Electrophonic Meteors
Meteors are what are commonly known as
"shooting stars". However, these are not stars at
all. Meteors are small grains of dust
left behind by comets as
they orbit the sun. Comets
are basically dirty snowballs that
heat up as they approach the sun. Some are periodic (like Halley
and Temple-Tuttle) returning every so often to visit us. As they
approach the sun they evaporate or sublimate, with tiny dust particles
ejected, remaining behind in space in the same orbit as the
comet. It may be hundreds or thousands of years before one of
these dust particles collides with a passing object such as the
Earth. And when this happens, we see these meteors burning up as
they enter the atmosphere. Larger particles and small rocks from
asteroids can result in huge
fireballs or bolides as they enter the
atmosphere, lighting up the whole sky [NASA movie].
For
centuries people have
claimed to
hear sounds while observing the large meteors (bolides) burning up as
they enter our atmosphere. But even Edmund Halley (from comet
Halley fame) in 1719 concluded that this was pure fantasy, since light
travels much faster than sound, and therefore there is no way that
anyone would be able to hear anything simultaneously as they see the
meteor tens to hundreds of kilometers away. But the anecdotes
kept coming in. Were these people really crazy, or imagig
things?
An Australian scientist Colin Keay suggested in the 1980s that perhaps
the meteors generate radio waves. As with our home radio, we hear
the news exactly at 8:00am no matter how far we are from the radio
station. The reason is that the radio waves carrying the
information travel at the speed of light to all of our homes, and then
our radio has a method of converting the radio waves (that we cannot
hear since they are electromagnetic waves) to sound waves (pressure
waves) that vibrate the speaker, and our ear drums. So if the
meteor sends out a radio wave together with the light we see, the radio
waves and the light would both arrive at the same time at the
observer. However, the radio waves then needs to be
converted to sound waves. Well this can be done by simply
vibrating any conduting object at the same frequencies of the radio
waves. This is how speakers produce sound in your home radio
system. Metal fences, glasses, hair, plants, etc. can all
be used as transducers to convert the radio waves to sound waves.
This conversion of electromagnetic waves to acoustic/sound waves is
known as electrophonics.
Now if the radio waves do exist, they need to be in the frequency range
that people can hear (20-20,000 Hz). This range of radio waves is
known as the extremely low frequency (ELF) and very low frequency (VLF)
range, and is also the range used for lightning research. Hence
my interest in this topic. If meteors do produce ELF/VLF
radiation as they enter the atmosphere, then perhaps our
sensitive
lightning detectors in the Negev desert (below) would be able to pick
up these signals. Below is also shown a dynamic spectrum of ourVLF
data for a 15 seconds period (x-axis) showing the frequency (y-axis) of
electromagnetic waves detected at our stations. The vertical
lines are from natural impulsive lightning sferics, while the
horizontal lines are mainly from man-made VLF transmitters used for
communications with submarines.
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During the 1999 Leonid meteor
shower (which occurs every year close to the 18/19 November), we were
ready for the challenge. We set up our instruments in the Negev
desert, as part of a large scientific experiment to study the Leonid
meteors called LeonidMAC.
The name of this meteor shower comes
from the region in the night sky (Leo constellation) where the meteors
enter the atmosphere. The meteors during the Leonids originate
from comet Temple-Tuttle that returns to visit Earth every 33
years. However, the meteors in any particular year can be from
visits of Temple-Tuttle hundreds of years ago since we do not pass
through the same part of the debris stream every year.
During the 1999 Leonids
we detected very brief (~10 msec) pulses of VLF radiation that did not
look anything like the normal lightning pulses we observe every
day. The pulses were longer, less impulsive, and actually had a
minimum in radiation in the 5kHz range, exactly where the lightning
radiation peaks. However, the pulses were very short relative to
the optical meteors themselves. Anyone who has seen a meteor
entering the atmosphere knows they can last up to 1 second or
more. The VLF pulses we found were much shorter. Were these
really from the meteors? So we decided to count them.
We knew that during the night of 18 November 1999 there was a huge peak
in optical meteor activity around 0200 UT. After counting these
strange ELF/VLF pulses for the night before the meteor shower (16/17
Nov) and the night of the shower (17/18 Nov) we found that the number
of these pulses tracked the optical counts very well (see below).
The night prior to the shower showed no significant changes through the
night, while during the meteor shower (black curve below) the ELF/VLF
counts reached a maximum, decaying together with the optical
counts. However, the ELF/VLF data showed another peak around 0045
UT not seen in the optical data. Could there have been another
weaker shower of meteors not bright enough to be seen, yet large enough
to produce radio waves? In addition, the ELF/VLF pulse counts
reached 15,000 per hour! Compare that with the 400 per hour observed
optically in the Negev. Could we be detecting many small meteors
that cannot be detected optically, or does each meteor produce a number
of pulses? This is still an open question.
But back to the
electrophonics. None of the observers out that night heard any
sounds while observing the Leonid meteors. There were also no big
bolides. However, we now think we have found the evidence that
can explain the reports of people hearing sounds while observing bright
shooting stars.
Another project we have worked on
in recent years is to see whether the meteors themselves may impact the
characteristics of the lower ionosphere (around 100km) on nights of
meteor showers. For this study we looked at the background VLF
noise levels (Vd values) to see if they change on nights of meteor
showers. The noise is generally produced by lightning
activity in our region, or even thousands of kilometers away from
Israel. What we noticed was that on nights of meteor showers, the
noise levels went down (Vd goes up). Why does the noise
decrease? We think that the meteors change the conductivity of
the upper atmosphere, resulting in more absorption of the distant
lightning signals before they arrived in Israel. The amount of
lightning activity did not change, but the VLF pulses arriving in
Israel did decrease due to the changing characteristics of the
Earth-ionosphere wavegiuide. Below we can see the plot showing
the value of Vd as a function of hour of day (x-axis) and
day of year (y-axis) for two meteor showers (red dots and dashed line
show peaks in optical observations of meteors). The plot shows
that the noise level changes are related to the meteor showers, while
the impact is large during nighttime than during daytime.
References:
Price,
C., and M. Blum. ELF/VLF emissions
detected from the Leonids ’99 meteorites, 2000: Earth, Moon and Planets, 82-83,
545-554.
Reuveni, Y., C.
Price,
Y. Yair and R. Yaniv, 2010: The connection between meteor showers
and VLF atmospheric noise signals, J.
Atmos. Electr.,
31 (1), 23-36.
Related websites:
University
of Kentucky
Australian
site
ELF/VLF meteor
signals
