During the North Pole–41 expedition, three components of the VLF electromagnetic field were simultaneously measured on a drifting ice-resistant platform and at the Lovozero and Barentsburg observatories. We consider three VLF events that occurred in magnetically quiet time. During two of them (the events on January 24, 2023 and March 12, 2024), auroral hiss bursts were recorded at three stations located in the auroral and circumpolar regions and spaced up to 2.600 km apart. The spectral and temporal characteristics of the bursts at all the stations were almost the same. The fact that hiss was recorded with the same properties at such large distances can be explained under the assumption of a homogeneous flow of auroral electrons with energies from 0.1 to 10 keV throughout the precipitation area, which generate quasi-electrostatic waves at altitudes 10–20 thousand km, along with the simultaneous presence of small-scale ionospheric irregularities in the vicinity of all three stations, where these waves are scattered into the propagation cone to the Earth surface. We examine the case of hiss recording (the January 25, 2023 event) demonstrating the locality of the hiss recording area during one day — a hiss burst is first observed at one station, then at another. This is probably due to the appearance/disappearance of local areas of small-scale irregularities, where quasi-electrostatic waves are scattered providing propagation to the Earth surface.

We study the spatiotemporal variations of ionospheric parameters over the regions of Eurasia by analyzing data from chains of high- and mid-latitude ionosondes during the extreme magnetic storm in May 2024. The analysis of ionospheric parameters allowed us to note strong latitudinal and longitudinal differences in variations of the analyzed parameters under quiet conditions before the onset of the magnetic storm and during its development. Almost immediately after the onset of the storm at 17:00 UT on May 10, 2024, according to data from all ionosondes, a sharp drop in the electron density at the height of the F2-layer maximum was recorded, regardless of the local time at the measurement point. Ionosondes of the high-latitude chain showed a complete absence of data (radio signal blackout) during the main and early recovery phases of the storm until the evening of May 12, 2024, i.e. more than one and a half days. Additional bursts of geomagnetic activity during the recovery phase of the storm were also accompanied by significant and prolonged decreases in the electron density according to ionosonde measurements at all longitudes of Eurasia. The recovery of ionospheric ionization began on May 14–15 at all longitudes of the mid- and high-latitude regions of Eurasia. A long-term negative disturbance of electron density covering a huge territory of mid-latitude Eurasia was caused by an extraordinary, catastrophic drop in the [O]/[N2] ratio according to satellite measurements of GUVI TIMED during the superstorm for almost three days. The response of the thermospheric composition of neutral gas to the processes developing at high latitudes of the Northern Hemisphere on May 10–15, 2024 was global, with penetration of the thermospheric disturbance at almost all longitudes up to the equatorial latitudes (~10° N) and with very low values of the [O]/[N2] ratio ~0.1÷0.4. Significant differences in the spatiotemporal variations of the thermospheric composition of neutral gas were revealed during the most extreme geomagnetic storms of the current 21st century — in May 2024 and October–November 2003 (Halloween storms). The magnetic superstorm in May 2024 was much more geoeffective than the superstorms in October–November 2003, and caused a significantly different ionospheric response at different longitudes and latitudes of the Northern Hemisphere.

An analysis is performed of the growth of scintillations of GLONASS and GPS satellite signals using the Septentrio GNSS receiver installed in the city of Apatity during a strong magnetic storm on March 23–24, 2023. Data from the ionosonde at the Lovozero station and data from the EISCAT radar in Tromsø are used to show that the growth of phase scintillations is due to an increase of the plasma density mainly in the E-layer of the ionosphere. The growth of phase scintillations is accompanied by the appearance of discrete aurora.

We have analyzed spatial and temporal variations in ionospheric parameters over high and middle latitudes of Eurasia, using data from chains of high- and mid-latitude ionosondes during a severe magnetic storm in March 2015. To analyze the ionospheric response to the severe geomagnetic disturbance of solar cycle 24, we have employed ionosonde data on hourly average values of the critical frequency foF2 of the ionospheric F2 layer, the critical frequency of the sporadic layer foEs, and the minimum reflection frequency fmin. There are strong latitudinal and longitudinal differences between the features of temporal variations in the analyzed ionospheric parameters both under quiet conditions before the magnetic storm onset and during the storm. We discuss possible causes of the observed spatial variations in ionospheric parameters. The source of spatio-temporal variations in ionospheric ionization parameters may be inhomogeneities generated in the high-latitude ionosphere under conditions of increased helio-geomagnetic activity. During the magnetic storm main and recovery phases, periods of blackouts of radio signals from ionosondes were observed at both high and middle latitudes. During these periods, there was a significant increase in the absorption of radio waves used in ionosonde sounding, as well as in the frequency of occurrence of screening sporadic Es layers. The long-term effect of the negative ionospheric storm over high and middle latitudes of Europe is explained by the movement of the vast region of the reduced density ratio [O]/[N2] at thermosphere heights from the Far East and Siberia westward to Europe during the late recovery phase of the magnetic storm. Increased ionization of the ionospheric F2 layer with foF2 exceeding the level for quiet days before the onset of the magnetic disturbance over the vast region of Eastern, Western Siberia and Eastern Europe after the end of the magnetic storm in March 2015 is a manifestation of the aftereffect of magnetic storms. The increase in ionization was especially pronounced, as measured by the chain of mid-latitude ionosondes.

During experiments on the modification of the high-latitude ionosphere by high-power HF radio waves of ordinary or extraordinary polarization of the EISCAT/Heating facility (Tromsø, Norway) in 2013, 2016, and 2019, Doppler measurements of diagnostic HF radio signals over long radio paths were carried out by the bistatic scatter method. We studied characteristics of Doppler frequency variations in bistatic scattered radio signals, using the experimental results obtained along radio paths of different lengths (up to ~8500 km) and orientation. We examined numerical dependences of the Doppler frequency variations in a radio signal on the azimuth of the wave vector of a radio wave incident onto an artificially disturbed region, on the bistatic scattering angle, and on the azimuthal direction of irregularity motion in an artificially disturbed region of the ionosphere. From simultaneous measurements of the Doppler frequency fD of the radio signal along two diagnostic radio paths, we numerically estimated the velocity vector of irregularities in the artificially disturbed region of the ionosphere. The total vector velocity of artificial ionospheric irregularities can be calculated from measurements of the Doppler frequency shift along several long diagnostic radio paths after preliminary analysis of experimental observations with the results of trajectory modeling of diagnostic HF radio signals.

. We have examined longitudinal-temporal variations in ionospheric parameters over Eurasia by analyzing data from a chain of high-latitude ionosondes along a latitude circle ~70° N (geomagnetic latitudes 58°

The work presents results of observations of the main ionospheric trough (MIT) in the latitudinal variation of the electron concentration at the height of the F2 layer in different geomagnetic conditions using an oblique HF ionospheric sounding network. A brief description of the equipment and its technical characteristics is presented. The geometry of the radio tracks is also given.

The paper considers the propagation of HF radio waves in the Russian Arctic in various heliogeophysical conditions in the E region of the ionosphere using the example of four inclined sounding routes (Sodankula – Barentsburg, Lovozero – Barentsburg, Lovozero – Amderma and Lovozero – Dikson) from January 2021 to May 2024. Automatic processing of the EEs reflection mode was performed, the probabilities of signal reflection from the sporadic Es layer were obtained for all frequencies of the radiated ranges of transmitters.

We present the result of the studies into characteristics of high-latitude ionospheric F-region longitudinal plasma waves (Langmuir and ion-acoustic), caused by the impact of powerful HF radio waves of ordinary (O-mode) or extraordinary (X-mode) polarization of the EISCAT/Heating facility, Tromsø, Norway. The powerful HF radio waves on October 20, 2012 and February 26, 2013 were emitted in the direction of the magnetic zenith with a step change in the effective radiation power (ERP). The radiation frequency fH of the EISCAT/Heating facility on February 26, 2013 was close to the F2-layer critical frequency foF2 (fH/foF2~1) and exceeded the electron gyroresonance frequency fH>5fce. On October 20, 2012, the conditions fH/foF2~0.85–0.95 and fH<6fce were fulfilled. Analysis of EISCAT measurements of an incoherent scatter radar (ISR) at a frequency of 930 MHz, spatially aligned with the heating facility for radiation conditions ERP<200 MW, has shown that parametric decay instabilities are excited at the ionospheric heights where the pump frequency is close to the plasma Langmuir frequency, fH≈fPL. We have studied peculiarities of excitation of the decay parametric instabilities as a function of height in the ionosphere and pump wave polarization, the ratios between fH and foF2, and also fH and nfce.

Physical experiments in natural free plasma (ionosphere) using controlled injection of powerful HF radio waves (HF pump waves) into the high latitude upper (F-region) ionosphere allow the investigation of various nonlinear phenomena. HF pump waves with ordinary (O-mode) polarization are commonly used for the modification of the upper ionosphere (F-region). This is due to the fact that extraordinary (X-mode) polarized HF pump waves are reflected from altitudes significantly below the reflection altitude of the O-polarized HF pump wave and the altitude of electrostatic plasma waves. Because of that they are not able to generate such waves or, as a consequence, cause artificial plasma turbulence and accompanying phenomena. However, the results of experiments carried out by AARI researchers at the EISCAT/Heating facility (Tromsø, Norway) have clearly demonstrated for the first time that X-polarized HF pump waves are able to produce artificial ionosphere disturbances which may be much stronger compared with O-mode disturbances. This opens up new possibilities for the investigation of nonlinear phenomena and ionospheric disturbances in the upper ionosphere, leading to the development of technologies allowing one to observe the processes in the Arctic zone ionosphere. In contrast to the traditional investigations of artificial ionospheric disturbances induced by O-mode HF pump waves, X-mode disturbances in the upper ionosphere are poorly investigated, the mechanisms of their generation are not understood. Therefore, such investigations require serious experimental and theoretical development. We present investigation results of the influence of the HF Phased Array beam width at the EISCAT/Heating facility (Tromsø, Norway) on the features of artificial disturbances in the high latitude upper (F-region) ionosphere induced by powerful HF radio waves. The paper analyzes the features, behavior, and spatial structure of electron density and temperature (Ne and Te), Langmuir and ion-acoustic plasma waves, artificial field-aligned irregularities (AFAIs), and narrowband (±1кHz relative to heating frequency) stimulated electromagnetic emission (NSEE) induced by X-mode HF pumping by phased Arrays with a narrow beam width of 5–6° (A1) and a wide beam width of 10–12° (at — 3 dB level) (A3). It is shown that the spatial size in the north-south direction of the Neducts and HF-enhanced plasma and ion lines (HFPL and HFIL) depends on the width of the HF Heating facility antenna beam. It corresponds to the angle width of 7° for the A3 antenna and 4° for A1, which is approximately two times less than the width of th pattern of A3 and A1. The relationship between the Ne duct transverse size and the size of the region occupied by the X-mode artificial irregularities is found. It has been established that the intensities of all the discrete components in the NSEE spectra are 10–20 dB higher when a powerful X-wave is emitted to the antenna A1, providing ERP = 820 MW, compared to radiation to the antenna A3, providing ERP = 230 MW. A comparison is made of the influence of the radiation pattern width of the antennas A1 and A3 on the characteristics of disturbances during O- and X-mode HF pumping. It is shown that Ne ducts and narrow band stimulated electromagnetic emission during O-mode heating, at frequencies below the critical frequency of the F2 layer, are not excited at all when the pump wave is emitted by both antennas A1 and A3. However, perturbations in the electron temperature, AFAI intensity, and the size of the region occupied by AFAIs are greater during O-mode heating than during X-mode heating.

The paper presents the results of investigating generation conditions and features of the narrowband stimulated electromagnetic emission (NSEE) induced by an extraordinary (X-mode) HF pumping into the high-latitude ionosphere F region. It was shown that NSEE spectral features were recorded at a distance of more than 1100 km from the heating facility. Distinctive features of NSEE discrete spectral lines at the pump frequencies fH below and above the F2 layer X-component critical frequency (fH < fXF2 and fH і fXF2) were analyzed. It was found that the NSEE spectrum contains the strongly pronounced discrete lines ordered by the electrostatic ion cyclotron frequency for the atomic oxygen ions (O+). When fH < fXF2, the multiple downshifted and upshifted spectral components (Stokes and anti-Stokes modes, respectively) were excited in the NSEE spectra. When fH і fXF2, the Stokes modes were only generated. The plausible mechanisms for the NSEE excitation by an X-mode HF pump waves are discussed.

Представлены результаты экспериментальных исследований явлений в высокоширотной верхней ( F -область) ионосфере, вызванных воздействием мощных радиоволн КВ-диапазона обыкновенной (О-мода) поляризации на частотах нагрева f H , существенно превышающих критическую частоту слоя F 2 ( f H – fоF 2 = 0.9–1.1 МГц). Результаты базируются на данных экспериментов, выполненных на КВ нагревном стенде EISCAT/Heating в г. Тромсё, северная Норвегия (69.6° N, 19.2° E). В период экспериментов мощная радиоволна КВ-диапазона О-поляризации излучалась в направлении магнитного зенита с максимальной эффективной мощностью излучения 350–550 МВт. Впервые обнаружено, что в условиях, когда мощная радиоволна КВ-диапазона О-поляризации не отражалась от ионосферы, происходит образование дактов повышенной электронной плотности Ne , генерация мелкомасштабных искусственных ионосферных неоднородностей и узкополосного (в полосе ±1 кГц относительно частоты нагрева) искусственного радиоизлучения ионосферы, регистрируемого на расстоянии ~1200 км от нагревного стенда. Выполнено сравнение характеристик мелкомасштабных искусственных ионосферных неоднородностей и спектральной структуры искусственного радиоизлучения ионосферы при альтернативном О-/Х- нагреве в магнитный зенит на частотах, существенно превышающих критическую частоту слоя F 2. Установлено, что в целом их поведение имеет одинаковый характер, однако эволюция развития рассмотренных явлений при О- и Х-нагреве отличается.

We have compared effects of geomagnetic disturbances during magnetic storms of various types (CME and CIR) and during an isolated substorm on scintillations of GLONASS and GPS signals, using a Septentrio PolaRx5 receiver installed in Apatity (Murmansk Region, Russia). We analyze observational data for 2021. The magnetic storms of November 3–4, 2021 and October 11–12, 2021 are examined in detail. The November 3–4, 2021 magnetic storm was one of the most powerful in recent years. The analysis shows that the scintillation phase index reaches its highest values during nighttime and evening substorms (σϕ≈1.5–1.8), accompanied by a negative bay in the magnetic field. During magnetic storms, positive bays in the magnetic field, associated with an increase in the eastward electrojet, lead, however, to quite comparable values of the phase scintillation index. An increase in phase scintillations during nighttime and evening disturbances correlates with an increase in the intensity of ULF waves (Pi3/Pc5 pulsations) and with the appearance of aurora arcs. This confirms the important role of ULF waves in forming the auroral arc and in developing ionospheric irregularities. The predominance of the green line in the spectrum of auroras indicates the contribution of disturbances in the ionospheric E layer to the scintillation increase. Pulsating auroras, associated with ionospheric disturbances in the D layer, do not lead to a noticeable increase in phase scintillations. Analysis of ionospheric critical frequencies according to ionosonde data from the Lovozero Hydrometeorological Station indicates the contribution of the sporadic Es layer of the ionosphere to jumps in phase scintillations. The difference between phase scintillation values on GLONASS and GPS satellites during individual disturbances can be as great as 1.5 times, which may be due to different orbits of the satellites. At the same time, the level of GLONASS/GPS scintillations at the L2 frequency is higher than at the L1 frequency. We did not find an increase in the amplitude index of scintillations during the events considered.

The paper is devoted to determination of the total electron content in the vicinity of the South geomagnetic pole using observations by global navigation satellite systems. Observations were carried out at the Russian Antarctic station Vostok in the periods February 2016 - January 2017, February 2018 - February 2019 and February 2020 - January 2021. Observations were made with satellites of GPS and GLONASS systems. Processing of observations was carried out by use of the TEC-suite software. Total electron content series were obtained for the specified time periods. Our results were compared with those of Center for Orbit Determination in Europe, there is a good agreement, based on which we conclude that our data are reliable. For all periods of observation, average daily profiles of changes in the total electron content in winter and summer were plotted. An excess of the winter total electron content measured from global navigation satellite systems observations over the model data provided by Center for Orbit Determination in Europe by about 5 total electron content unit was noted.

Estimates of excitation thresholds and the analysis of spectral features of narrowband stimulated electromagnetic emission (NSEE) depending on the electric-field intensity of an extraordinary polarized HF pump wave have been carried out. They are based on results obtained during power stepping EISCAT/Heating experiments. The HF pump wave was radiated toward the magnetic zenith at frequency of 5.423 MHz. The effective radiated power was changed from 55 to 360 MW. NSEE was recorded in the vicinity of St. Petersburg at a distance of ~1200 km away of the EISCAT/Heating facility. Calculations of the electric field of a powerful HF radio wave near the reflection altitude taking into account the nondeviation absorption in the underlying layers were performed. The threshold (minimum) values of the electric field required for the NSEE excitation were determined.

In this paper, we present the results of experimental studies of phenomena in the high-latitude upper ( F -region) ionosphere caused by the impact of high-power HF radio waves of ordinary (O-mode) polarization at heating frequencies f H that significantly exceed the critical frequency of the F 2 layer ( f H – fоF 2 = 0.9–1.1 MHz). The results are based on experimental data obtained at the EISCAT/Heating HF heating facility in Tromsø, northern Norway (69.6° N, 19.2° E). In the experiments, a high-power HF radio wave of the O-polarization range was emitted in the direction of the magnetic zenith with a maximum effective radiation power of 350–550 MW. It was found for the first time that under conditions when a powerful HF radio wave of the O-polarization was not reflected from the ionosphere, ducts of increased electron density Ne are formed, and small-scale artificial ionospheric inhomogeneities and narrow-band (within a ±1-kHz band relative to the heating frequency) artificial radio emission of the ionosphere that are recorded at a distance of ~1200 km from the heating facility are generated. The characteristics of small-scale artificial ionospheric irregularities and the spectral structure of artificial radio emission of the ionosphere with alternative O/X heating to the magnetic zenith at frequencies significantly exceeding the critical frequency of the F 2 layer are compared. It was established that, in general, their behavior has the same character, but the evolution of the development of the considered phenomena during O and X heating is different.

We present experimental results from the studies of large-scale inhomogeneities along the external magnetic field with increased electron density, electron temperature, and excitation of elongated plasma waves (Langmuir and ion-acoustic), induced by the ordinary (O-mode) and extraordinary (X-mode) HF heating near the F2-layer critical frequency, in the high-latitude ionospheric F-region. The experiments have been carried out at the EISCAT/Heating facility (Tromsø, Norway). Powerful HF radio waves radiated towards the magnetic zenith through a step change in the effective radiated power at frequencies fH near and below the F2-layer critical frequency fₒF2. The EISCAT incoherent scatter radar (930 MHz), co-located with the EISCAT/Heating facility, was utilized for diagnostics of ionospheric modification effects. We calculated the electric field of a powerful HF radio wave near the reflection altitude, taking into account the non-deflective absorption along the propagation path. We determined the conditions for electric field generation and its threshold (minimum) values required for electron density enhancements in a wide altitude range, excitation of Langmuir and ion-acoustic plasma waves under fH~fₒF2 and fH

The paper presents experimental results concerning disturbances of electron density in the high latitude ionosphere F-region, induced by powerfulHF radio waves (pump waves) with extraordinary (X-mode) polarization. The experiments were carried out at the EISCAT/Heating facility at Tromsø, Norway. The EISCAT UHF incoherent scatter radar (ISR), running at 930 MHz, co-located with a heating facility, was used to detect the disturbances of electron density. In the course of the experiments, the X-mode HF pump waves radiated into the F-region towards the magnetic zenith at different pump frequencies and ratios of the pump frequency to the critical frequency of the F2 layer.The effective radiated power was ERP = 360–820 MW. An increase in electron densities was found in a wide altitude range, giving rise to field-aligned ducts with enhanced electron density. The features and behavior of the ducts were investigated. It was revealed that the ducts are formed under quiet background geophysical conditions in a wide altitude range up to the upper altitude limit of EISCAT ISR measurements, when the pump frequencies were both below and above the critical frequency of the F2 layer (fH ≤ foF2 or fH > foF2). A plausible formation mechanism of the ducts is discussed.