[1] These transmitters operate in the high frequency (HF) range (3-30 MHz) at which radio waves are reflected from the ionosphere back to the ground.
With such facilities a range of plasma turbulence phenomena can be excited in a semi-controlled fashion from the ground, during conditions when the ionosphere is naturally quiet and not perturbed by for example aurora.
This stimulus-response type of research complements passive observations of naturally excited phenomena to learn about the ionosphere and upper atmosphere.
The optical emissions result from the excitation of atmospheric atoms and molecules by electrons that have been accelerated in the plasma turbulence.
Ionospheric HF pump facilities need to be sufficiently powerful to provide the possibility for plasma turbulence studies, although any radio wave that propagates in the ionosphere affects it by heating the electrons.