Excitation of Alfvén waves and vortices in the ionospheric Alfvén resonator by modulated powerful radio waves

A review of the artificial excitation of Alfvén waves and vortices in the ionospheric Alfvén resonator (IAR) is presented. In the framework of simplified models of the IAR and the Alfvén vortex instability, we discuss the main physical phenomena arising under the periodic heating of the ionosphere by a powerful HF radio signal with a modulation frequency F which lies in the range of short-period geomagnetic pulsations (F = 0.1–10 Hz). The amplitudes, frequency spectra, and polarization characteristics of artificial pulsations on the ground are found, and a brief comparison with experimental data is made. The Alfvén vortex instability in the IAR is analysed from the point of view of its artificial triggering. Two ways of such a triggering are discussed. The first suggests the use of two spaced transmitter antenna which produce an ionospheric current being in resonance with Alfvén vortices. Existing heating facilities are suitable for this experiment. The second method is based on the change of macroscopic parameters of the ionosphere, such as the conductivity in the instability region. This method is simple, but requires more powerful heaters.     

A mechanism of Pc 1 pearl formation based on the Alfvén sweep maser

A self-consistent model for the generation of Pc 1 pearl emissions based on the nonlinear coupling between the magnetospheric and ionospheric resonators for Alfvén waves is considered. Formation of pearls is attributed to the pulsating regime of the Alfvén sweep maser with nonlinear selective mirrors. Such mirrors are formed by the conjugate ionospheres: their reflection coefficient has an oscillatory frequency dependence due to eigenmodes of the ionospheric Alfvén resonator. Nonlinear magnetosphere/ionosphere feedback is provided by the dependence of the value and frequency of the reflection maxima on the flux of energetic protons precipitated into the ionospheres in the course of Alfvén wave generation in the magnetosphere. A nonlinear soliton-like solution of this model is found which corresponds to a single wave packet having the positive frequency drift and oscillating between the conjugate ionospheres. Properties of this solution are shown to explain many observational characteristics of Pc 1 pearls, such as their morningside predominance, correlation with low magnetic activity, spatio-temporal and spectral patterns.     

MHD evolutionary processes in the Earth’s model magnetic resonator

Specific features of the spatial-temporal dynamics of LF disturbances of the Earth’s magnetosphere have been analyzed by the method of numerical simulation of magnetic hydrodynamic equations taking into account plasma temperature variations. The effects related to the appearance of long-living magnetospheric jumps of density and temperature, MHD wave scattering on such inhomogeneities, Alfvén wave reflection from the near-Earth region, and magnetospheric plasma heating as a result of dissipative processes at a repeated propagation of MHD waves between magnetically conjugate regions have been considered. The problems of conformity of the discrete mathematical model with continuous equations of magnetic hydrodynamics are discussed.     

Sudden change in the resonance structure in the electromagnetic noise spectrum in the 0.1–10 Hz range during a substorm

The data of the geophysical observation complex at Barentsburg observatory on Spitsbergen archipelago, together with the data from other stations and satellite observations, were used to interpret a sharp increase in the frequency interval in the electromagnetic noise spectral resonance structure (SRS) in the 0.1–10 Hz range that took place during a substorm that occurred on December 24, 2005. It has been shown that such a change in SRS is related to a decrease in the electron density in the ionospheric F region, which agrees with the ionospheric Alfvén resonator theory. In turn, a decrease in electron density is probably related to the fact that the station was in the auroral cavity region related to the field-aligned current flowing into the ionosphere.     

A comparative Pc1 case study applying two modes of ionospheric Alfvén resonator modeling

The case study of four Pc1 subauroral pulsation events from Finland has been carried out on the basis of the full-wave numerical method. This method has been applied to simultaneous Scandinavian EISCAT radar measurements of the ionospheric plasma parameters, and their vertical (altitude) profiles have been utilized. Two alternative plasma profiles with different ion composition displays have been put to the test. A comparison between both types of the modeled ionospheric Alfvén resonator (IAR) ground signal frequency response and the frequency range of the Pc1 signal records has been studied. The results of the applied method can illustrate possible quiescent or disturbance changes in the upper ionosphere above the dense F2 layer. The ionospheric region up to ∼ 2000 km has been taken into account for this comparative analysis.     

Periodic Restructuring of Auroral Arcs as an Indicator of Alfvén Resonance in the Region of Substorm Onset

Using optical data from observatories of the Polar Geophysical Institutes, as well all-sky TV observations at Canadian stations of ground support for the THEMIS satellite mission, we clarify whether Alfvén resonance should necessarily be present in the region of subsequent substorm onset. If this is true, the diversion of magnetospheric cross-tail current to the ionosphere, which leads to substorm onset, may be due to resonant Alfvén (or flapping) oscillations that increase in duration. This possibility is believed to indicate optically the presence of Alfvén resonance via periodic restructuring of the preonset auroral arc 3–15 min before onset at T ₀ . At the latitudes of the observatories included in this study, auroral restructuring occurs as repetitive poleward excursions of the preonset arc (the periods of excursions are 1–3 min) and can be readily explained by the theory of Alfvén resonance. It is shown that this feature, while typically observed in strong substorm events, may be lacking for weaker substorms. As proved by conjugate satellite observations, the lack of auroral restructuring in the latter case may result from the weakness of the involved Alfvén resonance, which is still present but not accompanied by large field-aligned currents sufficient for visualization in the ionosphere of the apparent propagation of oscillation phase across the resonance layer.     

Determining the ionospheric irregularity velocity vector based on doppler measurements in the artificially modified <Emphasis Type="Italic">F</Emphasis><Subscript>2</Subscript> region of the polar ionosphere

The method for estimating the behavior of the ionospheric irregularity motion vector in the artificially disturbed HF ionospheric region has been proposed, and this behavior has been analyzed based on the simultaneous Doppler observations performed on several paths using the method of bi-static backscatter of diagnostic HF signals by small-scale artificial ionospheric irregularities. The Doppler measurements were performed during the modification of the auroral ionosphere by powerful HF radiowaves emitted by the EISCAT heating facility (Tromsø, Norway). It has been obtained that the dynamics of the ionospheric irregularity directions in the F region, calculated based on the Doppler measurements of the total vector of the ionospheric irregularity velocity above the Tromsø EISCAT radar at a frequency of 931 MHz, is in satisfactory agreement with such calculations performed using the three-position method.     

Specific features of resonance structures in spectra of ULF electromagnetic noise at high latitudes (Barentsburg observatory)

Results of studying resonance spectral structures (RSSs) of the noise background in the frequency range 0.1–5 Hz at Barentsburg high-latitude observatory (Spitsbergen) are presented. It has been indicated that the diurnal variations in the observation probability and the long-term variations in the RSS observation cannot be explained by variations in the illumination of the ionosphere by the Sun. Based on an analysis of the magnetic measurements, the conclusion is drawn that a decrease in the RSS registration probability in daytime hours and in winter months are related to an increase in the variability of ionospheric parameters. It has been indicated that the RSS source is located in the region above an observation point, and the horizontal scale of this source is 100–200 km, which agrees with the concepts of the role of the ionospheric Alfvén resonator in the formation of RSS.     

Peculiarities of artificial ionospheric radiation under the action of high-power HF radiowaves, emitted by the SPEAR facility, on the sporadic E layer of the polar ionosphere

The results of the experimental studies of the ionospheric effects originating under the action of high-power HF radiowaves, emitted by the SPEAR heating facility into the sporadic E _s layer of the polar ionosphere, are presented. The experiment was performed on March 2, 2007, simultaneously at two spaced points: Barentsburg (Spitsbergen, a distance of about 40 km from the SPEAR facility) and Gor’kovskaya observatory near St. Petersburg, located at a distance of about 2000 km from SPEAR. The distributions of the heating signal intensity in the 100 kHz frequency band were measured in Barentsburg. Bistatic backscatter of diagnostic HF signals by small-scale artificial ionospheric irregularities was observed at Gor’kovskaya observatory. Based on an analysis of the experimental data obtained in Barentsburg, it has been found out that a broadband noise-like component originated and additional maximums appeared in the heating signal spectrum. The broadband emission intensity was a factor of 1.5–3 as high as the noise level. The additional maximums were formed in the regions of the positive and negative frequency shift relative to the heating signal frequency and were observed when the heating frequency was lower than the critical frequency of the E _s layer; e.g., a high-power HF radiowave reflected from E _s . The expression for determining the frequency shift of the additional maximum in the heating signal spectrum at altitudes of the ionospheric E region, taking into account the ion-electron collision frequency, has been obtained. The heating signal spectrum registration was compared with the observations of small-scale artificial ionospheric irregularities and the trajectory modeling of signals scattered by the considered irregularities. The observation results have been analyzed and interpreted taking into account the magnetic and ionospheric data characterizing the background geophysical conditions.     

Auroral activity caused by high-power radioemission from the SURA facility

A series of experimental modifications of the ionosphere in the HF range, performed at the SURA facility base, together with optical measurements onboard the International Space Station (ISS), indicated that such impacts on the ionosphere are effective when the facility operational frequency is higher than the critical plasma frequency (for the main ionospheric F2 layer). The experimental measurements were supported by measurements at ground-based observatories, ISS, and the Demeter and GPS satellites. The analysis results of the entire data set are presented. The ray HF radio tracing for the experiment of October 2, 2007, has been calculated, and it has been indicated that the ionosphere to the north of the facility up to 60°–62° N latitudes was irradiated by the facility beam (the effects of ray redistribution and refocusing) due to refraction on the gradient of the F2 layer critical frequencies. An analysis of the ground-based and satellite measurements (both in the vicinity of a heater and in the magnetically conjugate region) indicates that it is possible to trigger a substorm in experiments with the Sura heating facility.     

Interaction between the Alfvén wave and turbulent sheet

The interaction between the Alfvén wave and turbulent sheet (TS) with an anomalous conductivity has been considered. High frequency turbulence causes the appearance of not only anomalous field-aligned plasma conductivity but also cross-field conductivity. Alfvén waves can be partially reflect from TS, be absorbed in this sheet, and pass through TS. When field-aligned conductivity is predominant, the relative effectiveness of these processes strongly depends on a cross-field wave scale. If TS is thin as compared to the Alfvén wavelength, the resistive Alfvén wave (λ _A ) characterized by the field-aligned resistivity and Alfvén velocity above the sheet is the characteristic parameter responsible for the wave-sheet coupling. A comparison of the loss, estimated using the analytical relationships for a thin sheet and numerically calculated based on the complete formulas for a sheet with a finite thickness, indicates that the approximation of a thin sheet results in reasonable estimates at all wave scales except very small ones. The developed model has been applied to the interpretation of the results of the works on Pi2 pulsation damping during the substorm expansion phase, which indicated that the damping decrement increases at large substorm amplitudes. The estimates indicate that this increase in damping is related to the appearance of anomalous resistivity in the case when field-aligned currents exceed the threshold values necessary for excitation of high frequency turbulence.     

Modeling the Effect of Mesospheric Internal Gravity Waves in the Thermosphere and Ionosphere During the 2009 Sudden Stratospheric Warming

The results of numerical experiments on the modeling of thermospheric and ionospheric disturbances under conditions of sudden stratospheric warming are presented to study the possible mechanisms of such disturbances. Local disturbances caused by a planetary wave with zonal wave number s = 1 and internal gravity waves (IGWs) propagating from the disturbed region in the stratosphere are taken into account as sources of disturbances. It is shown that the inclusion of an additional source of thermospheric disturbances caused by mesospheric variations of atmospheric parameters with IGW periods over the region of sudden stratospheric warming leads to significant changes in the parameters of the thermosphere and ionosphere, including a change in the global structure of the distributions of the gas components of the thermosphere and a shift in maximum concentrations of atomic oxygen to low latitudes of the Southern Hemisphere; there is an increase in the mean values, the diurnal and semidiurnal variations of the ion concentration in the F region of the ionosphere. These features of changes in the parameters of the thermosphere and ionosphere occurred with insignificant disturbances of tidal variations in the thermosphere.     

Modelling the diurnal evolution of the resonance spectral structure of the atmospheric noise background in the Pc 1 frequency range

We calculate the frequency spectrum of the electromagnetic background noise in frequency range 0.1–5 Hz based on the model attributing its formation to the ionospheric resonant filtration of the radiation from distant lightning discharges (Belyaev, P.P., Polyakov, S.V., Rapoport, V.O., Trakhtengerts, V.Y., 1989. Theory of formation of the resonance spectral structure of atmospheric electromagnetic noise background in the range of short-period geomagnetic pulsations, Izvestiya vuzov-Radiofizika 32(7) 802–810). Characteristics of the spectral resonance structure (SRS) formed due to the ionospheric Alfvén resonator are obtained and their dependence on ionospheric parameters is considered; the SRS variation during a day is discussed. The calculations are compared with the ULF ground based observations at Sodankylä and Nizhny Novgorod; consistency of theory and experiment is demonstrated. Opportunities to use such data to determine some of ionosphere parameters are discussed.     

Effects Stimulated by Radiation of the SURA Facility in the Geophysical Experiments Program at the International Space Station in 2017

The set of experiments in 2017 included the forecasting of geo- and heliophysical conditions in the ionosphere, the choice of heating regimes at a minimum critical frequency of the F2 ionospheric layer, analysis of the ionosphere radio sounding results and measurements of geomagnetic perturbations using ground-based facilities, study of the potential for higher efficiency and probability of artificial effects on the ionosphere, and the establishment of a correlation between geomagnetic field variations and the radio heating facility cyclogram. The experiments were carried out with a combination of new heating conditions (pulse ratio, period of action, two-frequency and continuous heating, variations in radiated power, polarization, shortwave radiation direction, etc.) in the intervals between recurrent magnetic storms. Geomagnetic field pulsations (in accordance with the cyclogram) and substorm effects were stimulated by the SURA facility against a background of quiet geophysical conditions.     

A determination of the hydromagnetic waves polarization from their perturbations on the terminator

The influence of the homogeneous and inhomogeneous ionosphere on the orientation angle of the horizontal magnetic vectors of the long-time geomagnetic pulsations is under consideration in this study. It was realized that this angle is small in the case of the homogeneous ionosphere for both the Alfvén and magnetosonic types of oscillations. An increase in the ionospheric electric field was discovered as the ionospheric conductivity changes during the switch from day to night conditions. It is valid only for the initial Alfvén wave. The ionospheric equivalent current systems excited by the initial magnetospheric waves of Alfvén and magnetosonic types as well as their behavior near the terminator were studied for different seasons. For the Alfvén source, seasonal variations of the orientation angle close to sunrise at the equator depend on the type of source: odd or even modes of Alfvén oscillations excite observable pulsations. It was found that the ionospheric two-vortex equivalent current system of the long-period pulsations arising in high-latitudes in the equatorial region alters not only its direction, but its intensity too. The largest anomaly (\sim25% of the source value) would be expected near the terminator. A new experimental method was suggested to recognize the type of incident magnetospheric waves by implementing observations either at a single observatory or at a couple of observatories. In the case of a single observatory it is proposed to study the frequency dependence of the orientation angle of their magnetic components close to sunrise. If the initial wave is magnetosonic, this angle must not be changed as a function of the local time within the wide frequency range of pulsations. When pulsations have an orientation angle sensitive to the presence of the terminator, they may be classified as both Alfvén and magnetosonic. For the Alfvén waves no frequency dependence of the orientational angle is peculiar. On the contrary, magnetosonic waves should be determined as oscillations with an orientational angle proportional to the frequency. These oscillations may be revealed at observatories located on the high-resistance cross sections. The example of the spectral-temporal analysis of pulsation at the equatorial observatory in Huancayo was demonstrated to confirm the proposed experimental technique. A weak dependence of the orientation angle anomaly on the frequency near the terminator was found. The latter is evidence for the dominant contribution of the Alfvén waves to low-latitude and equatorial oscillations.     

Space-time structure of Alfvén resonant disturbances generated by transversally localized FMA wave

This study describes the space-time evolution of the Alfvén resonant disturbance generated by a transversally localized fast magnetoacoustic (FMA) wave induced by impulse action on its localization area. We determine the conditions for the formation of qualitatively different space-time structures of Alfvén resonant disturbances under different relationships between parameters characterizing both dispersion/absorption of Alfvén disturbance and resonant absorption of transversally localized FMA waves. The spatial structures of Alfvén resonant disturbances and their time evolution are described analytically.     

Analysis of seismoionospheric disturbances at the Chain of Japanese stations for vertical sounding of the ionosphere

Seismoionospheric disturbances in the parameters of the ionospheric F ₂ and sporadic E layers at the chain of the Japanese stations for vertical sounding of the ionosphere before strong crustal earthquakes with M>6.5 during the period from 1968 to 1992 have been considered. The dependence of the disturbance time of appearance in the ionospheric parameters on the earthquake magnitude and epicentral distance, obtained for each specific earthquake using the selected series of ionospheric stations, made it possible to consider these disturbances among medium-term precursors of earthquakes. The velocity of the disturbance front apparent motion has been determined based on the model of horizontal radially-isotropic disturbance propagation from the projection of the impending earthquake epicenter to the ionospheric altitudes. The conclusion has been made that the distinguished seismoionospheric disturbances follow the boundary of the earthquake preparation region, expanding on the Earth’s surface.     

Effect of neutral particle density in the upper atmosphere on the generation of artificial magnetic pulsations in the Pc1 range

A series of experiments on modification of the ionosphere by a powerful ground HF transmitter was performed using the EISCAT heating facility in order to generate artificial magnetic pulsations in the frequency range 0.1–3 Hz. In several cases, the ionospheric electric field and the electron density vertical profile were measured with the EISCAT incoherent scatter radar. The measurement of the background values of the ionospheric parameters made it possible to verify the numerical model for generating artificial emissions. The calculated amplitudes of magnetic pulsations correspond to the values measured on the Earth’s surface. However, the model cannot explain the sporadic nature of artificial signals, which indicates that this model is incomplete. Disturbances of the neutral particle density in the upper atmosphere are one of the possible causes explaining a difference between the calculations and the experimental values. The numerical simulation indicated that the amplitude variations caused by such disturbances can be 20%. For artificial emissions whose intensity is comparable with the intensity of artificial noise, variations in the neutral components can result in the disappearance of an artificial signal on the spectrogram.     

Spatial structure of standing wave electromagnetic fields at the lower harmonics of the ionospheric Alfvén resonator

During the multiband wave Pc1 event on March 7, 2001 the EISCAT UHF and VHF incoherent scatter radars operated simultaneously covering an exceptionally wide altitude range of the ionosphere ～90—2000 km. This made possible to test the ionospheric Alfvén resonator (IAR) model over a large altitude range. The three lowest IAR eigenfrequencies, where the most of the Pc1 pulsation signal bands occur, were selected for the spatial analysis of the standing wave electromagnetic fields, applying the full-wave numerical simulation method. The altitude spread of amplitude maxima and nodes together with polarization characteristics of oscillation maxima in the horizontal plane are presented. The comparison of the standing wave oscillations on the altitude profile with the signal amplitude observed on the ground is also presented.