Tuning in to Radio Leonids - broadcasting the shooting stars symphony
From 14-18 November a team of ESA scientists will listen to the sound of Leonid meteors as they silently sweep across the night sky.
Instead of using cameras or simply observing with the naked eye, their aim is to repeat an experiment, first performed during last year's Leonids meteor shower, which uses digital signal processing to make audible the impact of the myriads of shooting stars as they hit the Earth's upper atmosphere.
During the nights of 14 - 18 November, as the annual Leonid meteor shower reaches its peak, the group from ESA's Research and Scientific Support Department at ESTEC in the Netherlands, will be glued to a radio receiver and their computer screens.
"The ionised meteor trails act like mirrors and reflect high frequency radio signals from stations that are below the horizon," explained ESA's Jean-Pierre Lebreton, "so we're going to listen to suitable signals from various radio stations around the world."
"The night time is much less effective in reflecting high frequency signals in the range above 10-20 MHz. The signals at those frequencies escape into space, so the high frequency radio stations are usually shut down after dark. However, meteor trails can also reflect high frequency radio signals for brief periods at night," explained Dr. Lebreton. This means that, at night, instead of no radio reception at all, the team can pick up brief transmissions every time they bounce off a meteor trail.
Figure 1: During the daytime, radio signals transmitted from Earth are reflected back by ionised layers of the atmosphere acting like mirrors. Ionised meteor trails also reflect radio signals, and since they are moving they produce a doppler-shifted signal, an echo. At night-time, the reflecting layers are fewer and less dense, so most of the high-frequency radio signals can escape into space. Echos are still produced by meteors intercepting the radio signals.
"For our experiment during night time, we have come to an arrangement with Merlin Communication, the service provider for the BBC, to continue their transmissions at 17.64 MHz," said Lebreton. "They will switch on one of their transmitters every night between 14 and 18 November. We will then use the short-lived meteor ionisation trails as mirrors to reflect the radio signals. In this way, we hope to be able to count the meteor echoes."
Not only will this technique enable the scientists to make an accurate count of meteors, but it will provide information on the winds in the upper atmosphere.
"Because upper atmospheric winds move the `mirror' a little bit, it induces a very small Doppler shift in the signal we receive," explained Lebreton. "These minute changes in the signal frequency allow us to separate the reflection from the meteor and the main signal. The fixed frequency of the high frequency carrier wave and the small changes caused by the reflection will be automatically recorded as dynamic spectrograms - in other words we will have a visual record of the audio signals."
Figure 2: Meteor activity is hard to miss! In these images the central horizontal line is the carrier radio frequency. 'Blips' on the graph represent the echos which are caused by reflections of radio signals by the meteor clouds. From left to right: low meteor activity during the daytime, high meteor activity during the daytime, and high meteor activity at night.
This recording of the 2000 Leonids shower was made by Jean-Pierre Lebreton and his colleagues.
"Anyone tuning in their radio to the BBC frequency can try this experiment for themselves," said Lebreton. "It should be possible to pick up the echoes - each lasting a few seconds - from all over Europe. This is the best way to learn about meteor showers - it works day and night, and in all weathers. It should be possible to listen to the radio broadcast and hear echoes that will last for more than a few seconds. During the peak activity on 18 November, we may even be able to listen continuously to the radio programme!"
"We will analyse the signals in real time, and also record them on digital audio tape so that we can analyse them later," said Lebreton. "Last year we were also able to record the sound of the Leonids shower."
So why are they going to all this trouble?
"We're hoping to complement the optical observations that are planned by some of our colleagues," said Lebreton. "And we wanted to do something that would be of interest to the public. One of the advantages of radio observing is that meteors can be detected when skies are cloudy or during daylight. Radio observing has some advantages at night, too. The human eye can only see shooting stars brighter than 6th magnitude, but radio methods can detect meteors that are at least 5 times dimmer."
"There are also some other scientific applications of this technique, but for the moment we are concentrating on fine-tuning our technique," Lebreton concluded.
Other scientists from ESA's Research and Scientific Support Department who will be involved in this radio campaign are: Trevor Sanderson, Udo Telljohann, Olivier Witasse and Andrea Toni.
How to tune in to the shooting stars
The experiment
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The Doppler method can be tried by anyone with a good shortwave receiver and a PC. Suitable software and a description of the method can be downloaded from the related link to "Instructions for Doppler experiment" at the Portugese Centro de Observação Astronómica no Algarve. The software uses the sound card of the PC to analyse the signal. All that is needed is a connection from the headphone output of the receiver to the PC's sound card input. Download the software, install, read the help file and you are ready to go. Tune to a station around 500 km or so away. Switch to SSB mode, and start the software. All you need now are the Meteors!
A simpler experiment
For this you use your FM receiver with an external aerial. Try to find a station a long way away (that's the difficult bit, as usually a nearby station gets in the way). Under normal circumstances the transmission should be difficult or impossible to detect, but when a meteor intervenes the signal jumps over the horizon and a brief fragment of the transmission can be heard. Depending on the type of transmission, it might sound like a tone, a fragment of music or voice, or simply noise. Contact lasts for as long as the meteor train persists, usually from 100 milliseconds to a few seconds.