ESA Scientists Listen to the Leonids
20 Nov 2001
(Source: European Space Agency)
ESA Science News
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 trails."
"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 destroyed."
"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
USEFUL LINKS FOR THIS STORY
[Image 1: http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=12&oid=29005&ooid=29008]
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: http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=12&oid=29005&ooid=29009]
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.