Some radio observations of the 2010 Geminids Meteor Shower
By David Knight
The Geminid meteor shower occurs when the Earth ploughs into the dust trail left by the asteroid 3200 Phaethon. Most meteor showers are associated with comets; but Phaethon has a highly elliptical 1.4 year orbit, causing it to cross the orbits of Mercury, Venus, Earth and Mars, and is therefore comet-like. Since Phaethon passes close to the Sun, it has lost any water it might have had, and so has no visible tail; but extreme thermal cycling is thought to have riddled the object with micro-fractures, thereby creating a cloud of rocky ejecta.
Geminid meteor activity peaks on December 13th - 14th each year. The rate of visible events has been high in recent years. peaking at around 100 / hour. The refractory nature of the particles emitted by Phaethon gives rise to long-lasting metoer trails and strong radar head echoes.
The radio spectrograms shown below were obtained during a public demonstration given at the NLO on the 14th December 2010 between 19:30 and 22:30 UTC. The radio receiver was an Icom IC275H (<1dB noise figure) in USB mode, tuned to 143.049 MHz (nominal, subject to calibration error) in order to translate signals on the Graves transmitter frequency into audio tones in the region of 1KHz (for more background info. see accompanying meteor detection article). The antenna was a 2m 6-element quad (ca. 9dBi) pointing SE, connected with a mix of RG213 and FSJ4-50 cables to give an overall noise figure of about 3dB.
The receiver was placed in the Members room and connected to a laptop computer on the podium in the lecture theatre via a 25m long audio lead. This arrangement worked, but was susceptible to interference from the GB2NLO HF radio station (some ferrite rings might have fixed it, but there was no time for troubleshooting). The spectrograms and audio recordings were made using the computer sound card and DL4YHF's 'Spectrumlab' software. Time setting was via the Windows 7 Internet time client, and should be within about ±1 sec. Subsequent processing to produce the audio clips given below was carried out using the sound editor 'Audacity'.
The audio output of the receiver was also connected to a wireless headphone base-station (865MHz) in order to seek visual corroboration of radio events, but unfortunately conditions were unfavourable on the night due to cloud.
The dashed line on the spectrogram above is the Graves transmitter direct, the keying 3.2s strong, 19.2s weak being associated with radiation-pattern switching. The Doppler shift for 143.05 MHz is about 347Hz / Mach, so in this spectrogram we see strong head echoes extending beyond Mach 4.7. The un-shifted frequency is around 950Hz in this spectrogram. The brigtening at twice the main trace frequency is due to audio distortion (second harmonic). Such audio overloading can be reduced by turning down the AF gain of the receiver; but the settings had been optimised for reception at the author's home and (with computer and receiver in differenct rooms) it would have created a distraction to re-optimise them during the demonstration.
Some doubling of the Graves signal is visible in the spectrogram above, but the absence of a 2.7s delay precludes Moon echo. Aircraft at ≤40000 ft (≤12.2Km ASL) are generally over the horizon for observers in the UK, but the signal is well within the aircraft doppler range.
mp3 audio clip 20:51:42 - 20:52:00 . mp3 audio clip 20:52:13 - 27 .
mp3 audio clip 20:57:07 - 16 . mp3 audio clip 20:57:52 - 58 . Good head echo.
mp3 audio clip 21:07:43 - 53 .
mp3 audio clip 21:19:53 - 59 . mp3 audio clip 21:20:12 - 19 .
mp3 audio clip 21:29:47 - 53 . Long head echo with pronounced overshoot, but no trail echo. This is probably due to a satellite in low Earth orbit.
Interference from GB2NLO from 21:33:34 -44 (probably pickup in the 25m audio cable between receiver and computer). mp3 audio clip 21:33:47 - 21:34:08 . Long lasting meteor trail.
mp3 audio clip 21:36:29 - 39 .
mp3 audio clip 22:02:08 - 21 . Two meteors.
mp3 audio clip 22:21:07 - 13 . Double head echo - see expanded trace below. mp3 audio clip 22:21:43 - 48 .
The event occurring at about 22:21:09 in the preceding spectrogram is here shown expanded in a spectrogram produced from the audio clip by Felix Verbelen. As Felix pointed out: it is in fact a double head echo, the two meteors being separated by less than 0.2s. The slopes are respectively -2193 and -2367 Hz/s, with an uncertainty of about 100 Hz/s.
mp3 audio clip 22:25:55 - 22:26:02 . Good head echo.
Shown below is the receiver output level from 20:42 to 22:28. Most of the spikes are meteor strikes.
Anyone wishing to obtain access to the original data, please contact the author:
