Features of the ELF/VLF Wave Generation and Propagation Processes during Ionospheric Modulated High-Frequency Heating

We consider the results of observations of the ELF/VLF horizontal magnetic field of the ionospheric source using a high-latitude network of PGI stations in the heating experiment of 2016 at the EISCAT/Heating facility. We have discovered that at frequencies close to the frequency of the first transverse resonance of the Earth–ionosphere waveguide (∼1.8 kHz), the amplitude of the horizontal magnetic field of the ionospheric source at the Barentsburg station (located 961 km from the heating facility) often exceeded the amplitudes at the mainland stations located closer to the heater (the nearest station Lotta is 395 km away). With all other conditions being equal, at frequencies considerably different from the frequency of the first transverse resonance, this effect was not observed. We identified and analyzed two possible physical mechanisms that can be responsible for the observed effect. The first is related to a fairly low decay of waves with frequencies close to the frequency of the first transverse resonance of the Earth–ionosphere waveguide when propagating above a highly conductive sea surface with a high reflection coefficient. The second is due to the non-uniformity of the radiation pattern of the ionospheric source. Numerical modeling of the generation and propagation of ELF/VLF waves in the Earth–ionosphere waveguide was carried out and the contribution of the proposed mechanisms was estimated. It is shown that the radiation pattern of the source does not have a notable effect and the experimental data are explained only by taking into account the high conductivity of the lower boundary of the waveguide along the EISCAT—Barentsburg path and low conductivity along the EISCAT—Lotta path. The state of the ionosphere is assessed and a family of electron density profiles for which a similar effect can be observed is obtained.