[meteorite-list] ESA Scientists Listen To The Leonids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:13 2004
Message-ID: <200111201802.KAA12402_at_zagami.jpl.nasa.gov>

ESA Science News

20 Nov 2001

ESA scientists listen to the Leonids

Although leaden skies over northern Europe last weekend were a
disappointment to most avid meteor watchers, scientists at the European
Space Research and Technology Centre (ESTEC) in the Netherlands are
happily recounting their successful Leonid observations. The reason for
their delight is that, despite the cloud blanket that obscured the
familiar glowing meteor trails, they were able to detect numerous radio
echoes from the famous swarm of shooting stars.

Building on their successful radio observations of last year's Leonids,
four scientists from ESTEC -- Jean-Pierre Lebreton, Udo Telljohann,
Trevor Sanderson and Olivier Witasse -- decided to carry out another
campaign 17-19 November, during the shower's annual return visit.

Although the Leonids are just one of many meteor showers that take
place each year, the dust particles can produce a spectacular display
as they are incinerated by friction in the upper atmosphere. This is
partly because of their large numbers and partly because the Earth and
the debris from Comet Tempel-Tuttle plough into each other almost
head-on. This results in extremely high speed collisions at more than
70 km/s (over 200 times faster than a rifle bullet).

Using receivers and antennae belonging to the amateur radio club at
ESTEC, the group attempted to pick up echoes of radio signals that
bounced off the ionised trails left by the incoming meteors.

"During the daytime, radio signals bounce off a layer in the atmosphere
known as the ionosphere," explained team leader Jean-Pierre Lebreton.
"However, during the night, the ionosphere disappears. This means that
high frequency (HF) radio signals will reach much higher in the
atmosphere -- high enough to bounce off the glowing, ionised trails
left by the Leonids."

"We had a special arrangement with a company that provides transmitter
services," said Udo Telljohann. "They kept the transmitter switched
on during the night (when it is normally switched off), and continued
sending BBC radio signals at a frequency of 17.64 MHz. This enabled
us to detect brief echoes of the BBC transmission from the meteor

"We detected two kinds of echoes," he explained. "Short-lasting echoes
(type 1), typically lasting 1 second, have a large shift in frequency
or Doppler shift (*). They are produced by high speed meteors during
their entry into the upper atmosphere."

"We also detected long-lasting echoes that continued for several
seconds to more than a minute," he added. "These type 2 echoes come
from the ionised clouds deposited in the upper atmosphere at about
90 km altitude, and they have much smaller Doppler shifts. The
clouds slowly drift with the wind (typically tens of metres per
second) until they disappear."

"We have many examples of type 1 radio echoes, although not all of
them came from the Leonids -- some would have been sporadic 'normal'
shooting stars," said Lebreton. "However, we are pretty sure some
of the echoes are coming from the Leonids as we saw an increase of
activity at the time of the Leonid peak."

"We also got lots of echoes from ionised clouds over several nights,
in particular during the night of 17/18 November," he said. "Some
may correspond to the Leonid fireballs that were observed at the
same time in Germany."

"Our radio spectrogram shows the shift in frequency of the radio
signal caused by the meteor that is entering the atmosphere at more
than 10 km/s," said Lebreton. "This is shown by a Doppler shift of
the carrier frequency line (straight line at 1 kHz) by more than
1 kHz. The signal then drops by 1 kHz over a period of about one
second, showing that the meteor had suddenly slowed as it was

"This is a very exciting result," said Lebreton. "By using this new
method, we have been able to detect the meteors while they were
still entering the upper atmosphere and study what happens to them."

"We still have a lot of analysis to do, but we are already looking
forward to carrying out more ambitious studies next year," he
concluded. "For example, at present we can only record their speed
along our line of sight, but with three stations we could fix their
positions and calculate their speeds very accurately."

(*) The Doppler shift is a very small change in the frequency of the
radio signals that is caused by the continuously changing position
of a meteor. The faster the meteor is moving relative to the
observer, the greater the frequency of the Doppler shift.

For more information please contact:

Dr. Jean-Pierre Lebreton
Huygens project scientist
ESTEC, The Netherlands
Tel: +31 71 5653600


* Listening to meteors- some background information
* Listening to meteors - some early results
* Leonids 2001 - The Adventure Down Under
* What is the Doppler effect?


[Image 1:
Listening to meteors -- how the technique works

During the daytime, high-frequency radio waves are reflected back to
Earth by the ionosphere. A meteor entering the Earth's atmosphere
is slowed down and deposits most of its ionised matter below the
ionosphere, at an altitude of about 90 km, in the process creating a
small ionised cloud which drifts slowly with the atmospheric winds
until it disappears. Radio waves reflect off this moving cloud
creating echos (known as type 2) which are detected by the receiver.

At night-time there are fewer and less dense ionospheric layers and
the high-frequency radio waves can escape into space. These can
intercept high speed meteors as they enter the upper atmosphere and
produce short-lasting echoes (type 1), typically lasting 1 second,
which have a large shift in frequency.

[Image 2:
An echo from a high speed meteor as it plunges through the upper
atmosphere. This radio spectrogram shows the shift in frequency of
a meteor that is entering the atmosphere at more than 10 km/s. This
is shown by the Doppler shift of the carrier frequency (the straight
line at 1kHz) by more than 1 kHz. The signal then drops by 1 kHz
over a period of about one second (characteristic of a type 1 echo),
showing that the meteor had suddenly slowed as it was destroyed.

Listen to the sound of the meteor: in this soundtrack
[http://sci2.esa.int/leonids/leonids2001/audio/multi.wav] you can
hear the meteor echo (about 2 seconds into the track) as the radio
waves reflect off it.
Received on Tue 20 Nov 2001 01:02:50 PM PST

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