Photoacoustic Sounds from Meteors

Abstract

Concurrent sound associated with very bright meteors manifests as popping, hissing, and faint rustling sounds occurring simultaneously with the arrival of light from meteors. Numerous instances have been documented with −11 to −13 brightness. These sounds cannot be attributed to direct acoustic propagation from the upper atmosphere for which travel time would be several minutes. Concurrent sounds must be associated with some form of electromagnetic energy generated by the meteor, propagated to the vicinity of the observer, and transduced into acoustic waves. Previously, energy propagated from meteors was assumed to be RF emissions. This has not been well validated experimentally. Herein we describe experimental results and numerical models in support of photoacoustic coupling as the mechanism. Recent photometric measurements of fireballs reveal strong millisecond flares and significant brightness oscillations at frequencies ≥40 Hz. Strongly modulated light at these frequencies with sufficient intensity can create concurrent sounds through radiative heating of common dielectric materials like hair, clothing, and leaves. This heating produces small pressure oscillations in the air contacting the absorbers. Calculations show that −12 brightness meteors can generate audible sound at ~25 dB SPL. The photoacoustic hypothesis provides an alternative explanation for this longstanding mystery about generation of concurrent sounds by fireballs.Concurrent sound associated with very bright meteors manifests itself as popping, hissing, and faint rustling sounds occurring simultaneously with the arrival of the light from the meteor [1][2][3][4][5][6][7] . Concurrent sound occasionally is generated by fireballs 8 with apparent magnitude (visual brightness) as low 8,9 as −9, and numerous occurrences have been documented 1,2 with apparent magnitudes of − 11 to − 13. These sounds cannot be attributed to direct acoustic propagation from the upper atmosphere for which the travel time would be several minutes. Concurrent sounds must be associated with some form of electromagnetic energy generated by the meteor, propagated to the vicinity of the observer, and transduced into acoustic waves. Prior to now, the means by which energy from meteors could be propagated to Earth and then converted into audible sound has not been adequately explained and validated by experiment. Here we present observational data, experimental results, and numerical models in support of photoacoustic coupling as the mechanism. Recent photometric measurements of fireballs reveal strong millisecond flares and significant brightness oscillations at frequencies of 40 Hz and higher 7,8 . Experiments and models show that strongly modulated light at these frequencies and light intensity on Earth from − 12 apparent magnitude meteors (same as full moon illumination ~10 −3 W/m 2 ) can radiatively heat common dielectric materials like hair, cloth, paint, etc. This heating can produce small pressure oscillations in the air adjacent to the absorber. These can be loud enou...

On the electrophonic generation of audio frequency sound by meteors

“…Here using a new idea in which ambipolar electric fields near the meteor track map to the highly conductive E region, we continue the thread of theoretical studies, arguing that radio waves are created by the interaction of meteors with the ionosphere. We also point out that, contrary to the comments in Spalding et al [], ELF radio waves were indeed detected during the Leonids storm of 1998/1999 by Price and Blum [] for nonfireball meteors.…”

“…Hence, audible sound by the electrophonics mechanism occurs when electromagnetic energy is converted to acoustic waves at the same frequency by metals at ground level. Dielectric materials have been suggested as transducers of either electromagnetic waves or even light waves [ Spalding et al, ]. At meteoroid ablation heights, ions and electrons are created by a variety of processes, mostly by impact ionization by atoms boiling off the meteor and some possibly by UV radiation.…”

“…Recently, Spalding et al [] suggested that the variability of light from incident meteors could induce radiative heating of dielectric material such as hair, clothing, and leaves. This, in turn, due to variability in the light emitted by the meteor, creates audible sound.…”

Rare Events Detected with a Bulk Acoustic Wave High Frequency Gravitational Wave Antenna

“…For the low energies we have observed ( / 20 meV), NAUTILUS would expect some > 10 6 events per day, however it is highly uncertain what fraction of the incident cosmic ray's energy would actually be deposited into the quartz. In future generations of this detector Cosmic ray events of this kind could be easily identified by employing muon (cosmic shower) detectors for coincidence analysis; (4) fireballs and other meteor type events in the atmosphere [41,42]. The detector used in this work should not be sensitive to atmospheric acoustic waves as it was shielded by two layers of vacuum.…”

A two year survey for VLF emission from fireballs

“…Spurny et al, 2016). Spalding et al (2017) tested this hypothesis and showed experimentally that the photoacoustic effect can generate noticeable sound when light with suitably modulated amplitude falls on certain materials. This is a promising explanation for anomalous sound, though clearly in need of more experiments and observational verification.…”