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Whistlers are very low frequency radio waves (typically between 1 kHz and 30 kHz) excited by lightning and then trapped in the ionosphere about the Earth. The ionosphere is a region of electrically charged molecules and electrons that envelopes the Earth at distances out into space up to 10 Earth radii. A radio wave passing through the ionosphere interacts with the free electrons there, causing them to move.  But the electrons can only move freely in the direction parallel to a magnetic field line which has the effect of trapping the radio waves and only allowing them to propagate along the magnetic field lines. The radio waves follow the magnetic field and reflect where the fields intercept the Earth. These waves then bounce back and forth along the Earth's magnetic field lines, sometimes bouncing up 20 times or more!  Since these are low frequency waves, they can be captured by radio antennas and then converted into an audio signal. 

The refractive index of the ionosphere varies rapidly with the frequency of the radio wave (a phenomenon known as dispersion), which means that different frequencies propagate at different speeds. The lightning generates a radio pulse containing many frequencies and as this wave bounces back and forth between the Earth's magnetic poles (like the dashed line in the figure above), the different frequency components travel at different speeds and begin to separate, with the higher frequency waves arriving first followed by the low frequency waves. When detected by a radio receiver and converted to an audio signal, they have a characteristic sound just like a whistle  - hence they are caused whistlers.  

The Radio Astronomy Section has built a whistler receiver that detects the low frequency radio waves and converts them to an audio signal that is output on the internet. You can listen to this radio and maybe you will hear a whistler too. This works best when there is lightning somewhere around Australia. 

Whistler Audio

Note that this curently does not work with Apple Safari ... :-(
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