Studies of the Electron Density Modifications in the Ionosphere Excited by the Powerful Radio Wave with Frequencies near the 4th Electron Gyroharmonic

We present results on plasma perturbations in the F-region ionosphere caused by HF heating in the vicinity of the 4
^th
electron gyroharmonic. The experiments were conducted at HAARP heating facility in June 2014. On the first time at HAARP the multi-frequency Doppler Sounder which measures the phase and amplitude of reflected sounding radio waves was used for the diagnostic of the plasma perturbations. These measurements were complemented by the observations of the Stimulated Electromagnetic Emission (SEE) for different pumping frequencies. We detected plasma expulsion from the reflection region of the pumping wave, and from the upper hybrid region. We found also how the anomalous absorption (AA) of the sounding waves, as well as the SEE spectral structure and intensity depend on the offset of the pumping frequency from the 4
^th
electron gyroharmonic. A review of studies of the ionosphere pumped by the powerful high-frequency radio radiation of the Sura facility, carried out by measuring the optical emission of the ionosphere, is presented.