Sferics – “Sferics” is short for atmospherics, the radio waves caused by lightning. They sound like the static you hear on an AM radio when thunderstorms are nearby.
Tweeks – Tweeks are sferics where the frequencies have been subject to dispersion by traveling a long distance through the earth-ionosphere waveguide. They have a metallic ringing sound and are usually heard between sunset and sunrise and are more prevalent in the winter.
Whistlers – Whistlers are a Natural Radio signal that originates from a lightning strike which propagates into the earth’s magnetosphere and follows a field line to the opposite hemisphere of the earth. It may be heard there or bounce back from the ionosphere to the hemisphere in which it originates. A whistler sounds like a descending tone similar to a slide whistle. Whistler’s can be pure tone, or they may be very hissy.
Chorus – Chorus or “Dawn Chorus” is a VLF emission that can sound like a flock of birds or the sound of a pond full of frogs or some combination thereof. The name “dawn” chorus may have originated because it sounds like birds, frogs and insects in the morning or because it is most often heard just after sunrise. Chorus is often heard in the early morning hours during periods of high geomagnetic activity.
VLF Emissions – VLF Emissions are Natural Radio signals that are thought to originate in the magnetosphere on the sun facing side of the earth near the geomagnetic equator and then propagate toward the poles along the field lines where they exit the ionosphere and propagate along the surface of the earth where they may be heard. While the mechanism is understood in the general sense, much still remains to be learned. VLF emissions include chorus, risers, hiss, and a variety of other signals. They are often heard during times of high geomagnetic activity.