HF Induced Modifications of the Electron Density Profile in the Earth's Ionosphere Using the Pump Frequencies Near the Fourth Electron Gyroharmonic

We discuss results on plasma density profile modifications in the F-region ionosphere that are caused by HF heating with the frequency f(0) in the range [(-150 kHz)-(+75 kHz)] around the fourth electron gyroharmonic 4f(c). The experiments were conducted at the HAARP facility in June 2014. A multi-frequency Doppler sounder (MDS), which measures the phase and amplitude of reflected sounding radio waves, complemented by the observations of the stimulated electromagnetic emission (SEE) were used for the diagnostics of the plasma perturbations. We detected noticeable plasma expulsion from the reflection region of the pumping wave and from the upper hybrid region, where the expulsion from the latter was strongly suppressed for f(0) approximate to 4f(c). The plasma expulsion from the upper hybrid region was accompanied by the sounding wave's anomalous absorption (AA) slower development for f(0) approximate to 4f(c). Furthermore, slower development and weaker expulsion were detected for the height region between the pump wave reflection and upper hybrid altitudes. The combined MDS and SEE allowed for establishing an interconnection between different manifestations of the HF-induced ionospheric turbulence and determining the altitude of the most effective pump wave energy input to ionospheric plasma by using the dependence on the offset between f(0) and 4f(c).