Features of the Spectra of Ultra-Low-Frequency Magnetic Fields from Horizontal Current Sources: Models of Planar and Spherical Earth–Ionosphere Waveguides

Abstract

We simulate the magnetic field components in the frequency range 0.1-30 Hz from a source such as the horizontal magnetic dipole located in the Earth-ionosphere cavity. Models of uniform spherical and planar waveguides are considered, the advantages of each model are determined, and a comparative analysis of the simulation results is performed. The results of numerical calculations lead to the conclusion about the impact of the IAR (ionospheric Alfvén resonator) and sub-IAR (resonator at altitudes of 80-300 km) on the amplitude spectra of the ultra-low-frequency (ULF) signal. It is also shown that the appearance of strong sporadic layers (with a plasma frequency exceeding 5 MHz at their maxima) along the entire propagation path of ULF fields from the source to the receiver can strongly change the amplitude spectra of artificial signals. A significant increase in the amplitude of the magnetic field at the IAR harmonic frequencies can be observed at night. The sub-IAR effect is responsible for the appearance of a dependence of the amplitude of the magnetic components, which increases with frequency. Phase shifts between the horizontal components of the ULF signal and the spectra of the polarization parameter are also analyzed. It is shown that ionospheric resonators have almost no effect on the phase characteristics of artificial signals in a uniform waveguide approximation, in contrast to the polarization of magnetic ULF fields from a vertical electric dipole. The dependences of the amplitude of the magnetic field of a signal on the distance r to the source and on the direction to the source are also analyzed. It is shown that the dependence of the decay of the magnetic field amplitude with distance can differ significantly from the 1/r 2 law. The radiation pattern is almost isotropic for k 0 r < 1, where k 0 is the wave number in free space. The lines of equal amplitude are stretched along the direction of the current source for k 0 r ≥ 1.

Generation of Artificial ULF/ELF Electromagnetic Emission in the Ionosphere by Horizontal Ground‐Based Current System

“…Electromagnetic field propagation from an ELF emitting dipole along the Earth‐ionosphere waveguide has been well‐modeled theoretically (Bernstain et al., 1974; Ermakova et al., 2021; Nickolaenko et al., 2016; Sobchakov et al., 2003; Tereshenko & Tereshenko, 2017). In these works, the ionosphere was modeled as a conductive homogeneous layer or an impedance wall.…”