Meteors imaged in radio glory

BY DR TAMMO JAN DIJKEMA (ASTRON)

For the first time, the LOFAR telescope has imaged the radio emission of several meteor showers, showing the versatility of this SKA pathfinder instrument.

Meteors are long known to reflect radio emissions beamed from Earth. In fact, this was one of the first subjects that Sir Bernard Lovell studied at Jodrell Bank in 1945. Observing radars’ meteor reflections is now a popular activity among many radio amateurs.

An all-sky radio image created with LOFAR / AARTFAAC12, during the Perseids meteor shower at 12 August 2020 21:25:51 (2-second integration). It shows the Milky Way, the Northern Galactic Spur, and broadband radio emission from a meteor (on the right). This radio meteor was visible for over two minutes.

What is special about the all-sky images that LOFAR captured of the Perseids and Geminids meteor shower in 2020, is that many of the meteor trails – at about 100km in altitude – were recorded in every channel selected between 30MHz to 60MHz. Because the signals covered such a wide frequency range, they are probably not reflected from Earth. In fact, astronomers aren’t sure yet just what causes these meteors to emit radio waves. This kind of emission was first detected in 2013 with the Long Wavelength Array (LWA) in New Mexico, USA. LOFAR is more sensitive and has a higher resolution than the LWA and can therefore observe meteors with weaker radio emissions.

To be sure that LOFAR was indeed observing meteors, we compared its observations with optical observations from the CAMS BeNeLux project. As part of that project, citizen scientists record the sky with low-light video cameras to capture meteors and compute their trajectory. During the night of 12-13 August 2020 when the Perseids meteor shower was in action, this network observed more than 700 meteors. Over 200 of those meteors matched LOFAR images we recorded that night! The radio meteors we saw followed the exact same path as the optical meteors. However, whereas the optical meteor lasted only fractions of a second, some radio trails persisted for minutes. We could even see the meteor trail being blown apart by the winds at that altitude.

The radio images were taken by 12 LOFAR stations within a 1.2km radius. A new sub-system of LOFAR that piggy-backs on its regular observations – called AARTFAAC12 – correlates all 576 dipole antennas of these stations every second. This alternative mode of LOFAR operation is useful for catching transient events such as meteors.

Our results appeared in the Journal of the International Meteor Organization: https://imo.net/papers/WGN-495-dijkema-radio-optical.pdf

You can view an intriguing timelapse of the whole night’s radio meteors onYouTube: https://www.youtube.com/watch?v=RlCfErtdY2M&feature=emb_logo