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Abstract—The possibility of remote studying the electromagnetic and ionospheric effects of the eruption of the Tonga volcano on January 15, 2022 is demonstrated. At distances up to 15000 km from the source, geomagnetic variations associated with Schuman resonance (SR) perturbations, propagation of Lamb wave and acoustic-gravity waves were detected. It is shown that the formation of a high-power source of thunderstorm activity caused by the eruption resulted in a significant (more than threefold) increase in the amplitude of geomagnetic disturbances at SR frequencies, which correlates with the number of lightning events. The effect of the eruption on the frequency characteristics of the SR was not detected. [ABSTRACT FROM AUTHOR]

The influence of path illumination on the amplitude of ultralong-wave radio signals during the passage of the solar terminator is discussed. Based on long-term measurements (from 2014 to 2020), it is shown that in winter a sharp drop in signal amplitude on mid-latitude paths occurs even when 15% of the path length is illuminated. An analysis of variations in the amplitude of ultralong-wave signals also made it possible to estimate the height of their reflection from the upper wall of the unilluminated part of the waveguide hN (the D-region of the ionosphere) and identify seasonal and annual variations of this value. An increasing hN trend was experimentally found by about 4 km in 7 years, caused by the decline in solar activity during this period. [ABSTRACT FROM AUTHOR]

The study is devoted to the analysis of geomagnetic field disturbances and the response of the Schumann resonance (SR) during the eruption of the Tonga volcano in 2022. The data on geomagnetic field variations at distances from 800 to 15,000 km from the volcano according to the INTERMAGNET network and parameters of SR signals recorded at Mikhnevo Observatory in Russia were used. The source of global geomagnetic disturbances are acoustic–gravity waves (AGWs), which caused changes in ionospheric conductivity, values of ionospheric currents, and the geomagnetic field. The propagation velocity of magnetic disturbances 263 ± 5 m/s, corresponding to the AGWs velocity, was determined and an independent estimate of the time of the eruption phase that caused the generation of the atmospheric wave (4:14 ± 10 UT) was obtained. A new method of processing the results of measurements of SR disturbance with a time resolution of 5 min instead of the usual 10–15 min allowed not only to detect but also to study this phenomenon in detail. The peculiarities of signals related to the number and energy of lightning discharges were revealed. Synchronous measurements of SR signals and geomagnetic field variations in a single observatory for the first time allowed to obtain an independent estimate of the eruption time and the electromagnetic disturbance propagation rate. Plain Language Summary: The eruption of the Tonga volcano on 15 January 2022 caused global disturbances in the geomagnetic field and Schumann resonances. The paper shows that the source of disturbance of Schumann resonances during the eruption was increased thunderstorm activity in the volcanic cloud. Disturbances in the geomagnetic field are caused by acoustic‐gravity waves resulting from the eruption. A comprehensive analysis of the data made it possible to determine the time of the beginning of the eruption. Key Points: The eruption of the Hunga Tonga‐Hunga Haapai volcano in January 2021 caused global atmospheric and electromagnetic perturbationsTime of geomagnetic disturbances at distances 800–16,000 km from volcano showed the velocity of their propagation 263 m/s typical for acoustic–gravity wavesThe time difference between geomagnetic disturbance and Schumann resonance response at one point gives an independent estimate of their propagation velocity [ABSTRACT FROM AUTHOR]

The interference of electromagnetic signals from lightning discharges in the frequency range below 100 Hz is the source of a global electromagnetic phenomenon in the Earth's atmosphere known as the Schumann resonance (SR). Changes in the parameters of SR signals caused by geophysical disturbances make it possible to study the state and dynamics of the lower ionosphere. When calculating the SR parameters, there are problems associated with the impact of electromagnetic interference of natural and anthropogenic origin. The main natural sources of interference are signals associated with the radiation of nearby lightning discharges, as well as the influence of the Alfvén ionospheric resonator. The paper presents a new method for calculating the SR parameters, which makes it possible to find the spectra distorted by interference, mention above, and exclude them from further processing. The developed technique significantly increased the temporal resolution of the obtained data on the frequency and amplitude of the SR. Due to this, it became possible to study the influence of fast heliogeophysical disturbances (such as solar X‐ray flares) on the lower ionosphere and, as a consequence, on the parameters of the SR. An analysis of the experimental data made it possible to establish a linear dependence of the SR frequency on the logarithm of the X‐ray flux in the range up to 0.2 nm during a class X solar flare. Key Points: A new method for calculating the parameters of Schumann resonance (SR) significantly reduces the impact of electromagnetic interferenceThe developed technique increased the temporal resolution of the SR frequency and amplitude dataThe new technique made it possible to establish the dependence of the SR frequency on the X‐ray flux with a wavelength of less than 0.2 nm [ABSTRACT FROM AUTHOR]

An experimental study was made of the dependence of the electron-density perturbations in the D‑regions of the midlatitude ionosphere during solar flares on the spectral composition of X-ray radiation in the wavelength range from 0.01 to 0.4 nm, i.e., outside the measurement range of the GOES satellite. For this, the variation in the brightness temperature of the emission of six flares in 2014–2017 were calculated, and the composition of their radiation was determined. It is shown that, although only a small percent of the total flash radiation energy is in the 0.01–0.2 nm wavelength range, it is the hard components that are the main factor leading to a change in the reflection height of very low frequency (VLF) radio waves. It is shown that the efficiency of solar flares significantly depends on the previous solar activity, not only for class-C flares but also for powerful class-M and -X flares. [ABSTRACT FROM AUTHOR]

Abstract—Studying the spatiotemporal dynamics of the disturbances in the atmosphere, upper, and lower ionosphere requires integrated investigation of interrelated processes at different heights in the different solar-geophysical conditions. The radiophysical complex at the Geophysical observatory "Mikhnevo" of the Institute of Geosphere Dynamics of the Russian Academy of Sciences (IDG RAS) which includes magnetic and electrophysical equipment, radio receiving systems, acoustic instruments, and ionospheric sounders provides data on the structure and dynamics of the ionospheric plasma in the midlatitude European part of Russia. The data from ULF/VLF receivers and measurements of GNSS signals are used to study synchronous variations caused in the lower and upper ionosphere by the magnetic storms, solar X-ray flares, and experiments on artificial modification of the ionosphere. The techniques are developed for processing the multi-instrumental measurements of the variations in the physical fields. They include the methods for reconstructing the absolute ionospheric total electron content (TEC) and electron density profile in the lower ionosphere. The obtained data are used for verifying the IDG RAS models of the lower ionosphere. [ABSTRACT FROM AUTHOR]

The paper presents the results of the reconstruction of the effective height h' and the slope of the profile β of the electron concentration in the D layer of the ionosphere during X-ray flares of M- and X-classes at the midlatitudes. The reconstruction was carried out with data from measurements at the observatory of the Institute of Geospheres Dynamics of the Russian Academy of Sciences at the Mikhnevo observatory of amplitudes and the phases of signals from VLF transmitters GQD (19.6 kHz) and GBZ (22.1 kHz) that propagate along one mid-latitude path. Based on the calculations, the empirical dependencies of the parameters h' and β on the flare energy according to the X-ray flux measurements on the GOES satellite are in the range 0.05–0.4 nm. Criteria are proposed for the selection of solar flares for the verification of theoretical computational models. [ABSTRACT FROM AUTHOR]

Changes in the state of the D and E ionospheric regions lead to variations in the amplitude-phase characteristics of VLF radio signals. The existing theoretical and empirical models of the propagation of low-frequency electromagnetic waves qualitatively describe the relative variations in the parameters of the lower ionosphere associated with strong heliogeophysical disturbances; however, these models do not allow estimation of the absolute value and distribution of the electron concentration. We used the measurement data for the amplitude-phase characteristics of VLF radio signals with different frequencies propagating along two closely spaced paths. This made it possible not only to quantify the parameters of the D region of the ionosphere on a spatial scale of thousands of kilometers during the powerful solar flare of September 10, 2017 but also to restore the electron concentration profile before the onset of X-ray radiation. [ABSTRACT FROM AUTHOR]

In 2012, remote measurements of electromagnetic signals in the ELF/VLF band were taken at different points in Russia during experiments on artificial ionospheric modification with the powerful HF wave at the EISCAT heating facility (Tromsø, Norway). The use of the new, highly sensitive magnetometric equipment allowed signals with an amplitude of a few femtoteslas to be recorded at a distance of up to 2000 km from the source. Analysis of the measurement results discovered substantial differences in the amplitude-phase characteristics of the signals, which were caused by a change in helio-geophysical conditions in the region of heating and along the signal passage route, and features of signal propagation, which are related to their mode of guided propagation, the directivity of the source, and angles of reception. [ABSTRACT FROM AUTHOR]

The statistical characteristics of the intensity of VLF-LF radio signals transmitted from the midlatitude radio stations and recorded by the receiver at the Mikhnevo geophysical observatory (54.94°N, 37.73°E; Institute of Geosphere Dynamics, Russian Academy of Sciences) in 2007-2010 are analyzed. The experiments revealed strong variations in the intensity of radio signals during the deep solar minimum conditions, when the medium does not experience impacts from above associated with solar and geomagnetic activity. We relate the observed variations to the disturbances from below, which are caused by the meteorological and wave processes occurring in the lower atmosphere. [ABSTRACT FROM AUTHOR]