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September 21, 2017 a bright fireball over Holland was reported by hundreds of people. Several all-sky cameras recorded it as well as a special radio beacon set up to capture radio signatures of meteors - the BRAMS beacon at Dourbes, Belgium.

I'm puzzled by what I see in the radio spectrogram from one of the BRAMS stations at Ophain, Belgium : BRAMS radio spectrogram from Ophain The orange blob in the center is the radio signature of this fireball. The horizontal line is the beacon signal (0 Hz), time runs horizontally, frequency runs vertically, positive above and negative below the beacon line. What is displayed are Doppler shifts.

The track for this fireball has been computed since and relative to the radio receiver at Ophain and radio beacon at Dourbes it is receding from both. However, the Doppler blob is above the beacon line which is only possible when the fireball recedes from Ophain but approaches the beacon, approaches both, or approaches Ophain but recedes from the beacon - NOT when it recedes from both!

The only way I can make sense out of this is to assume that the ionized gas trail from the fireball reflecting the radiowaves was NOT receding from the receiver and transmitter beacon - only the fireball was?!

Or are there any other explanations?

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    $\begingroup$ Have you seen other radio spectograms from the same event (different locations)? Do they show similar patterns? $\endgroup$
    – user1569
    Oct 11, 2017 at 19:24
  • $\begingroup$ The archive for the radio stations taking part in the BRAMS network can be searched at : brams.aeronomie.be/brams_viewer I looked at the extreme west and east points of the country (Ophain is centrally located) : brams.aeronomie.be/locations/index Unfortunately western stations Langemark and Harelbeke show no data, and the S/N signal quality of Gent is too poor to reveal anything. Easterly Liège shows a similar spectrogram to Ophain's. So, still puzzled. $\endgroup$
    – Petoetje59
    Oct 11, 2017 at 20:43
  • $\begingroup$ All other centrally located stations show a similar spectrogram to Ophain's. The track makes the fireball recede from ALL the stations, no matter where they are. $\endgroup$
    – Petoetje59
    Oct 11, 2017 at 20:49
  • $\begingroup$ Then the frequency shift can't be due to Doppler, or maybe you have the sign wrong? A positive Doppler shift usually means the target is approaching the sensor; a meteor falling to Earth would certainly be approaching all the stations. $\endgroup$
    – Carl Witthoft
    Oct 12, 2017 at 13:31
  • $\begingroup$ Is the signal maybe not a doppler shift, but a physical signal from Whistler or any other dispersive waves? I can imagine those get generated when the fireball ionizes a patch of air briefly, which would also explain the sharpness of the features. $\endgroup$
    – AtmosphericPrisonEscape
    Oct 12, 2017 at 13:50

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I checked with the people who run the BRAMS project and they confirmed my reasoning, and added another possible explanation.

Even if the fireball moves away from the receiver AND transmitter, the ionization trail it leaves behind can expand and be blown by mesospheric winds towards the receiver and/or transmitter. It's this trail that mostly reflects the radio waves.

And theoretically it's still possible but unlikely that the observed Doppler signal was coming from another meteor trail unrelated to this particular fireball, but coinciding with the exact time frame the fireball appeared in.

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