Ultra low frequency

The speed of the ULF waves is often associated with the Alfvén velocity that depends on the ambient magnetic field and plasma mass density.

[8] On December 9, 2010, geoscientists announced that the DEMETER satellite observed a dramatic increase in ULF radio waves over Haiti in the month before the magnitude 7.0 Mw 2010 earthquake.

[9] Researchers are attempting to learn more about this correlation to find out whether this method can be used as part of an early warning system for earthquakes.

NATO AGARD publications from the 1960s detailed many such systems, although it is possible that the published papers left a lot unsaid about what actually was developed secretly for defense purposes.

Radio amateurs and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced electrode pairs hammered into the soil.