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Astronomers Solve Puzzle of Mysterious Streaks In Radio Images of the Sky

Posted by timothy on from the smudges-on-the-lens dept.

KentuckyFC (1144503) writes 'Back in 2012, astronomers constructed an array of 256 radio antennas in the high deserts of New Mexico designed to listen for radio waves produced by gamma ray bursts, one of the most energetic phenomena in universe and thought to be associated with the collapse of a rapidly rotating stars to form neutron stars and black holes. The array generates all sky images of signals produced in the 25 MHz to 75 MHz region of the spectrum. But when researchers switched it on, they began to observe puzzling streaks across the sky that couldn't possibly be generated by gamma ray bursts. One source left a trail covering more than 90 degrees of the sky in less than 10 seconds. This trail then slowly receded to an endpoint which glowed for around 90 seconds. Now the first study of these transient radio signals has discovered that they are almost certainly produced by fireballs as they burn up after entering the Earth's atmosphere. The conclusion comes after the researchers were able to match several of the radio images with visible light images of fireballs gathered by NASA's All Sky Fireball Network. That solves the mystery but not without introducing another to keep astrophysicists busy in future. The question they're scratching their heads over now is how the plasma trails left by meteors can emit radio waves at this frequency.'

18 of 66 comments (clear)

  1. Meteor burst communications by Anonymous Coward · · Score: 5, Informative

    http://en.wikipedia.org/wiki/Meteor_burst_communications

    1. Re:Meteor burst communications by teridon · · Score: 5, Informative

      Yes, people use ionization trails to reflect their radio transmissions, but in TFA they discount this as the source of the emissions:

      Meteor trails are known to reflect radio waves and indeed this has been one way of spotting them in the past.

      But Obenberger and co reject this idea for a number of reasons. First, human radio transmissions are usually polarised and so any reflections ought to be polarised as well. The team found no evidence of this.

      At the same time, human radio transmissions have easily identifiable spectra but the team found no evidence of this either in the data from the Long Wavelength Array.

      "It is therefore our conclusion that ïreball trails radiate at low frequencies," they say.

      --
      I hold it, that a little rebellion, now and then, is a good thing. -- Thomas Jefferson
  2. Cyclotron Radiation? by bosef1 · · Score: 2, Interesting

    What about cyclotron radiation from the ions in the meteor plasma trails? Could charge carriers be orbiting in the plasma trail under the influence of the Earth's magnetic field, and radiating RF in the megahertz band?

    1. Re:Cyclotron Radiation? by Anonymous Coward · · Score: 4, Informative

      What about cyclotron radiation from the ions in the meteor plasma trails? Could charge carriers be orbiting in the plasma trail under the influence of the Earth's magnetic field, and radiating RF in the megahertz band?

      in a word: maybe

      From the paper: "If a magnetic eld of 10 to 15 G were present within the trail, it follows that cyclotron radiation would be emitted at the observed frequencies by the electrons in the plasma. However, the surface geomagnetic eld is only 0.5 G, so this would require the generation of a strong magnetic eld by a reball, an eect that has never been observed."

    2. Re:Cyclotron Radiation? by the_other_chewey · · Score: 4, Funny

      From the paper: "If a magnetic eld of 10 to 15 G were present within the trail, it follows that cyclotron radiation would be emitted at the observed frequencies by the electrons in the plasma. However, the surface geomagnetic eld is only 0.5 G, so this would require the generation of a strong magnetic eld by a reball, an eect that has never been observed."

      Hey, you! Your ligatures aren't showing!

    3. Re:Cyclotron Radiation? by xfade551 · · Score: 3, Interesting

      Let's see, meteor generates a plasma trail... that would be moving charged particles (both positive and negative). Moving charge means a changing electric field. A changing electric field causes a changing magnetic field, so we have a changing electro-magnetic field... i.e. radiowaves. The ions in the plasma trail don't stay that way, the electrons get recaptured at some point, which means a photon will be discharged, i.e. a radio wave-packet. As to frequency, just put a couple grad students to write a program to calculate photon emission frequencies of a couple million different ion configurations, and they'll probably find the correct correspondence somewhere in there.

    4. Re:Cyclotron Radiation? by PPH · · Score: 2

      Or the plasma trail forming a conductive channel through the atmosphere acting as a crude cavity resonator. The half wavelength of a 25 to 75 MHz signal should be 6 to 2 meters. So I'd start looking for structures in the meteor trail somewhere near these dimensions. A small (several cm) meteor's shock wave could conceivably ionize a column of air of this size.

      --
      Have gnu, will travel.
  3. Is this not the same as grass noise? by zoid.com · · Score: 4, Interesting

    I've heard reports of people laying on the ground and "hearing" meteors. What baffled scientist about this was people were hearing them realtime and not delayed due to the speed of sound. They finally realized that it was radio waves emitted by the meteors causing the grass to vibrate and they were hearing the vibrations.

    Maybe I dreamed it...

    --
    Zoid.com
    1. Re:Is this not the same as grass noise? by Peter+H.S. · · Score: 4, Informative

      I've heard reports of people laying on the ground and "hearing" meteors. What baffled scientist about this was people were hearing them realtime and not delayed due to the speed of sound. They finally realized that it was radio waves emitted by the meteors causing the grass to vibrate and they were hearing the vibrations.

      Maybe I dreamed it...

      I have heard meteors making real time sounds as they streaked across the sky. It was during the 2001 Leonids meteor storm. It was in a city with not a blade of grass in sight. This particular meteor storm was widely reported as emitting crackling and hissing sounds:
      http://www.spaceweather.com/me...

      There has been some speculation about what could cause such sounds. Some have suggested that “electrophonic meteors” can cause secondary lower frequency vibrations to be heard simultaneously, but it is just a suggestion since good quality data is missing.

      The RTFA discovery is just a part of a new series of discoveries that shows we know a lot less about meteors and comets than we thought we did.

    2. Re:Is this not the same as grass noise? by Tapewolf · · Score: 5, Informative

      That was sufficiently weird that I had to look it up: http://www.livescience.com/386...

  4. Reflected EM Waves? by Anonymous Coward · · Score: 2, Informative

    Chances are, the detected frequencies may be due to reflection of radio energy that's been transmitted by transmitters around the world. Radio amateurs have been using meteor-scatter as a way of reflecting radio frequency energy for short periods to make intercontinental contact - so this may be a contributor to the signals detected at "radio quiet" locations.

    1. Re:Reflected EM Waves? by dogsbreath · · Score: 2

      RTFA 25 to 75 mhz is a span that includes many Earth originated communication sources including radio amateurs, RC toys, CB, PTP business, and broadcast.

    2. Re:Reflected EM Waves? by Charliemopps · · Score: 2

      Chances are, the detected frequencies may be due to reflection of radio energy that's been transmitted by transmitters around the world. Radio amateurs have been using meteor-scatter as a way of reflecting radio frequency energy for short periods to make intercontinental contact - so this may be a contributor to the signals detected at "radio quiet" locations.

      They specifically rule that out if you read the article.

    3. Re:Reflected EM Waves? by Dan+East · · Score: 2

      One word: polarization. Man made radio waves for communication are almost always polarized. Further, reflected radio waves are often very polarized even if the source was not (which is why polarized sunglasses reduce glare). They did not see the polarization expected from reflected radio signals. It appears they are actually emitted.

      --
      Better known as 318230.
  5. Don't worry... by cirby · · Score: 2

    ...it's just the screams of alien robots burning up in the atmosphere.

    "Prepare to die, humans! Hey, this is sorta hot, isn't it? No, really, I should have thought this through. AAAHHHHHH!"

  6. Re:That's odd by Brett+Buck · · Score: 2

    Hmm, I am surprised that they don't know. I think there are plenty of other people who do. RF effect from meteorite trails is a well-known phenomenon from radio (people were using it to bounce messages in the 30s)

        Here are some people using it to track meteorites - very near the frequencies in question:

    http://spaceweather.com/glossa...

            The necessary condition for bouncing a particular frequency is that the path lengths of the plasma are the right length (say, half a wave length or maybe 2ish meters) which seems entirely plausible as a distance associated with the width of the plasma trail. It would not be at all surprising if a tiny amount bounced back and forth like a cavity resonator, OR, reflected ambient signals that the telescope wouldn't have otherwise detected.

    So it doesn't seem that mysterious.

  7. Re:That's odd by Anonymous Coward · · Score: 2, Interesting

    Except that they have reasons to thick it is not an echo, but being radiated by the trail itself. They explicitly discuss this. The ability of a plasma trail to radiate in the RF is not too surprising considering there are all sorts of frequencies and signals you can get from the vast zoo of waves in a magnetized plasma. Although figuring out exactly what type and source of waves you have in plasmas can be difficult, even on a table top plasma experiment where you have a lot more diagnostic access.

  8. Silicon ions by laughingskeptic · · Score: 2

    They reject most of my theories in their paper, but the don't mention silicon ions as a possible source so I'm going with that. With higher masses and higher charges the silicon ion part of the plasma will be denser and be more affected by the earth's magnetic field at that altitude.