Phenomena in the High-Latitude F Region of the Ionosphere under the Effect of Powerful HF Radio Waves at Frequencies above the Critical One of the F2 Layer

In this paper, we present the results of experimental studies of phenomena in the high-latitude upper (F-region) ionosphere caused by the impact of high-power HF radio waves of ordinary (O-mode) polarization at heating frequencies fH that significantly exceed the critical frequency of the F2 layer (fH – fоF2 = 0.9–1.1 MHz). The results are based on experimental data obtained at the EISCAT/Heating HF heating facility in Tromsø, northern Norway (69.6° N, 19.2° E). In the experiments, a high-power HF radio wave of the O-polarization range was emitted in the direction of the magnetic zenith with a maximum effective radiation power of 350–550 MW. It was found for the first time that under conditions when a powerful HF radio wave of the O-polarization was not reflected from the ionosphere, ducts of increased electron density Ne are formed, and small-scale artificial ionospheric inhomogeneities and narrow-band (within a ±1-kHz band relative to the heating frequency) artificial radio emission of the ionosphere that are recorded at a distance of 1200 km from the heating facility are generated. The characteristics of small-scale artificial ionospheric irregularities and the spectral structure of artificial radio emission of the ionosphere with alternative O/X heating to the magnetic zenith at frequencies significantly exceeding the critical frequency of the F2 layer are compared. It was established that, in general, their behavior has the same character, but the evolution of the development of the considered phenomena during O and X heating is different.