Vol 59, No 2 (2019)

This paper studies the position of the trapping boundary of electrons with energies of >100 keV relative to the equatorial boundary of the auroral oval during a large magnetic storm on December 19–22, 2015, with a minimum Dst of –170 nT as measured by the Meteor-M2–1 satellite. Energetic electrons with energies from 0.1 to 13 MeV and fluxes of low-energy electrons with energies from 0.13 to 16.64 keV have been measured. It is taken into account that the pitch-angle distribution of energetic electrons near the trapping boundary is almost isotropic. It is shown that the energetic electron trapping boundary during the considered storm is detected inside the auroral oval or near its polar boundary. The distance along the geomagnetic latitude between the energetic electron trapping boundary and the equatorial boundary of the auroral oval is determined. The dependence of this distance on time for crossings of the oval before and after midnight is analyzed. It is shown that the distance between the trapping boundary and the equatorial boundary of the oval during the storm decreases after midnight and increases before midnight. These values are almost equal near minimum Dst. The significance of the results obtained for a description of changes in the magnetospheric topology during magnetic storms is discussed.

Studies in the extreme ultraviolet (EUV) and X-ray ranges of the solar spectrum are important due to the active role of radiation of these ranges in the formation of the Earth’s ionosphere. Photons of the EUV range are completely absorbed in the upper layers of the Earth’s atmosphere and induce the excitation, dissociation, and ionization of its different components and, finally, atmospheric heating. From the archive data of the EUV Variability Experiment of the Solar Dynamics Observatory (SDO/EVE), we formed series of diurnal values of the background fluxes radiated beyond flares in the EUV lines HeII (30.4 nm), HeI (58.4 nm), CIII (97.7 nm), and FeXVIII (9.4 nm) in cycle 24 (from 2010 to 2017). These fluxes are compared to the corresponding values of the radio flux F_10.7 at a wavelength of 10.7 cm and the background radiation flux F_0.1−0.8 in the X-ray range between 0.1 and 0.8 nm measured onboard the GOES-15 satellite of the Geostationary Operational Environmental Satellite system. Comparative analysis has shown that the solar radiation in individual lines of the EUV range and the fluxes F_10.7 and F_0.1−0.8 are closely interrelated.

This article examines the effect of various large-scale solar-wind structures and streams on the occurrence of a special type of substorms—the so-called supersubstorms (SSS), which are very intense substorms defined by the indices SML <–2500 nT and AL < –2500 nT. The analysis covers 131 cases of SSS events from observations at the SuperMAG stations in 1998–2016 and 26 cases of SSS events from the IMAGE network. Analysis of the dependence of SSS events on different types of solar wind and different geomagnetic disturbances shows that these events are mainly observed during the approach to the Earth’s magnetosphere of solar-wind magnetic clouds (MC) (42%) and SHEATH plasma compression regions ahead of MCs or ahead of EJECTA. Supersubstorms may sometimes occur during EJECTA (8.3%). Thus, SSS events are caused by interplanetary coronal mass ejections and are, in fact, unassociated with high-speed streams from coronal holes. It is shown that SSS events mainly occur during magnetic storms (Dst < –50 nT). In the rare cases (13.4%) of SSS observations during intervals with Dst > –50 nT, the events occur mostly right after the sudden onset (SC) of a storm (11%) and, very rarely, happen late in the storm recovery phase (1.2%). The space weather conditions associated with SSS events differ sharply from those associated with other types of high-latitude substorms, such as polar and expanded substorms.

Regular and quasi-periodic local time and seasonal variations in the electron density in the maximum of the ionospheric F2 layer during low solar activity in 2016 are analyzed. System spectral analysis of temporal variations in the electron density in the range of 30–180 min is based on windowed Fourier transform, adaptive Fourier transform, and wavelet transform. In all seasons, the ionospheric F2 layer is dominated by oscillation with a period of 70–120 min, an amplitude of ΔN_a ≈ (2–10) × 10¹⁰ m^–3, and a relative amplitude of \({{\Delta {{N}_{a}}} \mathord{\left/ {\vphantom {{\Delta {{N}_{a}}} {\bar {N}}}} \right. \kern-0em} {\bar {N}}}\) ≈ 0.20–0.70. The duration of this oscillation varied from 6 to 17 h depending on the season. The amplitude of oscillations with other periods is significantly smaller. It has been confirmed that the regular local time and seasonal variations in the electron density in the F2-layer maximum are fully consistent with the existing views on physicochemical processes in the ionosphere. The main regularities in the behavior of quasi-periodic variations in the electron density have been revealed.

STARE Doppler data on electron drift velocities detected in the auroral ionosphere for weakly disturbed conditions are analyzed. The Doppler measurements are first averaged along each radar beam. The power spectral density (PSD) as a function of frequency is then calculated by discrete Fourier transforms of averaged signals for every radar beam. Deep stepwise drops (about 10 dB) in spectral powers of the ultra-low-frequency (ULF) range are revealed for all radar beams. These PSD drops are interpreted as manifestations of resonant ULF absorption, which occurs in the eigenfrequency continuum of standing Alfven waves excited on geomagnetic field lines. A variational analysis that models PSD decreases by stepwise profiles of the mean spectral powers is proposed. This analysis provides the least-squares fitting of model profiles to PSD decreases calculated by the data. The frequency of the mean PSD step determined for each radar beam is treated as the minimum frequency of resonant ULF absorption revealed by this beam during the given auroral event. Averaged over all beams, this frequency for the analyzed event is 4.9 ± 0.2 mHz.

The mechanism of the generation of the geomagnetic field disturbance accompanying tsunami wave propagation is considered. Electric currents in the marine environment and the ionosphere are the source of the disturbance. The current in the marine environment arises as a result of its motion in the tsunami wave, while the current in the ionosphere occurs due to the occurrence of an acoustic-gravity wave (AGW) propagating from the atmosphere on the ionosphere. The source of the AGW is the vertical displacement of the surface of the marine environment during tsunami-wave propagation in it. Although the ionospheric conductance is significantly smaller than the conductivity of the marine environment, the current value in it may considerably exceed the current value in the marine environment due to the exponential growth in the AGW amplitude during AGW upward propagation. The spatial distribution of a disturbance in the induction of the magnetic field of electric currents flowing in the marine environment and in the ionosphere is obtained with allowance for their mutual inductance. It is shown that the generation of the ionospheric electric current considerably changes the characteristics of the geomagnetic field disturbance induced by a tsunami wave. Calculations have demonstrated the possibility of space monitoring of tsunami waves with the use of satellites to record disturbances of the geomagnetic field.

It is possible that the radially independent, spatial-spectral components of the energy and power of the potential part of the main geomagnetic field were determined and studied for the first time. Energy is obtained by integrating its known radial density from the core of the Earth to infinity, and power is a time derivative of energy. The total and spectral variations of energy and power from 1840 to 2020 are analyzed based on three generally accepted observational models of the geomagnetic field. The total energy (~6 × 10¹⁸ J) and power (~10⁸ W) are determined by the sum of odd harmonics: dipole n = 1, octupole n = 3, etc. The dipole, the energy of which is close to the total energy symmetric with respect to the axis of rotation of the field, is predominant. The energy variations are ~10% and are similar for all models with the exception of the “burst” of the international geomagnetic reference field (IGRF) model in 1945–1950. Comparative spectral analysis showed that the “burst” is concentrated at n = 9 and 10, and the variations of the other harmonics are similar in all models. In this case, n = 3 dominates over n = 2. From n = 3 to 8, it decreases, and further n = 9 dominates over 8 and 10. The mean powers close to zero for n> 1 indicate an almost periodic behavior of the nondipole field, and significant power variations indicate a strong nonlinearity of the geodynamo. The results of the work are consistent with modern geodynamo-like models. The fact that such a significant IGRF “burst” that can have a non-linear geodynamic nature is a challenge. Alternatively, this may be some consequence of the imperfections of the IGRF model. Two other too-"quiet" models were subjected to excessive smoothing.