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General Principles
© Charles Chandler
 
When a meteoroid strikes the Earth, or anything else for that matter, it's going to explode. That much is easy to understand. But most meteoroids explode, or at least break up, before they actually impact the surface, and this is not so easy to understand. It is commonly believed that frictional heating causes the expansion of the meteoroid. But to get a break-up or an explosion, the meteoroid would have to be heated from the inside, developing a great internal pressure contained within a rigid shell. Yet friction would heat the meteoroid from the outside. Considering the extremely short period of time in which the meteoroid is exposed to extreme heat, and the low thermal conductivity of the rocky material, the internal temperature of the object shouldn't change. Thus the outer layers could just melt or boil away, with no effect on the internal strength.
 
Interestingly, coincident with the break-up of a meteoroid, there have been numerous reports of crackling, swishing, hissing, buzzing, and popping sounds. Researchers originally dismissed the reports, and for two reasons. First, the meteoroids were still tens of kilometers away when the sounds were heard, and such high-frequency sounds cannot travel such distances. Second, shock waves originating at the meteoroid propagate at the speed of sound, and therefore arrive at the observer long after the visible break-up (roughly 3 seconds for every kilometer of distance). So an audible popping sound that is coincident with a visible event tens of kilometers away isn't allowed by the principles of acoustics. The researchers concluded that the observers simply expected to hear associated sounds, and mistook environmental noises as related to what they were seeing.1
 
Yet further research revealed that the sounds are, indeed, caused by the meteoroids, so there must be more to it than just simple acoustics. Current thinking is that the break-up generates an EM wave that propagates at the speed of light, and which is somehow converted back to mechanical energy near the observer, as described in the following quote.2
The rapid movement of the electrons, with respect to the much more massive and slower moving ions, generates a sizable space charge (see e.g., 3). A transient electrical pulse is generated in response to the development of the space charge, and provided the resultant electrical field strength variations are large enough it is suggested, following 4, that they might trigger the generation of audible sounds through an observer localized transduction process. The shock wave is produced, 5 suggest, during the catastrophic break-up of the parent meteoroid.
Scientists use the term "electrophonic bolide" to refer to a meteoroid that produces an EMP that then gets converted back to mechanical energy on the ground (as opposed to the sonic booms, which travel at the speed of sound and therefore arrive much later).6 If the frequency of the EMP is in the ELF/VLF range, the electrophonics will be audible. The transduction could be some sort of piezo effect due to the electric field, or Faraday induction from the time-varying magnetic field, or both. An EMP is consistent with other related phenomena, such as in the following report.7
The Vitim bolide may be categorized as so-called electrophonic bolides. At the time of luminescence in the area of the settlement of Mama, eye-witnesses report sounds (rustling, buzzing). The employee of the Mama airport Georgy Konstantinovich Kaurtsev witnesses that the filament lamps of the chandelier glowed to half their intensity at the time of the bolide's flight, although the entire settlement was devoid of electrical power supply that night. The airport guards Vera Ivanovna Semenova and Lidiya Nikolayevna Berezan pointed to a scaring phenomenon: a bright luminescence at the upper ends of thin little wood poles of the fence surrounding the airport's meteorological ground. All that may be treated as resulting from a strong alternate electric current that was produced when the bolide was flying. It should be noted that the distance from the flight path in upper atmospheric layers to the settlement of Mama was several tens of kilometers.
The charge separation mechanism appears to be an artifact of shock-induced boundary layer separation ahead of the supersonic bolide.8,5 Some consider the nature of detached shock waves to be a mystery.9,10 Air molecules bouncing off of the surface of a moving object should sustain a thin boundary layer, where the thickness is defined by the distance a rebounding particle travels before its momentum is fully thermalized in collisions. At low mach numbers, the depth of this boundary layer is fairly consistent, because the rebound energy varies directly with the velocity of the oncoming air, though increasing velocities develop greater compression in the boundary layer, producing a shorter mean free path. These principles should hold true even at supersonic speeds, and the boundary layer should be further compressed. Yet at higher mach numbers, the shock front becomes "detached" from the object. (See Figure 1.)
 
 
Figure 1. Detached bow shock, courtesy NASA.
 
 
Others acknowledge the presence of charge separations around supersonic objects, and attribute them simplistically to triboelectric charging.11 But this doesn't account for the detached shock front, nor for the fact that air isn't on the triboelectric series.
 
The more complete explanation identifies a different charging mechanism. When oncoming air enters the boundary layer, some of the electrons are split off in collisions. The positive ions, with their greater masses, penetrate more deeply into the boundary layer than the free electrons. Hence the boundary layer becomes positively charged, and electrostatic repulsion bloats it into a detached shock front. So the critical speed is hit when the inertial forces become greater than the electric force binding electrons to atomic nuclei.12,3
 
This easily explains the EMP that is generated when the bolide breaks up, as the charge separation mechanism goes away, and all of the charges recombine, abruptly altering the EM fields. But it might also explain the break-up itself. A bolide surrounded by a positively charged sheath will lose electrons to the sheath, which are then whisked away at hypersonic speeds (i.e., "charge exchange ionization"). Since air is an insulator, the drift velocity of electrons is too slight to accomplish charge recombination, and the bolide develops a net positive charge. This weakens the crystal lattice holding the bolide together, and it introduces electrostatic pressure. If that pressure exceeds the hydrostatic force in the boundary layer, the bolide will break up. If it does, the friction will increase exponentially, as a function of surface area, which is much greater for many small pieces than it was for one big piece. The increase in temperature adds internal hydrostatic pressure to the existing electrostatic repulsion, and the bolide completely disintegrates.
 
Note that some bolides do explode (such as the one at Tunguska, Russia, 1908-06-30), but most of them simply break up. The extremely powerful shock wave, that most people interpret as the consequence of an explosion, is usually just a sonic boom.
 
 

References

1. Wylie, C. C. (1932): Sounds from meteors. Popular Astronomy, 40: 289

2. Beech, M.; Foschini, L. (2000): Leonid electrophonic bursters. Astronomy & Astrophysics, 367: 1056-1060

3. Zel'dovich, Y. B.; Raizer, Y. P. (1967): Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena. New York: Academic Press

4. Keay, C. S. (1980): Anomalous sounds from the entry of meteor fireballs. Science, 210: 11-15

5. Beech, M.; Foschini, L. (1999): A space charge model for electrophonic bursters. Astronomy and Astrophysics, 345: L27-L31

6. Keay, C. S. (1993): Progress in Explaining the Mysterious Sounds Produced by Very Large Meteor Fireballs. Journal of Scientific Exploration, 7 (4): 337-354

7. Grigoryev, V. (2002): The Vitim Bolide, 2002-09-25. Russian Academy of Sciences

8. Serezhkin, Y. G. (2000): Formation of ordered structures of charged microparticles in near-surface cometary gas-dusty atmosphere. Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 4137: 1-12

9. Kim, H. D.; Setoguchi, T. (2007): Shock Induced Boundary Layer Separation. 8th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows, Lyon, France

10. Burgess, D.; Möbius, E.; Scholer, M. (2012): Ion Acceleration at the Earth's Bow Shock. Space Science Reviews, 173 (1-4): 5-47

11. Spurný, P.; Ceplecha, Z. (2008): Is electric charge separation the main process for kinetic energy transformation into the meteor phenomenon? Astronomy & Astrophysics, 489: 449-454

12. May, H. D. (2008): A Pervasive Electric Field in the Heliosphere. IEEE Transactions on Plasma Science, 36 (5): 2876-2879


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