The paper considers the results of experimental studies in the wave zone of the field excited by a linear grounded antenna in the 0.4–95 Hz frequency range. As a result, the influence of the outer ionosphere on the magnetic field in the region of frequencies lower than 10 Hz is established statistically reliably. Measurements in the ultralow-frequency and lower-frequency range made it possible to estimate the state of the lower ionosphere and thus show that variations of the measured field in the frequency range less than 10 Hz are due to the influence of the outer ionosphere. Comparison of measurement results with natural electromagnetic noise, including its structured part, permitted an assumption about the relationship between an ionospheric resonator and the appearance of Alfvén resonances with the processes in the Earth’s magnetosphere. It is shown that the lithosphere, in particular, its heterogeneous structure in the field excitation region, plays a significant role in the measured frequency dependence of the field amplitude.

The paper addresses the effect of the ionosphere on the ELF and lower frequency waves excited in the Earth-ionosphere waveguide by a controlled source. The experiment carried out on the Kola Peninsula is described and the results of measurements in the frequency range 0.4–95 Hz are presented. Non-monotonic behavior of the magnetic field with time is revealed. It is shown that variations in the magnetic field are related to the state of the ionosphere and depend on the geomagnetic activity. The importance of the effect of the topside ionosphere on the structure of the studied field is discussed.

Excitation of electromagnetic field at ultralow, extremely low, and lower frequencies by a horizontal grounded antenna in a two-layer medium is considered. Approximate analytical formulas are derived to describe field evolution in the low-frequency range. Accuracy of the quasi-stationary approximation is analyzed. Substitution of the impedance of a plane wave that is normally incident on the interface for a real surface impedance is estimated. Frequency limits of the approximate approach in the estimation of the Earth conductivity are determined for several experiments performed on the Kola Peninsula in the framework of the FENIСS-2014 and FENICS-2019 International Projects.

The paper considers the excitation of a horizontal grounded antenna of the ELF-ELF electromagnetic field and a lower frequency range in a two-layer environment. Approximate analytical formulas are obtained that describe quite completely behavior of fields in the low-frequency range. The accuracy of using the quasi-stationary approximation is analyzed. The possibility of replacing the actual surface impedance with a plane wave impedance, normally falling on the interface. For a number of experiments carried out on the Kola Peninsula in the framework of the international cooperation (FENICS-2014, FENICS-2019), the frequency boundaries of the applicability of the approximate approach in assessing conductivity of the Earth.

An analytical expression for the vertical component of the magnetic field has been obtained, with the help of which calculations have been made showing the effect of the ionosphere on the low-frequency field in the Earth-ionosphere waveguide. At distances from the source that are less than the doubled waveguide height, in ELF, and a lower frequency range, noticeable changes in the field strength caused by the state of the ionosphere are found.

Experimental studies of signals from a controlled source in the middle zone under various geophysical conditions have been performed. In the experiment, five field electromagnetic components were measured in the range 0.3–95 Hz. Based on these observations, the effective conductivity of the underlying surface was calculated and an interpretation of the results was given.

Получены аналитические формулы для тангенциальных составляющих низкочастотного электромагнитного поля в плоском волноводе Земля–ионосфера, возбуждаемом заземленной линейной горизонтальной антенной. Исследовано поведение поверхностного импеданса в зависимости от электродинамических характеристик волновода и расстояния от источника. Показано, что поверхностный импеданс совпадает с импедансом плоской волны на земной поверхности на расстояниях от источника, превышающих толщину скин-слоя, при малости скин-слоя по сравнению с удвоенной высотой волновода. Теоретически обосновано влияние ионосферы на амплитуду магнитного поля крайне низкочастотного и более низкого диапазона, а соответственно и на импеданс, на расстоянии меньше двух высот ионосферы. Такого рода эффект наблюдался в экспериментах, выполненных на Кольском полуострове, которые позволили из-за низкой проводимости Земли обнаружить влияние ионосферы на амплитуду магнитного поля в низкочастотном диапазоне.

The analytical formulas are obtained for the tangential components of an extremely low-frequency electromagnetic field in the Earth–ionosphere plane waveguide excited by a grounded linear horizontal antenna. The behavior of the surface impedance is studied as a function of the electrodynamic characteristics of the waveguide and the distance from the source. It is shown that the surface impedance coincides with the plane wave impedance on the Earth’s surface at distances from the source larger than the skin depth provided that the skin layer is thinner than double the waveguide’s height. The influence of the ionosphere on the amplitude of an extremely low- and lower frequency magnetic field and, thus, on the impedance at the shorter distances than two ionospheric heights is theoretically substantiated. This type of effect was observed in the experiments conducted on the Kola Peninsula, where the low conductivity of the Earth allowed the detection of the effect of the ionosphere on the amplitude of the magnetic field in the low-frequency band.

Based on the experimental studies on measuring the controlled source signals in the near zone under different geophysical conditions, it is established that the amplitude of the field experiences variations in the lower part of the ELF band and at lower frequencies. At the same time, variations in the VLF range are absent. For identifying the factors responsible for this peculiarity in the behavior of the field, excitation of the ELF and lower frequency electromagnetic field in the Earth-ionosphere waveguide with different conductivities of the Earth and the ionosphere is considered. The theoretical calculations are proposed showing that at low conductivity of the Earth, the effect of the ionosphere in the near zone can be significant.

AbstractPolarization characteristics of the field of an on-Earth emitter located at the Kola Peninsula are experimentally measured at a distance that is no greater than the height of an effective ionospheric waveguide in the FENICS-2014 experiment. Variations in the field amplitude and orientation of the major axis of polarization ellipse are observed at lower frequencies upon significant changes of the K index of geomagnetic activity. Polarization characteristics of the horizontal component of magnetic field calculated with allowance for the ionosphere and two-layer Earth structure prove the observed sensitivity of the ultralow- and lower-frequency filed in the near-field zone to the state of ionosphere at lower conductivity of underlying medium. Theoretical results are compared with the experimental data. The results are important for deep sounding of the Earth and monitoring of ionosphere with the aid of controlled low-frequency ground sources.

Polarization characteristics of the field of an on-Earth emitter located at the Kola Peninsula are experimentally measured at a distance that is no greater than the height of an effective ionospheric waveguide in the FENICS-2014 experiment. Variations in the field amplitude and orientation of the major axis of polarization ellipse are observed at lower frequencies upon significant changes of the K index of geomagnetic activity. Polarization characteristics of the horizontal component of magnetic field calculated with allowance for the ionosphere and two-layer Earth structure prove the observed sensitivity of the ultralow- and lower-frequency filed in the near-field zone to the state of ionosphere at lower conductivity of underlying medium. Theoretical results are compared with the experimental data. The results are important for deep sounding of the Earth and monitoring of ionosphere with the aid of controlled low-frequency ground sources.

The shape and dynamics of an unusual disturbance in the Pc 1 geomagnetic pulsation range recorded by ground-based induction magnetometers during the morning hours (03–06 MLT) on September 11, 2017, in the late recovery phase of a strong magnetic storm were analyzed in the context of changes in the parameters of the interplanetary medium. Pulsations were observed in the auroral and subauroral zones, as well as at midlatitudes, and had a complex structure in the form of multiplet “pearls” (1–1.5 Hz range) and two series of narrow-band bursts (2–3 Hz range) with a repetition period of ~5 and ~20 min. Pulsations in the form of a series of bursts are a rare event and are recorded mainly during the daytime hours. Comparison of the dynamics of Pc 1 pulsations with the parameters of the interplanetary medium was carried out with data from the DSCOVR and THEMIS satellites. A change in the carrier frequency and intensity of multiplet pearls was a response to a jump in the plasma density of the solar wind. A series of bursts with a period of ~5 min could be initiated by a simultaneous increase in the solar-wind velocity and the By -component of the interplanetary magnetic field, which was observed ~40 min after the density jump. We associate bursts with a period of ~20 min either with the response of the magnetosphere to the short-term excursion of the daytime magnetopause to the Earth or with oscillations of a close period in the transition region ahead of the front of the density perturbation. The frequency, period, and amplitude of the pulsations are estimated.

Excitation of electromagnetic fields at extremely low and lower frequencies in the near-field zone of the Earth–ionosphere waveguide is considered. Variations in the field amplitude in the lower part of the extremely low frequency (ELF) range and lower frequencies are experimentally determined under different geophysical conditions in the absence of variations in the ultralow frequency (ULF) range. The effects related to such variations are analyzed with the aid of theoretical calculations that show significant effect of ionosphere in the near-field zone at relatively low terrestrial conduction.

Влияние ионосферы на возбуждение электромагнитного поля диапазона КНЧ и более низких частот в ближней зоне© Е.Д

Excitation of electromagnetic waves in a two-layer medium by a horizontal antenna or an antenna with flooded electrodes that is situated on the water surface is investigated. The region below the interface between two media is considered. The general solution of the problem is presented in the form of the well studied modified Bessel functions within the framework of a quasi-stationary approximation. In contrast to the scheme, which is widely applied in geoelectricity and connected with the calculation of the field on an interface, the Watson rather than Fok integrals are used. The Watson integrals make it possible to determine the field both on the interface and outside of it. The limit passage is made to the values of the potential and fields, when the interface is approached from below, as well as in the lower conducting medium at the distance equal to the thickness of several skin layers from a source.