Dawn chorus (electromagnetic)
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The electromagnetic dawn chorus is an unexplained phenomenon that occurs most often at sunrise or shortly after, that (with the proper radio equipment) resembles the sound of the birds' dawn chorus. It is thought to be caused by high-energy electrons that get caught in the Van Allen radiation belts of the Earth's magnetosphere and fall to the Earth's surface in the form of audible radio waves. Dawn choruses occur more frequently during magnetic storms.
This phenomenon also occurs during aurorae, when it is termed an auroral chorus.
External links
.WAV format recordings of the electromagnetic dawn chorus (http://image.gsfc.nasa.gov/poetry/sounds/sounds.html)