Meteoric Airbursts © Charles Chandler
When a meteoroid strikes the Earth, or anything else solid 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 strong solid shell. Yet friction actually heats the meteoroid from the outside, and the outer "shell" should be the first to fail, begging the question of what contains the pressure necessary for an explosion. 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 at all. 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 possible. 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., Zel'dovich et al. (1967)). 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 Keay (1980), that they might trigger the generation of audible sounds through an observer localized transduction process. The shock wave is produced, Beech et al. (1999) 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).3 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.4
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.5,6 Some consider the nature of detached shock waves to be a mystery.7,8 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.9 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.10,11
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.
Meteors that explode in the air are more difficult to explain {than those that explode on land or sea}, but I don't think that megalightning is the answer. A meteor will certainly be charged, having passed through the ionosphere, which is positively charged. But any net charge is always around the outside of an object, due to electrostatic repulsion. Discharging the potential might char the surface, but it isn't going to blow the thing apart. If you want an electrical explosion, the current has to pass through the center, like a transformer blowing up when struck by lightning, because the wires lead through the center. In a monolithic charged body, this shouldn't be possible. …
{With} the Tunguska event, interestingly, there wasn't just one explosion -- there was a series of them, and some of the accounts mentioned that they occurred at regular intervals. In EM terms, I'd call this "sputtering". In my analysis of earthquake waves, I considered the possibility that an electric current flowing through microcracks in the crust creates ohmic heating, which causes the rock to expand. Interestingly, the expansion also closes the microcracks, and thus shuts off the current. If a rupture occurs, a negative pressure wave propagates back through the rock, re-opening the cracks, and the current flows again. Hence the current sputters. Similarly, an electric current through the Tunguska impactor might have sputtered before finally blowing the thing apart.
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. Keay, C. S. (1993): Progress in Explaining the Mysterious Sounds Produced by Very Large Meteor Fireballs. Journal of Scientific Exploration, 7 (4): 337-354 ⇧
4. Grigoryev, V. (2002): The Vitim Bolide, 2002-09-25. Russian Academy of Sciences ⇧
5. 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 ⇧
6. Beech, M.; Foschini, L. (1999): A space charge model for electrophonic bursters. Astronomy and Astrophysics, 345: L27-L31 ⇧
7. Kim, H. D.; Setoguchi, T. (2007): Shock Induced Boundary Layer Separation. 8th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows, Lyon, France ⇧
8. Burgess, D.; Möbius, E.; Scholer, M. (2012): Ion Acceleration at the Earth's Bow Shock. Space Science Reviews, 173 (1-4): 5-47 ⇧
9. 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 ⇧
10. May, H. D. (2008): A Pervasive Electric Field in the Heliosphere. IEEE Transactions on Plasma Science, 36 (5): 2876-2879 ⇧
11. Zel'dovich, Y. B.; Raizer, Y. P. (1967): Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena. New York: Academic Press ⇧
Chelyabinsk Bolide / Trails © Charles Chandler
A recent event introduces a new form of data — the Chelyabinsk bolide left twin trails. The trails were relatively symmetrical, started simultaneously, and remained split even after a dramatic reduction in the size and spacing of the trails.
Figure 1. Twin trails left by meteoroid in Chelyabinsk, Russia, 2013-02-15.
Figure 2. The start of the smoke trails, courtesy Nikita Plekhanov.
Figure 3. The end of the smoke trails, courtesy Nikita Plekhanov.
This has never been observed before, so the topic is open to speculation. While two trails could have been caused by two fragments, they would not have both undergone the same dramatic reduction in size at the same time, nor would they have gotten closer together at precisely the same time. So this can only be evidence of one bolide that had some sort of bipolar process.
Figure 4. Spherical object moving in a medium with a density gradient develops a spin.
One idea, which was posted to the Internet before this event, offers a possible explanation. A meteoroid entering the atmosphere at a shallow angle will start spinning, due to the density gradient in the air. (The air underneath is denser than the air overhead. See Figure 4.) The difference might be slight, especially for a meteoroid only 17 meters in diameter, and high in the atmosphere, where the density gradient isn't steep. But the detached bow shock is much larger, and if there is more friction on the bottom than on the top, the meteoroid will "roll" through the air, developing an angular velocity no greater than the meteoroid's overall velocity (i.e., 18 km/s for the Chelyabinsk bolide). If the meteoroid is charged, and if it is spinning, it's a dynamo, generating a solenoidal magnetic field. And just as the Earth's magnetic field deflects charged particles toward the poles, the meteoroid's far more powerful magnetic field might deflect charged particles toward its axis of rotation (parallel to the ground, and perpendicular to the direction of travel). This means that electrons, split off by the detached bow shock, and attracted to the positively charged bolide, might have been directed by the magnetic field into the bolide at the solenoidal poles.
For the sake of clarity, the following diagrams illustrate the various constructs.
Figure 5. Without electric charges taken into account, the deceleration should be dramatic.
Figure 6. Inertial charging creates a positive sheath that could create a Coulomb explosion.
Figure 7. Top view of a bolide "rolling" across the atmosphere.
The only model capable of producing twin smoke trails is the last one. The sudden appearance of smoke trails in Figure 2 would then indicate that the bolide had begun rolling as it entered the steeper density gradient in the lower atmosphere, and the angular velocity had achieved the threshold for a magnetic field strong enough to strip electrons out of the bow shock, which otherwise would have trailed away in the coma, but began to get directed into the bolide. Once the arc was struck by these electrons, smoke was produced. We can also note that the smoke trails waver up and down at a regular rate. Perhaps once the arcs were struck, their footpoints on the bolide remained the same, which might have been on the windward side of the bolide when they were first struck, but which moved to the back and then to the front again as the bolide rotated, leaving a smoke trail in a cycloid curve.
Chelyabinsk Bolide / Flashes © Charles Chandler
Video data were also collected at Chelyabinsk, one of the best of which was this one, taken by a dash-cam at the intersection of Beyvelya and Professora Blagikh streets (55° 13' 16" N, 61° 17' 45" E). The clock on the dash-cam wasn't correctly set, since its time-stamp starts at 2012-12-31, 18:30:40, though the event actually occurred on 2013-02-15. Regardless, the time-stamps are referenced in Figure 1 to more surely identify the frames in question. The video has a resolution of 480 × 360 pixels, at 30 frames per second, with good color depth.
The video captured several flashes of light, and the smoke trails left by the bolide, giving us a lot more information about the distinctive processes at work. Figure 1 shows 2 frames per second, over the relevant 16 second span. The largest flash occurred in frame 630. Determining the actual location of the bolide relative to the characteristics of the trail it left cannot be done just from that frame, since it was almost completely saturated. So the position was interpolated from previous and successive frames.
The selected frames were aligned horizontally by a common reference point (i.e., the leftmost of two smokestacks, visible in frame 346).
A high-contrast copy of the aligned frames was generated on which to do the photogrammetry.
The green curve passes through the observable locations of the bolide. If the bolide was traveling at a consistent speed, the positions should have all fallen on a straight line. But it was surely decelerating. Moreover, the video was taken from a moving car; the car went around a turn; and the dash-cam had a fish-eye lens, distorting the geometry. Regardless, the curve is smooth enough that additional mathematical analysis isn't necessary for a reasonable interpolation.
At the end of the sequence (i.e., frame 754), a trail of highly active plasma is still glowing brightly. The blue curves project the location of this plasma back to frame 630.
In frame 630, the red dashed line denotes the path of the bolide, based on its vertical position in previous and successive frames.
The intersection of the red dashed line and the green line is the location of the bolide in that frame.
This shows that the brightest flash occurred when the bolide was at the end of the thickest portion of the plasma trail.
We can also note that the bolide flared up again (i.e., frame 660), leaving a smaller trail of glowing plasma. But the color shift in frame 673 is suggestive of a lens flare, meaning that the brighter flash occurred once again when the bolide was done expelling plasma. So it wasn't that the brightest flashes created the trails of glowing plasma. Rather, the brightest flashes were events that terminated the process that was expelling the plasma.
Figure 1. Photogrammetry of the Chelyabinsk bollide.
These data can then be correlated with still images taken a couple of minutes later.
Figure 7. The bolide's trail shortly after its passing. (AP Photo/Chelyabinsk.ru)
This is interesting because the large step-down at frame 630 shows no evidence of an explosion, nor does the end of the trail at frame 673. This suggests that there was a physical separation between the flash and the smoke-producing process. If the bolide disintegrated due to Coulomb forces inside an ionized bow shock, the flashes were at the edge of the bow shock. This would not have disrupted the smoke coming off the bolide itself. (See Figure {5} and Figure {6} {above}.)
We should also acknowledge that any net charge is around the outside of an object. For this reason, a Coulomb explosion can reduce the mass of an object, without thoroughly destroying it. Obviously, this bolide maintained its general form and processes through the largest flash, which is consistent with a Coulomb explosion, and not with disintegration due to internal hydrostatic pressure.
Previous Events © Charles Chandler
Something similar might have been observed previously, in two cases (2011-06-29 over Mexico, and 2011-08-25 over Peru), but no information other than the amateur videos has been located, so these objects have not been positively identified as bolides. Some of the cases reported as possible bolides were actually just vapor trails from commercial airliners.
And here's another one from 2003-01-28:
This might also help solve one of the riddles concerning the most famous bolide of all — the one that exploded over Tunguska, Siberia (1908-06-30). The effects of the airburst formed a butterfly shape, which has defied explanation in Newtonian terms.1 But if arc discharges were drilling in from the sides of the bolide, and both exploded at the same time, this is the pattern that would have been created.
Figure 8. Direction of shock wave from the airburst over Tunguska.
- Why do [shock waves] get detached from the objects themselves, and stand off by quite a distance?
- By Newtonian standards, the shock front should never become "detached" from the object, and molecular rebounds should be fully absorbed in the first dozen collisions, producing an extremely thin buffer between the object and the oncoming air.
- Maybe the shock front isn't a fluid dynamic phenomenon, but an electrostatic one.
- High-velocity atoms in the approaching air are getting embedded in the boundary layer, stripped of their electrons, and therefore building up a positive double-layer around the supersonic object.
- The greater the speed, the thicker this positive double-layer, with electrostatic pressure pushing against the hydrostatic pressure of the oncoming air.
- For impacters getting into the thicker atmosphere, the implication is that this "detached positive double-layer shock front" might be highly charged, and therefore, might be responsible for an enormous amount of electrostatic repulsion within the supersonic object.
- In other words, if the meteor is surrounded by a layer of highly charged air, the air is going to suck all of the electrons out of the meteor.
- Then the whole thing will come unglued.
- The absence of valence electrons will weaken the crystal lattice of the solid object, and electrostatic repulsion will generate an outward force that wasn't there before.
- Once the meteor disintegrates into smaller pieces, the friction goes up exponentially, as that is a function of surface area, which is much greater for a bunch of small pieces than it was for one big piece.
- The increase in temperature adds hydrostatic pressure to the existing electrostatic pressure, and ba-boom!