Recorded for centuries, people can hear and see meteors nearly concurrently. Electromagnetic energy clearly propagates at the speed of light and converts to sound (called electrophonics) when coupled to metals. An explanation for the electromagnetic energy source is suggested. Coma ions around the meteor head can easily travel across magnetic field lines up to ~120 km. The electrons, however, are tied to magnetic field lines, since they must gyrate around the field above ~75 km. A large ambipolar electric field must be generated to conserve charge neutrality. This localized electric field maps to the E region then drives a large Hall current that launches the electromagnetic wave. Using antenna theory and following, a power flux of over 10
8 W/m2 at the ground is found. Electrophonic conversion to sound efficiency then needs to be only 0.1% to explain why humans can hear and see meteors nearly concurrently.

1. On the electrophonic generation of audio frequency
sound by meteors
Michael C. Kelley1,2
and Colin Price3
1
School of Electrical and Computer Engineering, Cornell University, Ithaca, New York, USA, 2
Now Professor Emeritus,
3
Department of Geosciences, Tel Aviv University, Ramat Aviv, Israel
Abstract Recorded for centuries, people can hear and see meteors nearly concurrently. Electromagnetic
energy clearly propagates at the speed of light and converts to sound (called electrophonics) when coupled
to metals. An explanation for the electromagnetic energy source is suggested. Coma ions around the meteor
head can easily travel across magnetic field lines up to ~120 km. The electrons, however, are tied to magnetic
field lines, since they must gyrate around the field above ~75 km. A large ambipolar electric field must be
generated to conserve charge neutrality. This localized electric field maps to the E region then drives a large
Hall current that launches the electromagnetic wave. Using antenna theory and following, a power flux of
over 10À8
W/m2
at the ground is found. Electrophonic conversion to sound efficiency then needs to be only
0.1% to explain why humans can hear and see meteors nearly concurrently.
1. Introduction
For many thousands of years, humans must have noticed that some meteors could be heard nearly as soon as
they were seen. To our knowledge, the first published account of this phenomenon was made by Halley
[1714] and, in that same century, by Blagdon [1784]. Olmsted [1834] followed with the first scientific study
during the 1833 Leonids meteor storm.
The theoretical work presented here is based on experimental studies by Hawkins [1958], Keay [1980a,
1980b], Keay and Ostwald [1991], Keay [1992a, 1992b], Keay and Ceplecha [1994], Garaj et al. [1999],
Price and Blum [2000], and Zgrablić et al. [2002]. The Zgrablić et al. [2002] article has many useful references.
Price and Blum [2000] measured the magnetic field on the ground during the 1999 Leonids event, which they
reported as having an integrated ELF/VLF magnetic field of several picoteslas. Earlier, Beech et al. [1995]
reported VLF detection due to fireball meteors. The sounds recorded by Zgrablić et al. [2002] at around
50 Hz correspond to a sound energy flux of 10À8
W/m2
. For reference, the threshold for human hearing is
10À12
W/m2
[Durrant and Lovrinic, 1984].
Recently, Spalding et al. [2017] 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. This is an intriguing idea and was verified by
observations made of fireball meteors. However, these observations were made at meteor brightness
levels of À12. The report by Drummond et al. [2000] and by Kruschwitz et al. [2001] discussed meteor
brightness levels of only À1 to À8, some of which were heard by Air Force observers at the Starfire
Optical Range, located at Kirtland AFB near Albuquerque, NM [Drummond et al., 2000]. 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. [2017],
ELF radio waves were indeed detected during the Leonids storm of 1998/1999 by Price and Blum [2000] for
nonfireball meteors.
2. Previous Theoretical Work
The first serious attempt to explain this phenomenon was published by Keay [1980a] and developed further
by Bronshten [1983a, 1983b]. In this theory, the magnetic field behind the meteor becomes twisted by turbu-
lence in the wake. But this only applies to slow bolides, which can reach 20 km altitude. Keay [1992b]
suggested explosive disintegration of a fireball as the source of VLF. Beech and Foschini [1999, 2001]
KELLEY AND PRICE METEOR-GENERATED AUDIO FREQUENCY SOUND 2987
PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2017GL072911
Key Points:
• Meteors can create audible sound at
the ground that is nearly concurrent
with visual detection.
• ELF radiation from the ambipolar
electric field mapping to the E region
causes electrophonic sound.
• A model for current flow and an
antenna calculation show that EM
energy can generate audible sound.
Correspondence to:
M. C. Kelley,
mck13@cornell.edu
Citation:
Kelley, M. C., and C. Price (2017), On the
electrophonic generation of audio
frequency sound by meteors, Geophys.
Res. Lett., 44, 2987–2990, doi:10.1002/
2017GL072911.
Received 1 FEB 2017
Accepted 21 MAR 2017
Accepted article online 23 MAR 2017
Published online 9 APR 2017
©2017. American Geophysical Union.
All Rights Reserved.

2. proposed a charge separation
driven by a shock wave. Another
charge separation mechanism was
suggested by Bronshten [1991]
during ablation of the meteor.
These models are relatively complex.
Here we discuss a much simpler idea
and test it using the full wave calcula-
tion of Yagitani et al. [1994], who
used it to explain electromagnetic
waves launched by a powerful HF
radio wave impacting this same
altitude region. To our knowledge,
all models involve propagation of an
electromagnetic wave from the
meteor deposition region, which
couples to an audible sound wave
when it interacts with a metal object
at ground level in a process called
electrophonics. The reentry of a large
satellite did indeed generate both
electromagnetic and audible sound
at ground level [Verveer et al., 2000].
This is the only known simultaneous
observation of a body entering the atmosphere and creating both electromagnetic and audible signatures.
We argue that the electrical current associated with the charged coma around the head of a meteor is
responsible for the electromagnetic pulse observed on the ground. Hence, audible sound by the electropho-
nics 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., 2017]. At meteoroid ablation heights, ions and electrons are cre-
ated by a variety of processes, mostly by impact ionization by atoms boiling off the meteor and some possibly
by UV radiation. The ions move freely across the magnetic field and hence follow the meteoroid flight. The
electrons are magnetized and cannot move easily with the ions in the D and lower E regions. To keep ni = ne,
an electric field builds up such that the ion current perpendicular to B equals the electron current. Parallel to
the magnetic field, the electrons can easily match the ion/meteor velocity in that direction. Thus, the ions and
electrons both move along the meteor track at the same velocity as the meteor. The resulting plasma density
pulse is responsible for the radar head echo, which has been studied for over 50 years [Evans, 1965]. This
dense ionization, which causes the radar head echo, is localized within a few meters behind the meteor, as
opposed to the meteor trail, which causes classic radar echoes when the trail is oriented perpendicular to
the radar beam [McKinley, 1961; Jones et al., 1988; Pellinen-Wannberg, 2005, and references therein]. The
meteor trail is much longer (by many kilometers) and has a much lower plasma density than the coma near
the meteor head, which causes the head echo.
Figure 1 shows the geometry in the plane containing the magnetic field, B, and the Cowling current system,
which we argue launches the electromagnetic wave. The component of the meteor velocity perpendicular to
B, V⊥ m, is out of the page. To satisfy charge neutrality, the ambipolar electric field, E⊥ m, is into the page. This
electric field component maps along the conducting magnetic field lines to the highly conducting E region
[Farley, 1959].
Using the electron conductivity in the D region [Kelley, 2009], we can solve for the ambipolar E field using the
relation
Ex ¼ nceVm=σepð Þ^x;
where Ex is the electric field perpendicular to B (see Figure 1 for the geometry we use), Vm is the meteor
Figure 1. A schematic of the interaction between the meteor and the atmo-
spheric/ionospheric system Vxm is the component of the meteor velocity
perpendicular to the magnetic field (B) and Exm is the needed ambipolar
electric field for quasi-neutrality, which maps up to the highly conducting
lower ionosphere. As shown by Haldoupis et al. [1996], if the source electric
field is localized (as it is due to the finite meteor track), this field generated a
Cowling electric field Eyc into the page, which drives an intense Cowling
current Jxc out of the page. This current source creates the audio frequency
electromagnetic field.
Geophysical Research Letters 10.1002/2017GL072911
KELLEY AND PRICE METEOR-GENERATED AUDIO FREQUENCY SOUND 2988

3. velocity, nc is the coma plasma density, ^x is the unit vector parallel to the component of Vm perpendicular to
B, and σep is the Pedersen conductivity. The coma density for a large meteor must be greater than 1016
mÀ3
since gigahertz radars see a head echo. Substituting yields Ex > 106
V/m. This result is much greater than the
critical field for neutral atmospheric discharge that is known to occur over thunderstorms, creating the
phenomenon of Sprites [Pasko et al., 1997]. For the purpose of this calculation, we suggest that the E field
saturates at the critical value for discharge, which, at meteor ablation height (between 80 and 100 km), is
2–100 V/m [see Pasko et al., 1997, and references therein]. This hypothesis yields the minimum value of Ex,
which easily maps up the magnetic field lines to the E region [Farley, 1959] where it drives a large Hall current.
Although the Cowling current is usually discussed in the equatorial region, for a localized region at other
latitudes, there is also a Cowling effect [Haldoupis et al., 1996], which amplifies the electric field Ey = (1–10) Ex.
Yagitani et al. [1994] studied the effect of an external energy source on the currents flowing in the iono-
sphere. This is accomplished by absorption of HF high power radio waves incident on the ionosphere, waves
that are known to launch ELF radio waves by modulating the Earth’s ionospheric current system. Their
analysis involves a full wave analysis of the currents that is appropriate for comparison to our hypothesis.
Their approach involves a current system near 70 km where the radio waves are absorbed [Robinson, 1989]
and exposed to an external electric field of 5 mV/m due to magnetospheric electric fields. The pulsed heater
modulates the conductivity, causing the ELF generation. They used the beam width of the typical transmit-
ting antenna at ionospheric heights, which is about 40 km [see Robinson, 1989]. Using the ambient electric
field and the conductivity modulation of the current, Yagitani et al. found a current system of 0.2 A, leading
to a magnetic field at the Earth of 0.1 pT. The horizontal size of the heated region is similar to the track of a
meteor traversing the lower ionosphere.
In our case, the currents are at 100 km and driven by a much larger electric field, which is a minimum of 2 V/m.
In turn, this field is magnified by the Cowling factor [Kelley, 2009], which is between 2 and 20 [Haldoupis et al.,
1996]. Since the sounds from meteors have been detected at midlatitudes, we use the conductivities
measured over the Arecibo Radar (courtesy of R. Burnside and published in Kelley [2009]). The height-
integrated Pedersen conductivity in the E region is therefore about 0.1 Ω. Using a moderate Cowling factor
[Ey = (
P
H/
P
P) Ex] of 6, the current in the E region is thus [(
P
H/
P
P)2
+ 1)]
P
P/x = 37
P
PEx, and following
the Arecibo data, the current/meter is ~8 A/m. The width of the ionized coma is several meters, so the current
is about 20 A. This leads to a ground magnetic field of (70/100)2
× (20/0.2)2
× (0.1 pT) = ~1 nT. This idealized
meteor track result is completely consistent with the observations of Price and Blum [2000] for their randomly
oriented meteor trail of various sizes and orientations. We note also that the choice of 2 V/m for the ambipolar
electric field is a lower limit to the field, based on our idea that it is limited by the breakdown voltage.
The Poynting flux corresponding to this magnetic field is a few times 10À8
W/m2
. This is comparable to the
sound measurements made by Zgrablić et al. [2002]. More importantly, the threshold flux for human hearing
is 10À12
W/m2
, so the efficiency of the electrophonics effect need only be ~0.1%.
The ambipolar field develops extremely fast and creates a pulse. The fundamental frequency of the electric
field pulse is related to the few meter size of the coma and the meteor velocity of about 40 km/s. The pulse
is thus a few milliseconds long and propagates across the E region for about 1 s. The frequency range is thus
1 Hz to 500 Hz, in good agreement with the Price and Blum data.
We have no idea how large the meteors were that created the magnetic field measured by Price and Blum
[2000]. The meteor actually heard by young airmen in the Leonids storm [Drummond et al., 2000] was from
a meteor with an estimated mass of 560 g, but less bright meteors were also heard. So it might be the case
that just such an ELF/VLF electromagnetic power flux is needed to create a sound strong enough to be heard
by humans.
3. Conclusions
We propose a relatively simple explanation for the fact that humans can occasionally hear a meteor as soon
as they see it. This phenomenon has been known for millennia and entered the scientific discussion as far
back as 1714. Unique in this model is a direct current driven by the head echo plasma, which generates an
electric field antiparallel to the meteor track. This, in turn, generates a Hall current that extends to the local
E region, which is quite large and generates the ELF/VLF signal and propagates to the ground. Then,
Geophysical Research Letters 10.1002/2017GL072911
KELLEY AND PRICE METEOR-GENERATED AUDIO FREQUENCY SOUND 2989

4. metallic structures convert this audio frequency EM wave to sound waves by the electrophonics mechanism.
The conversion efficiency for electrophonics need only be 0.1% to exceed the human hearing threshold. Our
EM radiation prediction is quite sufficient to generate the electromagnetic signal detected by Price and Blum
[2000] in a Leonids storm.
Finally, it is well known that the aurora creates intense ELF and VLF radio waves that reach the ground. We
think that it is quite possible that the many reports of audible aurora by high-latitude, indigenous peoples
are, in fact, true. Hopefully, this paper will inspire research on this topic.
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KELLEY AND PRICE METEOR-GENERATED AUDIO FREQUENCY SOUND 2990
Acknowledgments
Work at Cornell was supported by the
School of Electrical and Computer
Engineering. The work at Tel Aviv
University was unfunded. The data sup-
porting our conclusions are available in
the published literature. This paper is
primarily theoretical.