors	Earth-approaching NEAs with orbits exterior to Earth's but interior to Mars' (named after asteroid 1221 Amor)	a>1.0 AU, 1.017<q<1.3 AU
PHAs	Potentially Hazardous Asteriods: NEAs whose Minimum Orbit Intersection Distance (MOID) with the Earth is 0.05 AU or less and whose absolute magnitude (H) is 22.0 or brighter	MOID<=0.05 AU, H<=22.0

Historical and current NEO information

• http://star.arm.ac.uk/impact-hazard/
• http://star.arm.ac.uk/movies/2010/Risk-and-Natural-Catastrophes.html
• http://star.arm.ac.uk/neos/1980-2010/
• http://www.arm.ac.uk/paseg/
• http://www.spaceguarduk.com/images/articles/full_report.pdf

On 6 March, 2014, NASA's Hubble Space Telescope confirmed the break-up of asteroid P/2013 R3 into 10 smaller fragments, each with comet-like tails. The four largest fragments are up to 400m in diameter. Asteroid P/2013 R3 is disintegrating due to the effects of sunlight causing its rotation rate to gradually increase. This is known as the `YORP effect` and is considered to be the cause of rotational bursting in smaller, 1-2km, asymmetrical asteroids. The component pieces are reacting to centrifugal force and are slowly pulling apart. For this to occur it must have a weak, fractured interior, probably as the result of numerous non-destructive collisions with other asteroids. The YORP effect was first detected in 2007 by the Northern Ireland astronomer Dr Stephen Lowry and his colleagues by measuring the increasing rotation rate of asteroid 54509 2000 PH5, now designated 54509 YORP. Most small asteroids are now thought to have been damaged in this way. P/2013 R3 is likely to be the result of such a collision, occurring sometime in the last billion years.

The possibility of this type of disruption had been postulated but never reliably observed. Crumbling asteroid P/2013 R3 was first observed on 15 September 2013 by the Catalina and Pan STARRS sky surveys. A follow-up observation on 1 October 2013 by the W. M. Keck Observatory on the summit of Mauna Kea revealed three bodies moving together in an envelope of dust nearly the diameter of Earth.

Combined with the previous discovery of an active asteroid with six tails, P/2013 P5, more evidence emerged that the pressure of sunlight may be the primary force causing the disintegration of asteroids less than1.5 km in diameter. P2013 R3`s remnant debris, weighing ~ 200,000 tonnes, will in the future provide a rich source of meteoroids. Most will eventually plunge into the sun but a small fraction of them could one day collide with the Earth.

• http://science.nasa.gov/science-news/science-at-nasa/2013/07nov_6tails/
• http://neo.jpl.nasa.gov/orbits/

Also being tracked is a 1.2km diameter asteroid, 2003 QQ47. First observed on 24 August 2013 by Lincoln Near Earth Asteroid Research Program (LINEAR), based in Socorro, New Mexico. It has a mass of ~2,600 billion kg and is regarded as `an object meriting careful monitoring`. Although the probability of this asteroid hitting Earth on 21 March 2014 is over 1 in a million, with the risk of impact decreasing as orbital information is updated. Currently It`s orbit calculations are based on just 51 observations obtained during a seven-day period. This NEO will be observable from Earth for the next two months and astronomers will continue to track it over this period.

• http://arpc65.arm.ac.uk/~spm/neo_map.html
• http://szyzyg.arm.ac.uk/~spm/local_map.html
• http://newton.dm.unipi.it/neodys2/index.php?pc=1.0

NASA: NEO close-approach tables and Impact risk assessment

• http://neo.jpl.nasa.gov/ca/
• http://neo.jpl.nasa.gov/risk/doc/sentry.html

Conclusion

Almost 300 years since its first scientific assessment and rejection, the phenomenon of sounds perceived simultaneously with meteors is still without a robust physical explanation or unified model. According to accepted knowledge, audible meteor `sounds` are simultaneously created by meteor `radio emissions` in the audio frequency range reaching ground level. How the field fluctuations at audio frequencies then emit acoustically or are perceived or heard passively is mostly speculative. There is no convincing theory that fully explains why some meteors appear to enable this type of field, while subsequent, apparently identical meteors or most others do not.

It also remains undetermined whether reported `audible sounds` coincide with specific instrumental ELF electric and magnetic field spectra, attributed unambiguously to the observed meteor. It is necessary to clarify what is meant by `simultaneously`, since sounds perceived during several continuous seconds of visual observation may not coincide exactly with observed events, i.e. flaring, fragmentation: perceptible delays strongly suggesting additional, almost immediate, intermediary processes.

An augmenting theory associated with electric field production of Ozone to explain some hissing `electrophonic sounds` is introduced in section 5.2. Unlike previous theories, a field fluctuation rate is not required to occur in the audio frequency range for this process to acoustically emit hissing and intermittent impulsive sounds; removing the requirements of direct conversion, passive human transduction, or excited localised `emitters`

The perceived improbability of electrophonic meteors has prevented substantial experimental work; consequently it remains on the margins of scientific interest. This is unjustified, since these audibly perceived electric field affects indicates complex, inconsistent and still unresolved electric-magnetic coupling and charge dynamics; interacting between the meteor; the ionosphere; mesosphere; stratosphere and the surface of the earth. Meteor Interaction with the Ionosphere/neutral atmosphere in the complex 80km to 100km Mesopause transition region might create other processes which facilitate `electrophonic sound` production. There is possibly a dominant and currently unrecognised diurnal, seasonal and yearly variance in the occurrence of electrophonic meteors, caused by Sun dependent changes in ionosphere composition. These unexplored aspects merit appropriate observation, research and discussion.

At Armagh Observatory, sensors with continuous coverage from ~5 Hz to 24 kHz have been utilised simultaneously with our meteor cameras. The resultant ELF recordings are subject to detailed spectrum analysis. Since the Earth's geo-electric-magnetic ELF background is intensely active involving natural phenomena such as Sferics, Tweeks, Whistlers and Sprites, and also strong man-made emissions, distinct effects or disturbances directly attributable to individual bright fireballs remain elusive. Exceptionally powerful disintegrations, such as the Chelyabinsk super bolide airburst, indisputably confirm all of the historically reported `electrophonic sound` phenomena.