Dependence of stimulated electromagnetic emission on the ionosphere and pump wave

Abstract

Electromagnetic emission, stimulated by a powerful high frequency radio wave injected into the ionospheric F region, exhibits a rich spectral structure in the sidebands of the reflected pump wave. Results from such experiments at the Heating facility near Tromsø, Norway, are presented. Spectral features in a steady state, a few seconds after the onset of the pump, are shown to depend on ionospheric conditions, the pump frequency, and power. Different types of stimulated radiation spectra may be obtained when the ionospheric critical frequency is well above or near the pump frequency. Further, the appearance of the spectral features is very sensitive to the pump frequency near harmonics of the F region electron cyclotron frequency. Our interpretation is that the considered stimulated electromagnetic emissions are generated primarily in two interaction regions, the plasma resonance region at the pump reflection height and the upper hybrid resonance region, typically a few kilometers below the reflection height.