Detection of Artificial ULF Signals at Staraya Pustyn Magnetic Station during the FENICS-2019 Experiment

In September 2019, two out-of-service power transmission lines were used as a horizontal emitting antenna in an experiment conducted on the Kola Peninsula to generate ultra-low-frequency (ULF, 0.38–6.4 Hz) and extremely low–frequency (ELF, 9.4–194 Hz) signals during nighttime hours. The current ranged from 240 A at low frequencies (LF, 0.382 Hz) to 20 A at the highest frequencies (194 Hz). The results of the recording of ULF signals at Staraya Pustyn magnetic station, which is 1610 km away from the transmission line, are presented. The recorded signals had frequencies of 0.6–6.4 Hz and amplitudes normalized to the value of the emitter current ~0.4–0.7 fT/A. Three models were used for theoretical estimates: (1) formulas based on the theory of ELF field excitation over a conducting surface; (2) a numerical model of the ULF field in the atmosphere and ionosphere generated by a linear surface current of infinite length, and (3) a numerical model of a horizontal dipole in the multilayer Earth–atmosphere–ionosphere medium. Model 2 is based on the numerical solution of the system of Maxwell equations in the vertically inhomogeneous atmosphere and ionosphere; its fundamental feature is that it takes into account the contribution of ionospheric waveguide propagation to the excited field at large distances. Model 3 demonstrated the best agreement with the amplitudes of the recorded signals. However, contrary to the predictions of models 1 and 3, the frequency dependence of the amplitude of artificial signals in the 2–8 Hz range is non-monotonic, which may be a manifestation of the effects of waveguide propagation along the ionosphere.