Vol 65, No 4 (2025)

A new physical mechanism of flare energy release in force-free magnetic flux ropes is described: as the top of the magnetic rope-loop enters the corona, the external pressure G, which keeps it from expanding, steadily decreases. At its critically low value, the longitudinal field tends to zero on the magnetic surface where the currents change sign, and the force-free parameter and azimuthal current near this surface grow without limit, approaching the rupture. This excites plasma turbulence in the rope and serves as a flare trigger. Rapid dissipation of the field and currents on the anomalous plasma resistance induces an electric field much higher than the Dreiser field. The forces acting on the flare filament in the corona are described, and the conditions for its transition to the dynamic CME regime are discussed.

We investigated the correlations between the cutoff rigidity of cosmic rays and the parameters of interplanetary space, solar wind, and geomagnetic activity during a strong magnetic storm on March 23–24, 2023. The cutoff rigidity of cosmic rays was obtained by calculating the trajectories of particles in the magnetic field of the solar wind according to the Tsyganenko Ts01 model. The analysis showed that the changes in the cutoff rigidity is controlled mainly by changes in the indices of geomagnetic activity Dst (correlation coefficient k ≈ 0.95), as well as electromagnetic parameters — the total value of the interplanetary magnetic field B, its component Bz, the azimuthal component of the electricfield Ey and the plasma parameter β (│k│≈ 0.6–0.75). The parameters of the solar wind such as velocity V, density N, and dynamic pressure P have little effect on the variations of the cosmic ray cutoff rigidity (│k│<0.45).

The results of the analysis of long-term changes in the relationship between solar activity indices for 1957–2022 are presented. For this purpose, the smoothed (using a 24-month filter) indices F10, F30, Ly-α, MgII, Ri and IG were used: the solar radio emission flux at the wavelengths of 10.7 and 30 cm, solar radiation in the Lyman-alpha line of hydrogen (121.567 nm), the ratio of the central part to the flanks in the magnesium emission band of 276–284 nm on the Sun, the international sunspot number and the ionospheric index, which is determined from ionospheric data as an analogue of the sunspot number. It was confirmed that the entire measurement period can be divided into the intervals 1957–1980, 1981–2012 and 2013–2022, in which the relationships between the solar activity indices are clearly different. In the interval 1957–1980, these relationships are stable over time, i.e. there is practically no linear time trend in the dependence of one solar activity index on another. In the 2013–2022 interval, such trends are usually significant. In this interval, the trend ∆IG(X) = IG – IG(X) is negative and significant for X = F10, F30, MgII, Ly-α, or Ri, where IG(X) is the average dependence of IG on X for this interval.

Long-term variations (trends) in two parameters of the ionospheric F2 layer (foF2 and hmF2) based on the observations at the Sverdlovsk (Arti) ionospheric station and the calculated values of TEC over that station are analyzed. Five near-noon moments of LT are considered. Principal analysis is performed for three winter months, but for the sake of comparison, the values of the trends in foF2 and hmF2 for the spring month April, summer month July and fall month October are also presented. A confirmation of the existence of seasonal variations in the foF2 trends is obtained: negative trends are the strongest in winter. The obtained results agree with the results obtained earlier for midlatitude stations of the Northern and Southern hemispheres. No substantial seasonal variations are found for the hmF2 trends. Trends in the ТЕС values over the aforementioned station are also considered. It is demonstrated that there is a reasonable agreement between the relative trends (in percent) in foF2 и ТЕС.

The paper presents a comparative analysis of diurnal variations in the monthly median critical frequency of the regular ionospheric F2 layer (foF2) for all “standard” seasons during the period of low solar activity in 2007–2008. The hourly data from manual processing of measurements taken at mid-latitude ground-based vertical ionospheric sounding stations Wakkanai (Japan) and Hobart (Australia) located almost symmetrically relative to the geographic equator were analyzed. It is found that the relative differences in the median of the third month of any of the “standard” seasons compared to the median of the second month of the season are significantly higher than the differences in the median of the first month of the season relative to the median of the second month of the season. It is also shown using a specific example of these two mid-latitude stations that, according to their characteristics, the median of November (the last month of the autumn season) corresponds to the beginning of the winter season, and the median of May (the last month of the spring season) – to the beginning of the summer season. It has been established that the relative contribution of solar radiation to changes in the value of the electron concentration at the maximum of the F region (NmF2) under conditions of low solar activity in equinoctial seasons, associated with variations in the value and rate of change of the solar zenith angle, is no less than half of the relative contribution to changes in NmF2 made by variations in the composition of the neutral atmosphere.

Data on the content of the cosmogenic isotope ¹⁴C in the Earth’s atmosphere make it possible to study variations in solar activity in past epochs. However, changes in the Earth’s climate over time also lead to a distortion of information about solar activity. This paper examines the time interval from the end of the 17th to the beginning of the 10th millennium BC. During this time, as is known, there were several periods of climate change on Earth. There were several warming events (Mayendorf, Allered) and cooling events (the Oldest, Ancient, and Late Dryas). The reasons for these changes have not yet been determined. It is shown that a possible cause of the cooling during the Ancient Dryas could be a decrease in solar activity.

The possibility of explaining the causes of global climatic changes in the Late Pleistocene of Northern Eurasia on the basis of the astronomical theory of climate change is shown.

In the Late Pleistocene, the effect of dividing seasonal radiation intensity by phases of annual Earth radiation intensity was found, which explains the mechanism of manifestation of the 100-millennial cycle in the Earth’s natural system. Solar tuning (modeling) of the climatic epochs of the Late Pleistocene of Northern Eurasia has been performed. Based on the model, the solar conditions and the mechanism of development of cover glaciations in Northern Eurasia in the Late Pleistocene are determined. The cause of global climate change is related to the dynamics of the radiation factor, the representative characteristics of which are the intensity of summer radiation and the intensity of winter meridional radiation heat transfer in the Northern Hemisphere. The chronological discrepancies between the model and actual climatic epochs, reflecting the nonlinear response of the natural system to the dynamics of irradiation, average about 7 thousand years. There is a weak response of the oxygen isotope composition (δ¹⁸O) of bottom foraminifera (maximum range of 0.2% fluctuations) to fluctuations in radiation factors of global climatic changes in Northern Eurasia.: the intensity of summer radiation in the phase division of seasonal radiation (the average range for the summer half-year is 0.486%, for July – 0.785%) and in the phases of climatic precession (the average range is 4.336%).