However, since power line radiation (PLR or PLHR) is much weaker than transmitter signals, observations of their effects depend strongly on amplification in the magnetosphere. PLR effects are usually observed when good echoing conditions exist, allowing weak input waves to be amplified repeatedly. The nonlinear amplification process produces complicated output wave forms showing frequency broadening, side bands, and stimulated free-running VLF emissions.
Example of PLHR stimulated VLF emissions. The horisontal lines between about 3 and 6 kHz are the power line harmonics.