The term is most commonly used when referring to the scattering of an electromagnetic wave (usually light or radio frequency) by random fluctuations in a gas of particles (most often electrons).
The most well known practical application is known as incoherent scatter radar theory, a ground-based technique for studying the Earth's ionosphere first proposed by Professor William E. Gordon in 1958.
Since the positively-charged ions also present in the ionosphere are orders of magnitude more massive, they are not as readily excited by the incoming electromagnetic wave in the way that the electrons are, so they do not re-radiate the signal.
[2] Since each of the individual electrons and ions exhibits random thermal motion, the received echo will not be at the exact frequency it was transmitted.
[citation needed] If the ionospheric plasma is in motion as a whole, then there will also be an overall Doppler shift in the received data as well.