Characteristics of ULF Magnetic Fields in the 3D Inhomogeneous Earth‐Ionosphere Waveguide

We develop an algorithm for calculating the characteristics of the magnetic field of signals in the frequency range of (0.1–20) Hz in the horizontally and vertically inhomogeneous spherical Earth‐ionosphere waveguide. This algorithm involves calculations of the surface impedance of an inhomogeneous ionosphere. The frequency dependences of the amplitude, phase and polarization of the magnetic field generated by a horizontal magnetic dipole were analyzed. It is shown that the horizontal inhomogeneity of the Earth‐ionosphere waveguide can significantly affect the frequency dependence of the amplitude and polarization of the magnetic field, increasing the degree of circular polarization. The horizontal inhomogeneity only slightly influences the results in daytime conditions. The most noticeable difference in the amplitude dependences of the magnetic components and large phase shifts (up to 40°) are observed when the terminator passes through the geodetic line connecting the source and receiver, and in the presence of sporadic layers with a horizontally inhomogeneous structure along the low‐frequency wave propagation path. We compare the calculation results with the magnetic pulsations recorded at Nurmijarvi station during the 2001 campaigns of Kola Peninsula ultra low frequency emitter operation. The obtained results make it possible to qualitatively explain peculiarities of the amplitude and polarization spectra of the artificial magnetic signals recorded in this campaigns that are not explained within the framework of the flat homogeneous waveguide model. Plain Language Summary Our numerical calculations have shown that the horizontal inhomogeneity of ionospheric parameters during the daytime did not lead to significant changes in the characteristics of artificial signals compared to a homogeneous Earth‐ionosphere waveguide. The phase difference between the longitudinal and transverse magnetic components were close to zero like for homogeneous waveguide at night, the spectra of the characteristics of the signal magnetic field are formed under the influence of sub‐ionospheric Alfvén resonator structure. It was found that namely the horizontal inhomogeneity of ionospheric plasma leads to an increase in phase shift between the components. The most notable difference in the character of magnetic component amplitude spectra and large phase shifts between them (up to 400) are observed when the terminator passes through the line connecting the source and receiver.