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

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.'

Comments

Meteor burst communications

[Anonymous Coward]

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

Re:Meteor burst communications

[teridon]

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.

Pffft

[Tablizer]

So there are no giant human-eating bird-dragons, it was just a moth on the lens.

Cyclotron Radiation?

[bosef1]

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?

Re:Cyclotron Radiation?

[Anonymous Coward]

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."

Re:Cyclotron Radiation?

[the_other_chewey]

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!

Re:Cyclotron Radiation?

[xfade551]

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.

Re:Cyclotron Radiation?

[PPH]

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.

Re:Cyclotron Radiation?

[calidoscope]

That sounds like a reasonable guess as to what is going on. The hot plasma is conductive and the Johnson noise may produce enough current to emit detectable radiation. My recollection was that the sky noise temp at 50 MHz was on the order of 3,000K, and the plasma from the meteor trail is likely to be considerably hotter than that.

Re:Cyclotron Radiation?

[budgenator]

Don't forget that the ion trail behind a meteor is also pretty turbulent so there would also be all kinds of doppler shifts, trying to calculate an emission freq is likely meaningless.

Re:That's odd

[meerling]

Solving one mystery reveals several more.

Is this not the same as grass noise?

[zoid.com]

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...

Re:Is this not the same as grass noise?

[Peter H.S.]

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.

Re:Is this not the same as grass noise?

[Tapewolf]

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

Re:Is this not the same as grass noise?

[PPH]

Something like very low frequency sound (pressure) waves which are beyond human hearing causing the grass to move, creating rustling noises.

The Chelyabinsk meteor created intense enough pressure waves (actually supersonic shock waves) to blow out windows, so I guess rustlng some grass isn't out of the question.

Re:Is this not the same as grass noise?

[volmtech]

I remember hearing a sizzling sound from a few meteors. There was no delay in the sound as the meteor whizzed overhead. I remember thinking to myself, "Did I just hear a meteor?" I have also heard lightning "crackle" many seconds before the thunder boom.

Re:Is this not the same as grass noise?

[Optali]

Not what I heard.

I heard reports from people smoking grass while listening to The Meteors

Re:Is this not the same as grass noise?

[vandamme]

I've heard a meteor too. however, I was in a middle of a lake on a boat, and there was no grass shaking out there.

Re:That's odd

[narcc]

I assume that the key bit here is "at this frequency".

Surely, there's a physicist around who can elaborate on that.

Reflected EM Waves?

[Anonymous Coward]

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.

Re:Reflected EM Waves?

[brainboyz]

Not many people transmitting x and gamma rays.

Re:Reflected EM Waves?

[dogsbreath]

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.

Re:Reflected EM Waves?

[Charliemopps]

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.

Re:Reflected EM Waves?

[Dan East]

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.

Fission is happening?

[TheRealHocusLocus]

Some type of triggered fission in the heavy elements of the meteorite? And gamma emitter with a short half life of minutes is being created and left in the trail, such as Barium-137?

a planet 4000 miles away

[rossdee]

"I meant to say 4000+ light years"

Whew I was a bit worried there. If there was another planet within 4000 miles of the earth the tidal effects would wipe us out.

Re:a planet 4000 miles away

[lgw]

Well, there's definitely a planet 4000 miles from me. It's not another planet, but that's really OK by me.

Don't worry...

[cirby]

...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!"

Obviously they are alien space probes...

[LF11]

..burning up in the upper atmosphere, like ours will someday on far-off planets, long after we are gone.

Re:That's odd

[Brett Buck]

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.

Re:That's odd

[Anonymous Coward]

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.

Re:That's odd

[currently_awake]

We know that plasma (hot ionized air) generates radio waves, that's how lightning can disrupt your radio reception. Yes it's mostly in the HF spectrum, but the lower VHF band is not immune to atmospheric noise.

Silicon ions

[laughingskeptic]

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.

Re:That's odd

[osu-neko]

So it doesn't seem that mysterious.

Once again demonstrating the principle: the less you understand a problem, the more obvious the answer seems. (Related to the old programming adage: Confidence is the feeling you have before you understand the problem.) No actual problem has an obvious solution. If it did, it wouldn't be a problem to begin with. Whenever you feel something is obvious, it's a dead-giveaway that you're missing something important...

Re: That's odd

[codegen]

For every problem there is a solution that us simple, neat, and wrong -- H.L.Mencken

25-75 MHz

[manu0601]

There is something wrong with the summary: 25 to 75 MHz is not within the gamma ray spectrum.

Re:That's odd

[RockDoctor]

TFA isn't talking about picking up echoes from the trails, but about actual emission from the trails, which is a different kettle of fish. Or even a different plasma of electrons, if you like your analogies mixed. As you say, people have been "bouncing" radio signals off meteor trails for decades. People have also been using omni-directional antennae hooked to fairly basic wide-band radio receivers as a too for detecting meteors in flight for a number of decades too. It's one of the easiest ways of getting a count of the actual rate of meteor arrival during a meteor shower, as it doesn't involve spending hours sitting out watching the sky, and the frailties to which the human observer is prone. Unsurprisingly, it correlates well with the ZHR records kept by experienced human observers, to the point that it's a valuable research tool.

So the fact that meteors produce Rf noise is hardly new ; being able to show that some of the RF meteor trails trails is fairly big news though. I still remember with a degree of awe the sight of the 100-odd degree long tail of Comet Hyakutake streaking across the sky in late 1996 (or was it 1997 and I'm getting confused with Hale-Bopp? Two good comets almost exactly a year apart. Was Slashdot even in existence then?)

I don't awe easily ; that was an incredible sight. To know that there are (broadly) similar events gracing the skies of our new, RF-seeing Overlords makes me feel happy for them and I extend my heartiest welcome to them.