Here’s an interesting mystery. Back in 2012, astronomers finished the construction of an array of 256 radio antennas in the high deserts of New Mexico called the Long Wavelength Array. The array is designed to listen for radio waves with frequencies of between 25 and 75 MHz 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.

Gamma ray bursts are usually followed by an afterglow of radiation at longer wavelengths, from x-rays through to radio waves. But of these, the radio component is the least well observed. So there is significant interest in the data from the Long Wavelength Array, which produces images of the whole sky as it appears in the radio part of the spectrum.

Earlier this year, this data threw up something of a mystery. Having observed gamma ray bursts at other frequencies, astronomers were combing through the data from the Long Wavelength Array looking for afterglows in this part of the spectrum.

In some 200 hours of data, they found two transient radio signals of interest. But to their surprise neither of these signals were actually associated with gamma ray bursts. That caused some head scratching: what else could have produced these kinds of signals?

Today, Ken Obenberger at the University of New Mexico in Albuquerque and a few pals say they have solved the mystery and say the source of these radio signals is much closer to home. Having combed through thousands of hours of data from the Long Wavelength Array, they have found good evidence that these radio bursts are produced by fireballs or large meteors as they burn up in the Earth’s atmosphere.

Obenberger and pals analysed some 11,000 hours of all-sky images produced by the Long Wavelength Array since it began gathering data in April 2012. This process resulted in the discovery of 49 transient radio signals with a duration of several tens of seconds.

The signals show a very definite pattern. Most are point sources, meaning they’re limited to an area of the sky less than about 4° across. But some of them extend much further. On 21 January 2014, one source left a trail covering more than 90° of the sky in less than 10 seconds. This trail then slowly receded to an endpoint which glowed for around 90 seconds.

This type of signal points to a certain origin. “The only known source that could cover this distance across the sky in less than 10 seconds and leave a persistent trail is a ﬁreball,” say Obenberger and co.

To test this idea, the team compared the position of all the transient signals with data from NASA’s All Sky Fireball Network, a system of 12 cameras that record the three-dimensional position, speed, absolute magnitude and mass of fireballs as they pass across the sky. Significantly, two of these cameras are situated in southern New Mexico and so monitor some of the same part of the sky as the Long Wavelength Array.

As it turned out, 39 of the radio transients could not have been picked up by the NASA network because they were either out of view or occurred during the day. But of the other signals, five correspond more or less exactly to fireballs detected by the NASA network. These fireballs are all relatively bright with magnitudes greater than four.

“The chance overlap in space and time of 5 events found randomly in [the Long Wavelength Array] data with ﬁreballs brighter than magnitude −4 within the 11,000 hours of data is about 1 in 10^28,” say Obenberger and co. In other words, the match cannot be a coincidence. “This implies that a large fraction of the events not seen by the network are most likely meteors as well.”

That’s a fascinating discover that raises various other questions. For a start, how do fireballs produce radio waves at this frequency? One possibility is that the trails they leave are simply reflecting radio waves produced on the ground. 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 ﬁreball trails radiate at low frequencies,” they say.

Obenberger and co go on to discuss how meteor trails might produce radio waves at these frequencies. They calculate that the total radio energy emitted is a tiny fraction of the kinetic energy of a typical fireball, perhaps one part in 10^12. So there is no problem with energy.

The fireballs also produce a trail of plasma that the team calculates would be too cool to generate radio waves at this frequency from thermal emissions alone. So something else must be causing the trails to radiate.

Exactly what this might be, nobody quite knows. But Obenberger and co are hoping that data from the Long Wavelength Array will allow them to study the process in more detail.

They end their discussion by raising an interesting link with another mysterious radio phenomena known as Perytons, radio bursts of unknown origin that some astronomers believe are produced in the atmosphere. Obenberger and co suggest that the fact that meteors are now known to produce radio signals means they ought to be considered more seriously as the source of Perytons.

So there’s plenty more work to be done here for radio astronomers. In the meantime, if anyone has any suggestions as to how meteor trails can produce radio emissions of this kind, please post your ideas here.

Ref: arxiv.org/abs/1405.6772 : Detection of Radio Emission from Fireballs