—An analytical solution is obtained for the problem of excitation of electromagnetic waves by a horizontal dipole located at the interface between vacuum and a conducting two-layer medium, based on the smallness of the wavelength in a conducting medium compared to vacuum. Numerical calculations have been carried out, which made it possible to evaluate the effectiveness of this approach and show that the approximate formulas describe the behavior of the field with high accuracy. An asymptotic representation is obtained for the magnetic field at the interface between a flat-layered Earth and the atmosphere, which has the form of a wave in a homogeneous half-space with an excitation coefficient depending on the effective conductivity of the lower half-space, which is useful for determining the electrical conductivity of the lithosphere based on experimental data recording the electromagnetic field of an active source. A high sensitivity of the vertical component of the magnetic field to the conductivity of the medium in comparison with the horizontal is noted.

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.

The paper considers methodological issues of solving the inverse problem of electromagnetic sound, based on the results of measuring the low-frequency magnetic field from two orthogonal antennas located on the Kola Peninsula in areas with different conductivity of the lithosphere. The polarization characteristics of the tangential magnetic field components are determined. They are compared with theoretical calculations using a plane-layered model of a wave propagation medium with homogeneous layers. Comparison of the theoretical calculations with the experimental data on the major axis of the polarization ellipse yields results consistent with existing concepts of the conductivity of the Kola Peninsula. Differences in the direction of the ellipse and ellipticity require complication in the model: the inclusion of faults in the lithosphere, as well as the anisotropy of the structures forming it.

We present the results of comparing the total electron content measurements based on GLONASS satellite signals and the EISCAT UHF incoherent scatter radar (Tromsø, Norway) during modification of the high-latitude ionosphere in the magnetic zenith direction by high-frequency radio waves of the EISCAT/Heating facility (Tromsø, Norway). The measurements were performed during two experiment campaigns in October 2013 and in October 2018. In general, the total electron content variations obtained from the radar data in the altitude range 100–400 km were consistent with the total electron content variations from the GLONASS satellites. The efficiency of using GLONASS satellites for observations of high-latitude phenomena was shown. The anomalous increase in the total electron content by 4 TECU obtained from the incoherent scatter radar when the ionosphere was heated in the region close to the magnetic zenith is considered. The GLONASS satellite data show the total electron content reduction in the same region. To explain the disagreement between measurements by these two methods, the effect of smallscale electron-density irregularities arising in the region modified by high-power HF radio waves is considered. It is shown that when the electron density in artificial irregularities exceeds the background density of the medium by 2 ・ 10−3 times in relative units, scattering by irregularities with spatial scales of the order of 16 cm becomes predominant in the reflected signal.

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.

The theoretical explanation and experimental confirmation of the previously observed discrepancies in the measurements of total electron content by the method of incoherent scattering and the method of radio sounding by the influence of small-scale artificial ionospheric irregularities, leading to the occurrence of a coherent component in the power of the scattered signal are given.

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.

We present the results of studying the characteristics of the artificial plasma structures excited in the ionospheric F2 region modified by high-power HF radio waves. The experiments were carried out at the Sura heating facility using satellite radio sounding of the ionosphere. The plasma density profile was reconstructed with the highest possible spatial resolution for today, about 4 km. In a direction close to the magnetic zenith of the pump wave, the following phenomena were observed: the formation of a cavity with a 15% lower plasma density at the altitudes of the F2 layer and below; the formation of an area with plasma density increased by 12% at altitudes greater than 400 km. With a long-term quasiperiodic impact of the pump wave on the ionosphere, wavy large-scale electron-density perturbations (the meridional scale λx ≈ 130 km and the vertical scale λz ≈ 440 km) are also formed above the Sura facility. These perturbations can be due to the plasma density modulation by an artificial acoustic-gravity wave with a period of 10.6 m, which was formed by the heat source inside a large-scale cavity with low plasma density; there is generation of the electron density irregularities for the electrons with ΔNe/Ne ≈ 3% in the form of layers having the sizes 10–12 km along and about 24 km across the geomagnetic field, which are found both below and above the F2-layer maximum. The mechanisms of the formation of these plasma structures are discussed.

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.

This study presents an analysis of geomagnetic disturbances and ionospheric electron density distribution during the 2015 St. Patrick's Day geomagnetic storm. To study those we have used the satellite-borne and ground-based observations. The St. Patrick's geomagnetic storm covers the interval of 15–23 March 2015, when solar eruptive phenomena (a long-enduring C9-class solar flare and associated CME's on 15 March) and a strong geomagnetic storm on 16–18 March (Dst dropped as strong as –228 nT) were reported. This geomagnetic storm is still the strongest one observed in the current solar cycle. The severe geomagnetic storm on 17-18 March 2015 led to complex effects on the ionosphere. We consider major features of the positive and negative ionospheric storms development at European mid- and high-latitudes. One of the interesting phenomena was observation of the positive ionospheric disturbances during the recovery phase. Using the Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) we examined the main physical processes that played a major role in dramatic changes of the total electron content and the F2 layer peak electron density during this storm event.

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.

Results of observations of variations of temperature, electron concentration and total electron content of the high-latitude region of the ionosphere during its modification by powerful short radio waves of the heating complex EISCAT/Heating (Tromsø, Norway) according to signals of the GLONASS satellites and the incoherent scattering UHF EISCAT radar (Tromsø, Norway) have been provided. The geometry of passes of the GLONASS and GPS satellites for operating conditions of the heating complex in Tromsø has been considered. It has been shown that during the experiments on the EISCAT/Heating complex for the study of the modified structure of the high-latitude ionosphere it is more convenient to use the GLONASS satellites. Parameters of orbits of these satellites allow researching changes of total electron content in the direction along the geomagnetic field line at the place of observation. It has been shown that during heating of the ionosphere by powerful short radio waves its structure is becoming an irregular one. Operation of the heating complex in the mode "switched on – switched off" has caused appearance of wavy variations of total electron content with the periods close to the heating period. The main features of behavior of the total electron content in the case of the continuous heating of the ionosphere in the direction of the magnetic zenith according to the GLONASS satellite are: reduction of total electron content in the central zone of the antenna diagram, i. e. in the direction of the magnetic zenith, and presence of the increased values of total electron content at the edges of the heating zone. According to the incoherent scattering radar the heating of the ionosphere by the powerful short radio wave has created the region of the increased electron temperature and electron concentration along the direction of the magnetic zenith. The behavior of total electron content according to the GLONASS satellite and the radar of incoherent scattering in many respects corresponds each other except the central part. Reasons of the mismatches have been considered.