The dawn chorus is an electromagnetic phenomenon that occurs at or around sunrise, and can only be heard with VLF(very low frequency) radio equipment. It's named so because it sounds like the chorus of birds that usually arises at the same time. It's thought to be caused by electrons getting caught in the earth's radiation belts and then falling to earth as radio waves. They can also happen during auroras.
Short recording (WAV) here.
More atmospheric recordings here.
This is also my first post here, so I'm scared. Be gentle :(
EDIT: Here's the Wikipedia link.