Effects of the May 10–13, 2024 extreme magnetic storm in the Asian region of Russia have been studied using experimental data from vertical and oblique sounding of the ionosphere with a continuous chirp signal. Features of ionospheric disturbances induced by the magnetic storm have been revealed: the long-lasting negative ionospheric disturbance that was manifested as a significant decrease in F2-layer critical frequencies and maximum observed frequencies of radio paths; the absence of HF signal reflections from F-region due to sporadic Es layer and increased absorption of HF signals; recording of auroral and oblique Es layers; the long-lasting G-effect during local daytime during which the F1-layer critical frequency exceeded the F2-layer critical frequency; the dusk enhancement of electron density and F2-layer peak height. We have found a correlation of variations in ionospheric parameters and the maximum observed frequencies of HF radio wave propagation modes with spatial location of the main ionospheric trough and the equatorial boundary of the diffuse electron precipitation zone.

In the paper, we examine the atmospheric part of the global electric circuit. When studying large-scale currents in the atmosphere flowing from the ionosphere to the ground, the ionosphere and Earth’s surface can be considered as ideal conductors with high accuracy. These currents are determined by the ground-ionosphere voltage and the spatial distribution of conductivity in the atmosphere. We employ a one-dimensional model of atmospheric electric fields and currents in which currents are assumed to be nearly vertical. Then it is possible to reduce the spatial distribution of conductivity to longitude and latitude distribution of conductivity of atmospheric columns. By integrating the conductivity over the entire Earth surface, we obtain the total conductivity of the atmosphere. Inside clouds, air conductivity decreases due to the ion attachment to water drops. Using available data on decrease in local conductivity within individual clouds, we analyze the effect of cloud density in latitude, longitude, and height on geographical distribution of conductivity and total conductivity of the atmosphere. By the example of 2009, it is shown that cloudiness reduces the total conductivity of the atmosphere by 20 %. Its variations during the day and year are so small that the model fair-weather electric field varies only by 2 % due to cloudiness. Judging by the results obtained, the influence of clouds on atmospheric conductivity does not explain the diurnal and seasonal cycles of the fair-weather electric field strength (Carnegie diagram).

The paper presents a mean-field model for large-scale flows in convection zones of the Sun and solar-type stars. The model extends former differential rotation models by allowance for variations of the flow with time and its deviation from axial symmetry. The model is realized as a numerical code, which combines the spectral method of decomposition in spherical functions with second-order accurate finite-difference method in time and radius. First computations show close agreement of the axially symmetric part of the computed flow with helioseismological detections of differential rotation and meridional circulation. Patterns of the time-decaying non-axisymmetric flow computed with the model qualitatively agree with the Rossby waves observed on the Sun. The paper also formulates a problem for further development of the large-scale flow theory.

A device for measuring geomagnetically induced currents (GISs) has been created which is installed at the Ininskaya power substation in the Altai Republic. Since April 2024, periodic monitoring of GIS in the 110 kV power transformer grounding neutral has been carried out. GISs were registered during geomagnetic disturbances up to 138 mA, which, taking into account the parallel grounding of the Ininskaya substation and the Ininskaya solar power plant, means the presence of 1.3 A total GIS in the grounding of both objects. GISs are shown to occur during Pc3 and Pc5 geomagnetic pulsation observations. The qualitative agreement has been found between the GIC measurement results and the model values calculated from Baigazan magnetic station data in the approximation of the homogeneous Earth’s crust conductivity. The grounding resistance is shown to exert an effect on recorded GICs.

A statistical analysis of vertical ionospheric sounding data from the Yakutsk station (62.01° N, 129.43° E, 57.12° MLAT) for the period from 1956 to 2017 encompassing six solar cycles has been carried out to identify long-term changes in the F2 layer of the subauroral ionosphere and their relationship with solar and geomagnetic activity. We examined variations in one of the main parameters of the ionospheric F2 layer, the critical frequency. A high correlation was found between the F2-layer critical frequency and the solar activity index F10.7. It is shown that during six solar cycles (cycles 19–24) there were negative trends in annual average F2-layer critical frequencies both at midday and at midnight. It has been revealed that foF2 trends depend on the season and time of day. Absolute values of the trends are higher in equinoctial and summer seasons. Peak negative trends are observed at midday during equinoctial months, reaching approximately –11 kHz/year.

The paper analyzes experimental data on the position of the polar wall of the main ionospheric trough under low geomagnetic activity at Kr=0–1 from measurements made at the Yakutsk chain of vertical and oblique sounding ionosondes. The northern boundary of the trough under these conditions shifts to high magnetic latitudes 67–70°. This corresponds to the position of the geophysical structure “contracted oval” or compressed oval. Critical frequencies at the polar wall of the trough have high values ​​of about 6–8 MHz. At this time, the DMSP satellite records intense 200–300 eV electron precipitation that can create the observed ionization in the F-region of the ionosphere.

Using a novel signal processing technique — the method of correlation function of amplitude and phase fluctuations (APCF) — we have obtained the first spectra of equidistant frequency (EF) groups in records of background seismic oscillations. This method has previously been successfully applied to the analysis of resonant oscillations in Earth’s magnetosphere-ionosphere system. In the case of seismic oscillations, all peaks in the EF spectrum can be divided into two groups corresponding to eigenfrequencies of vertical standing waves of P and S types in the Earth’s interior. The upper turning point of these waves lies at the free upper boundary of the Earth’s crust, whereas the lower turning point is located at the boundaries between subsurface layers. We demonstrate that this signal processing technique can serve as a new method for probing the layered structure of the Earth’s subsurface. Specifically, it enables the determination of the depth and thickness of each layer, as well as the estimation of elastic properties (such as Poisson’s ratio) of the geological material composing the layer. The findings have revealed that at the depth of ~2.7 km there are two layers of different solid substance 58 m and 140 m thick with Poisson’s ratios of 0.231 and 0.187.

The paper presents the results of modeling of spatial and temporal perturbations of the thermosphere during a strong meteorological disturbance. The modeling was performed using the Global Self-Consistent Model of the thermosphere, ionosphere, and protonosphere (GSM TIP). The impact of tropospheric/stratospheric sources on the thermosphere during dissipation of acoustic and internal gravity waves, generated in the meteorological storm region, was considered in GSM TIP by specifying an additional thermal source. The results of modeling of ionospheric effects of the meteorological storm in October 2017 have shown that the action of a local additional source of heating of the thermosphere leads to perturbations of the thermosphere and ionosphere parameters both directly above the source region and at a significant distance from it. In additional heating of the thermosphere, a decrease is observed in the total electron content (TEC) values, reaching 20 % in the daytime compared to a meteorologically quiet day. To the south and east of the source region, there are positive TEC perturbations with relative amplitudes 5–10 % during the daytime. The physical processes determining the ionospheric response directly in the source region are conditioned by heating of the thermosphere and its influence on changes in the neutral composition and circulation of the neutral wind. The TEC perturbations in the regions remote from the source region are determined by dynamic processes, which lead to the eastward transport of plasma and displacement of ionospheric perturbations to low latitudes.

During the active experiment FENICS-2024 on the Kola Peninsula using a decommissioned power transmission line as a horizontal radiating antenna, ultra-low-frequency signals of the 1–6 Hz range were recorded at magnetic stations located from ~1600 km to ~2100 km from the transmission line with normalized amplitudes from ~0.3 fT/A to ~0.8 fT/A. Observational results are compared with approximate analytical estimates of the magnetic field excited by the magnetic dipole. The calculations turned out to be in qualitative agreement with the observational results. To assess the possible response in the upper ionosphere, a numerical model of the ULF field in the atmosphere and ionosphere generated by the horizontal surface current was employed. The model is based on solving the system of Maxwell equations in the vertically inhomogeneous atmosphere and ionosphere. The fundamental feature of this model is that it correctly takes into account the contribution of ionospheric waveguide propagation. The observational results supported by numerical simulation have shown the potential of active experiments of the new type for signal generation for large-area magnetotelluric sounding and for modification of near-Earth plasma with artificial signals.