Using a new Radio-Doppler technique to analyse the reflection of broadcast radio stations from meteors, scientists in ESA's Space Science Dept, in Noordwijk, the Netherlands and in the Centro de Observação Astronómica no Algarve (COAA) in Portugal were able to listen to the sound of the reflections from the recent Leonids meteor shower. The technique, developed by UK Radio Amateur Peter Martinez, uses DSP software to measure the minute changes in frequency of the carrier of a short wave broadcast station due to Doppler shift caused by the the motion of the meteor trail in the upper atmosphere. The method of course works both by night and day, and under cloudy skies, and was recently used with dramatic effect during the Leonids.

Figure 4. Spectrogram of a fireball exploding

The top spectrogram, taken in Portugal by COAA, shows a fireball exploding. Time is plotted along the x-axis, and frequency along the y axis. The straight line shows the signal from the carrier, which in this case was a shortwave station in the 15 MHz broadcast band. Each time a meteor impacts the atmosphere it leaves behind it a trail of ionisation which may last for a second or two. This trail reflects a part of the radio signal. The frequency of the reflected signal is Doppler shifted by a few Hz due to the changes due to motion of the trail caused by the upper atmosphere winds, or in this case by the expansion of the cloud. The very small vertical trace is a meteor. The slightly ragged shape of the trace is due to the different layers of the upper atmosphere moving in different directions. Also seen in the top left is the signature of an aeroplane as it slowly crosses the sky.

Figure 5. Spectrogram showing signatures of two fireballs

The second plot, taken in the Algarve by COAA on the morning of November 17, shows the signature of two fireballs disintegrating over the sky in Portugal. When the first fireball exploded, a cloud of ionisation was released, which drifted off in several directions. The three yellow patches are probably due to patches of ionisation in different layers of the atmosphere which drift in different directions, and therefore impart three different Doppler shifts to the reflected signal. The second fireball signature is much more complicated, and shows evidence of some stratified motion prior to the final disintegration. As well as displaying being able to display this data in a spectrogram, it is possible to listen to the signal reflected by the meteor. If the received signal is fed to a loudspeaker, a more or less constant tone, dominated by the carrier, is heard. By removing the tone of the carrier using a notch filter, only the sound of the reflection of the meteor is left. The next spectrogram is of a short 1.25 minute period of data taken around the time of maximum of the Leonid shower on the morning of November 18. The signal has been notched out, and all that is left is the sound of the Leonid. This sound is available as a .wav file. This is the sound of a small fireball exploding.

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