Ionospheric Whistler

Whistlers aren't utilities, or man-made signals at all, but they're interesting. These audio-frequency electrical sweeps are believed to be generated by distant lightning interacting with the ionosphere. They are best heard by special direct-conversion receivers, though a standard audio amp will work if noise permits, which it usually doesn't.

Whistlers are spooky. Something like 10 different sweeps are visible in this unusually large one. Notice how the amplitude builds as they kick in, then drop off. Several university professors have made very good recordings of whistlers and other atmospheric electrical phenomena, and these are available online.