Vol 64, No 5 (2024)

The accuracy of the M. Aschwanden’s (2020) model for short-term (24 h) prediction of the maximum X-ray class of solar flares based on the power-law dependence on the energy of potential magnetic field of active regions is checked and assessed. For this purpose, a sample of 275 flares (253 M-class and 22 X-class) in isolated active regions on the solar disk in 2010−2023 is analyzed. Magnetic field extrapolations are made in the nonlinear force-free and potential approximations using the GX Simulator based on photospheric vector magnetograms from the Helioseismic Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). It is found that in 6% of cases Aschwanden’s model underestimates the predicted maximum flare class relative to the observed one (maximal underestimation by 4.4 times). The accuracy of this model (the average ratio of the observed to predicted maximum flare class) is 0.31 ± 0.47. Four other statistical models are proposed, two of which, like Aschwanden’s model, are based on the power-law dependence of the maximum flare class on the energy of potential magnetic field, and the other two are based on the power-law dependence on the free magnetic energy of active regions. These models give fewer (or no) underestimations of the maximum flare class, but two to three times lower forecast accuracy, ranging from 0.11 to 0.17. Additionally, based on the obtained statistical sample, estimates of the limiting X-ray class of solar flares are made. The five models give different limits ranging from ~X14 to ~X250. The realism of these values and the possibility of refining the models by expanding the sample of events is briefly discussed.

We analyzed 214 cases of “polar” substorms on the Scandinavian meridian IMAGE, i.e. substorms recorded by magnetometers located at geomagnetic latitudes above ~70° MLAT at 19−02 MLT under magnetically quiet time in the absence of negative magnetic bays at lower latitudes. The Harang Discontinuity, which separates the westward and eastward electrojets by latitude, is a typical structure for the indicated MLT sector of the high-latitude ionosphere. The global distribution of ionospheric electrojets and the location of the Harang discontinuity during the development of “polar” substorms were studied by the maps constructed from the results of spherical harmonic analysis of the magnetic measurements on 66 simultaneous ionospheric communications satellites of the AMPERE project. Based on these maps analysis, it is shown that the instantaneous location of the equatorial boundary of the ionospheric current of a “polar” substorm determines the instantaneous location of the polar boundary of the Harang Discontinuity, and the polar boundary of the eastward electrojet determines its equatorial boundary. It has been established that the appearance of 90% of the “polar” substorms is observed simultaneously with increasing of the planetary substorm activity according to the AL-index and the development of a magnetospheric substorm in the post-midnight sector. At the same time, the development of the evening “polar” substorms is associated with the formation of near-midnight magnetic vortices at geomagnetic latitudes of ~70° MLAT (near the “nose” of the Harang discontinuity), indicating a sharp local enhancement of the field-aligned currents. This leads to the formation of a new substorm in the evening sector of near-polar latitudes, called a “polar” substorm with typical features of the onset of a substorm (Pi2 geomagnetic pulsation bursts, an abrupt onset of the substorm close to the equatorial boundary of the constructed oval (the development of a “substorm current wedge” – etc.)

The first results of identifying trends in the annual average ionospheric indices ∆IG₁₂ and ∆T₁₂ are presented, which were obtained after excluding the dependence of these indices on solar activity indices from IG₁₂ and T₁₂. In this case, the solar activity indices are F10 and F30 – solar radio emission fluxes at 10.7 and 30 cm. It was found that for the interval 1957–2023 all analyzed linear trends are negative, i.e. the values of ∆IG₁₂ and ∆T₁₂ decrease with time, and these trends are significant. In absolute value, they are maximum for ∆IG₁₂, taking into account the dependence of IG₁₂ on F10₁₂, and minimum for ∆T₁₂, taking into account the dependence of T₁₂ on F30₁₂. Taking into account the nonlinearity of trends shows that, for example, after 2010 they intensified. Relationships are presented that allow, based on the data of trends in ionospheric indices (∆IG₁₂ or ∆T₁₂), to judge the nature of the ∆foF2 trend over a specific point. For this purpose, using the IRI model for foF2, a coefficient was obtained that gives the relationship between the trends of the ionospheric index and ∆foF2 over a given point. Based on a comparison with experimental data at mid-latitudes, it was found that trends in ionospheric indices make it possible to correctly determine the sign of the ∆foF2 trend and the general tendency for this trend to change, but the calculated trend value over a specific point may differ markedly from the experimental data.

Based on the data of seventeen midlatitude ionospheric stations for 1958–1988, analysis of seasonal features of the F2 layer maximum concentration (NmF2) at different longitudes with enhanced (48 > ap(τ) > 27) geomagnetic activity, where ap(τ) – the weighted average (with a characteristic time of 14 hours) ap-index of this activity. As the characteristics of NmF2 variability, the standard deviation of NmF2 fluctuations for relatively quiet conditions and the average shift of these fluctuations x_ave during daytime (11–13 LT) and night (23–01 LT) were used. It was obtained that at all analyzed stations, the dispersion σ² for enhanced geomagnetic activity is greater than for quiet conditions, and, other things being equal, it is maximum in winter at night. For enhanced geomagnetic activity in all seasons, the difference in x_ave values between the analyzed stations is quite large. One of the reasons for this difference is associated with the dependence of x_ave on geomagnetic latitudes. To select these latitudes, approximations of the geomagnetic field with tilted dipole (TD), eccentric dipole (ED) or using corrected geomagnetic (CGM) coordinates were used. It has been obtained that the x_ave dependence on the ED-latitude is more accurate in comparison to the x_ave dependence on the TD-latitude or CGM-latitude during all seasons at night and during equinoxes and winter – in the daytime. In the summer, in the daytime hours x_ave dependence on ED-latitude and CGM- latitude are comparable in accuracy, and they are more accurate in comparison to x_ave dependence on the TD-latitude. Consequently, ED-latitudes are optimal for taking into account the effects of storms in the F2 layer maximum concentration at middle latitudes during all seasons. This conclusion was apparently made for the first time.

The results of the “Terminator” space experiment on board the International Space Station are given. Images of the Earth atmosphere are obtained in the near IR spectral range at limb-geometry of observations under the full Moon. The calculated vertical profiles of volume emission/scattering rate point that the aerosol layer occurs within the height region of 80 – 100 km in the Earth atmosphere. It is proposed that this layer is of meteoric origin. Estimations show that the size spectrum of aerosol particles lies within the region of 1 – 100 nm.

During analyzing a narrowband signal, the Hilbert transform is often used, which makes it possible to describe the process through slowly changing functions: the envelope (amplitude) and, weakly dependent on time, the characteristic frequency of the signal - the “instantaneous” frequency. Based on the smoothness of these characteristics, one can evaluate the process and compare it at different periods. This approach was used to analyze the spectral components of a series of average monthly Wolf numbers. This description of the main and second harmonics, supplemented by the properties of the long-period component, gives a fairly complete picture of the entire series of monthly averages. The work examines the correspondence of the characteristics of reliable data, with this approach, to the accepted description through the parameters of cycles (maximum of the cycle, duration of the cycle and its growth branches) and constructs an “envelope” of the maxima of the cycles. The time dynamics of the “instantaneous” frequencies of the fundamental and second harmonics of the entire series are also presented and significant differences in their behavior are noted in the intervals corresponding to the reconstructed and reliable parts.