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Listening to Meteors

The View from the Front Yard 3:30 AM, August 30, 2014

Texas A&M, hot on the heels of their newfound football success has upgraded Kyle Field, their legendary football stadium where undergrads aren’t allowed to sit.  They’re allowed into the stadium mind you, it’s just considered poor form to sit.   We found out early one morning last week that a portion of the upgrade was a brand new jumbotron.  This became somewhat blindingly apparent when the pup and I were treated to technicolor clouds on our morning walk.  The jumbotron sits atop the newly refurbished end zone bleachers over a mile away from our house.  We wouldn’t be treated to such displays if it wasn’t for Johnny Football, and of course, our rather persistent low-lying early morning cloud cover that wafts in from the gulf.  The same circumstance that made for our polychromatic morning walk , (the cloud cover, not Johnny Football), obscured most of the Perseids meteor shower a few weeks ago.  As it turns out though, we could have listened to the meteor shower uninhibited.

When meteors hurtle through the atmosphere they leave a column of ionized gas in their trail.  Radio signals are reflected by this column, and so, while listening to a far-away mostly unintelligible TV or radio station, using a shortwave AM radio receiver, you can actually hear meteors whistle as they fly overhead.   The whistle is caused by the falling meteor Doppler shifting the frequency of the station you are listening to.  Most people are familiar with audio frequency Doppler shifts like the change in pitch of a moving fire engine siren.  As a source of waves, (sound waves in the case of the fire engine siren), moves, the frequency of the waves  changes with respect to a stationary listener.  Thus, as a fire engine approaches its siren goes first up in pitch, and then down as it passes by, and drives away.  Radio waves bouncing off the ion cloud created by a falling meteor exhibit the same effect.  Their frequency changes in proportion to the speed of the meteor.

Dr. .William Lonc of St. Mary’s University in Halifax, Canada[1] suggests listening for the video carrier of a television channel using an AM shortwave radio.  See Wikipedia for a list of television channel frequencies.  As a meteor cruises by, the Doppler shifted television signal will heterodyne with the local oscillator signal in your radio, and create a beat signal, or whistle in your speaker with a frequency determined by the speed of the meteor,

During a meteor shower like the Leonids, coming up in November, you should hear several pings, (in the older literature, they’re called whistlers), per minute.  Give it a try and please let me know how it goes.  I’ll try to do the same here at A&M and keep you posted.

 

References:

1.  William Lonc,Meteors by radio: Getting started,The Physics Teacher 37, 123 (1999); doi: 10.1119/1.880168

 

 

 

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