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.     

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.     

On the latitudinal structure of the magnetohydrodynamic Alfvén resonances

According to the data of the IMAGE network of magnetometers the latitudinal profile of the amplitude of the Pc5 geomagnetic pulsations is constructed, which are excited in the Earth’s magnetosphere in the form of the resonance Alfvén magnetohydrodynamic pulsations. The approaches to the solution of two problems are studied on a specific example. The first concerns the anharmonicity of Alfvén’s resonances. The displacement of the peak of the resonance curve towards to the north with the reduction of the amplitude of the pulsations is discovered. Based on the results of measurements, the nonlinear distortion coefficient of the latitudinal profile is determined. The second problem is connected with the magnetotelluric sounding. Information about the resonance structure of the Alfvén pulsations is useful for magnetotelluric sounding. This information gives the possibility of evaluating the accuracy of the sounding with the application of the local impedance relationship and of introducing corrections if necessary.     

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.     

Non-stationary Alfvén resonator: vertical profiles of wave characteristics

A Pc1/IPDP event recorded by the Finnish search coil magnetometers on 15 December 1984 was analyzed in a companion paper (Mursula et al., 2000. Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events. J. Atmos. Solar-Terr. Phys. 62(4), 299–309) using numerical simulations of the ionospheric Alfvén resonator (IAR). EISCAT incoherent scatter radar data were used to determine the vertical profiles of ionospheric plasma parameters. In this paper, the detailed altitude profiles of several wave characteristics at the IAR eigenfrequency are computed up to 1000 km height, including, e.g., the real normalized amplitude of the magnetic wave field component, ellipticity and orientation of the polarization ellipse in the horizontal plane. We also calculate the altitude profile of the energy flux density (Poynting vector). These features illustrate in detail the ionospheric effects on the wave spectral structure in a non-stationary IAR, and their significance in the formation of the Pc1/IPDP signal on the ground.     

Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events

We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on 15 December 1984, which subsequently developed into a structured IPDP after a substorm onset. The EISCAT radar was simultaneously monitoring the mid- to high-latitude ionosphere. We have calculated the ionospheric resonator properties during the different phases of the event using EISCAT observations. Contrary to the earlier results, we find that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the ground corresponds to the maximum of the transmission coefficient rather than that of the reflection coefficient. This casts strong doubts on the bouncing wave packet model of Pc1 pearls. Instead, we present evidence for an alternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, whereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator properties can be modified during the event so that the resonator eigenfrequency increases according to the observed frequency increase during the IPDP phase. We suggest that the IPDP signal on the ground is a combined effect of the frequency increase in the magnetospheric wave source and the simultaneous increase of the resonator eigenfrequency. The need for such a complicated matching of the two factors explains the rarity of IPDPs on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.     

The ionospheric response to the cme event of 6–11January 1997

An active aurora was observed at Eureka, Canada (88.9° N magnetic) following the arrivalat the magnetopause of the shock front resulting from the solar Coronal Mass Ejection of 6January 1997. This onset at 02:20 UT on 10 January marked the beginning of an aurora whichcontinued until at least 15:00 UT on 11 January, as viewed from both Eureka and the CANOPUSsite at Ft Smith (67.8° N magnetic). There were enhanced OI 630 nm polar F-region emissionsthroughout this period, with the IMF B_z controlling their form. When B_z was positive, there were continuous polar arcs; when B_zturned negative there were F-layer patches on open field lines. While the strong H_β observed over Ft Smith (240 R) in addition to the extended 630 nm emissions and theirpersistence over two days suggested a red aurora of global proportions, it apparently did notextend to latitudes below 60°. There was a moderate magnetic storm associated with the aurora,but the D_st index reached only −81 nT on 10 January.     

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.     

Formation of accelerated ion flows in Alfvén disturbances of the magnetotail

This paper shows that there exists a mechanism of longitudinal plasma acceleration which is inherent in the process of the process of resonant conversion of a fast magnetosonic wave freely propagating along the magnetic field into an Alfvén wave. This mechanism is caused by the Ampere force arising due to the interaction between the poloidal component of the current of the compressible disturbance and the generated toroidal disturbance. It is shown that plasma acceleration takes place at the stage of increase in the Alfvén wave amplitude and that the accelerated flow retains its velocity when the process of resonant conversion is over. We describe spatiotemporal structures of plasma flows arising with the transformation of fast magnetosonic waves into Alfvén waves. An interpretation of the presence of fast ion flows in the magnetotail as a consequence of the action of the plasma acceleration mechanism considered in this work is proposed.     

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.     

On the seismic fortification intensity during succeeding use of the existing structure

Introduction Fortification intensity is the seismic fortification criterion of engineering structure against an earthquake of a certain magnitude in a certain service period. Seismic risk level is taken into con-sideration, so do the importance of engineering structure, the function and efficiency of structure, and the ability of bearing the fortification investment. Fortification intensity is constrained by both the present scientific level and social economic level, which makes fortification…     

Characteristics of the Energy Transfer by Alfvén Waves in the Solar Atmosphere

Geomagnetism and Aeronomy - The propagation of linear Alfvén waves with periods of 10–200 s from the photosphere to the solar chromosphere under the conditions of an isothermal...     

Characteristics of Arc-Polarized Alfvén Waves in the Near-Earth Solar Wind

Geomagnetism and Aeronomy - The arc-polarized Alfvén waves observed in the near-Earth region of the heliosphere are studied with instruments of the WIND spacecraft in 1995–2011....     

Effects of soil cracking on the seismic response of soil–structure systems

A numerical study on the influence that cracks and discontinuities (closed cracks) can have on the seismic response of a hypothetical soil–structure system is presented and discussed. A 2-D finite-difference model of the soil was developed, considering a bilinear failure surface using a Mohr–Coulomb model. The cracks are simulated with interface elements. The soil stiffness is used to characterize the contact force that is generated when the crack closes. For the cases studied herein, it was considered that the crack does not propagate during the dynamic event. Both cases, open and closed cracks, are considered. The nonlinear behavior was accounted for approximately using equivalent linear properties calibrated against several 1-D wave propagation analyses of selected soil columns with variable depth to account for changes in depth to bed rock. Free field boundaries were used at the edges of the 2-D finite-difference model to allow for energy dissipation of the reflected waves. The effect of cracking on the seismic response was evaluated by comparing the results of site response analysis with and without crack, for several lengths and orientations. The changes in the response obtained for a single crack and a family of cracks were also evaluated. Finally, the impact that a crack may have on the structural response of nearby structures was investigated by solving the seismic-soil–structure interaction of two structures, one flexible and one rigid to bracket the response. From the results of this investigation, insight was gained regarding the effect that discontinuities may have both on the seismic response of soil deposits and on nearby soil–structure systems.     

Spectral expressions for modelling the gravitational field of the Earth’s crust density structure

We derive expressions for computing the gravitational field (potential and its radial derivative) generated by an arbitrary homogeneous or laterally varying density contrast layer with a variable depth and thickness based on methods for a spherical harmonic analysis and synthesis of gravity field. The newly derived expressions are utilised in the gravimetric forward modelling of major known density structures within the Earth’s crust (excluding the ocean density contrast) beneath the geoid surface. The gravitational field quantities due to the sediments and crust components density contrasts, shown in numerical examples, are computed using the 2 × 2 arc-deg discrete data from the global crustal model CRUST2.0. These density contrasts are defined relative to the adopted value of the reference crustal density of 2670 kgm⁻³. All computations are realised globally on a 1 × 1 arc-deg geographical grid at the Earth’s surface. The maxima of the gravitational signal due to the sediments density contrast are mainly along continental shelf regions with the largest sedimentary deposits. The corresponding maxima due to the consolidated crust components density contrast are over areas of the largest continental crustal thickness with variable geological structure.     

Nonzonal structure of the response of the global field of the Earth’s atmospheric temperature to solar activity

The work describes the results of calculations obtained with the Atmospheric Research Model (ARM) general circulation model. The temperature response of the troposphere and middle atmosphere to variations in UV solar radiation were found to have a large-scale wave structure when planetary waves at the lower model boundary were taken into account. In the present paper, the results from the processing of global temperature fields with three databases (ERA-20C, NOAA-CIRES 20th Century Reanalysis, v2, and NCEP/NCAR Reanalysis I) are provided. Analysis of the differences of the mean monthly temperature global fields (January and July) between the maxima and minima of three solar activity cycles (21, 22, and 23 cycles) also demonstrated their nonzonal structure. It was shown that the amplitude of this difference in January in the stratosphere (10 hPa) can be 7–29 K in the Northern Hemisphere. In July, this effect is prominent in Southern Hemisphere. In the troposphere (500 hPa), a nonzonal temperature effect is present in both the Northern and Southern Hemispheres; the amplitude of the effects amounts to approximately 5–12 K. In conclusion, we discuss that the mechanism of solar energy impact on atmospheric temperature discovered by numerical modeling is supported after reanalysis data processing.     

On the hydro－spectrum of deep wells and the response ability of well－aquifer system

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Dynamics of Ionospheric Alfvén Resonances from the End of Cycle 21 through Cycle 24 of Solar Activity

Geomagnetism and Aeronomy - The results of the study of the dynamics of ionospheric Alfvén resonances (IARs) in a range of 0–10 Hz based on data from magnetic field observations at the...     

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.