Power Line Harmonic Radiation (PLHR)

spaceweb@oulu.fi - last update: 24 November 1998, 1500 UT

It was discovered during 1970's that radiation from electrical power transmission lines is one source of whistler-mode waves in the magnetosphere. Most of the world's power systems use 50 Hz or 60 Hz, but their harmonics in the kHz range (due to mechanical imperfections, nonlinear loads, etc.) can be radiated by transmission lines and leak into the magnetosphere with sufficient intensity to stimulate wave growth and emission generation, much like the VLF transmitter signals.

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.