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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]

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]