We present the results of multi-instrument experiments related to studying the phenomena in the high-latitude ionosphere affected by high-power radio waves using the EISCAT technical facilities. It was found for the first time that strong small-scale artificial field-aligned irregularities (AFAIs) are excited when the ionospheric F region is heated by a high-power HF radio wave with X-mode polarization near the altitude at which the critical frequency f xF 2 of the F 2 layer is equal to the frequency f H of the heating accompanied by an up to 50% increase in the electron temperature. The spatial structure of the artificially perturbed ionospheric F region is examined in detail using an incoherent scatter radar operated in the regime of scanning over elevation angles from 92 • to 74 • with a 2 • step. It is shown that the spatial size of the heated patch strongly depends on the angle of the HF pumping relative to the Earth's magnetic field. The phenomena occurring in the artificially modified ionospheric F region heated at frequencies near the third electron gyroharmonic, i.e., at f H = 3f ce = f UH , where f UH is the upper-hybrid frequency, are explored on the basis of multi-instrument observation data.