http://www-pw.physics.uiowa.edu/space-audio/sounds/EarthWhistlers/ewhist.htmlWhistlers are produced by lightning and travel along Earth's magnetic field line from one hemisphere to the other, as shown in this illustration. In the ionized gas that exists in this region of space, the high frequencies travel faster than the low frequencies, thereby dispersing the wave from the lightning stroke into a whistling tone that decreases in frequency with increasing time, hence the term "whistler."