Artificial optical emissions in the high-latitude thermosphere induced by powerful radio waves: An observational review

High-power high-frequency radio waves beamed into the ionosphere with O-mode polarization cause plasma turbulence, which can accelerate electrons. These electrons collide with the F-layer neutrals causing artificial optical emissions identical to the natural aurora. The brightest optical emissions are O( 1D) 630 nm, with a threshold of ∼2 eV, and O( 1S) 557.7 nm, with a threshold of ∼4.2 eV. The optical emissions give direct evidence of electron acceleration by plasma turbulence, the non-Maxwellian electron energy spectrum as well as the morphology of the accelerating region with high spatial resolution. HF pumping of the ionosphere also causes bulk electron temperature enhancements, but these alone are not sufficient to explain the optical emissions. We review the published radar and optical observations of high-latitude pump-induced artificial optical emissions and introduce new data.