VLF Emissions are Natural Radio signals that are thought to originate in the magnetosphere on the sunward 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.