Variations in the amplitude and phase of VLF radiowaves in the ionosphere during the August 1, 2008, solar eclipse

Time variations in the amplitude and phase of signals of the Russian telecommunication station (the frequency is 25 kHz) on the Arkhangelsk—Kharkov path with a length of about 1600 km on the day of the August 1, 2008 solar eclipse (SE) and on the adjacent days are analyzed. Two types of effects are detected. An increase of the signal amplitude by approximately 32% in comparison with the background days and the 2.1 μs time shift of the signal during 2—2.5 h is referred to the first type. Changes in the spectral composition of the quasiperiodic disturbances in the ionosphere presented the second type of the effects. For spectral analysis of the quasiperiodic variations in the amplitude and phase of the radio signal, the window Fourier transform, adaptive Fourier transform, and wavelet transformation were applied simultaneously. In the period of SE and after it, oscillations with periods of 10—15 min (according to the amplitude data) and also about 10 and 18 min (according to the phase data) were intensified. Based on radio signal characteristics, the parameters of ionospheric disturbances are estimated.     

Studying geomagnetic pulsation characteristics with the local approximation method

A local approximation method based on piecewise sinusoidal models has been proposed in order to study the frequency and amplitude characteristics of geomagnetic pulsations registered at a network of magnetic observatories. It has been established that synchronous variations in the geomagnetic pulsation frequency in the specified frequency band can be studied with the use of calculations performed according to this method. The method was used to analyze the spectral–time structure of Pc3 geomagnetic pulsations registered at the network of equatorial observatories. Local approximation variants have been formed for single-channel and multichannel cases of estimating the geomagnetic pulsation frequency and amplitude, which made it possible to decrease estimation errors via filtering with moving weighted averaging.     

Patterns of simultaneous observations of high-latitude magnetic impulses (MIEs) and impulsive bursts in the Pc1–2 band

The analysis of simultaneous observations of 128 cases of high-latitude magnetic impulse events (MIEs), as well as geomagnetic pulsations in the Pc1–2 band observed in the area of the dayside cusp, was carried out. We investigated magnetograms from the Mirny Observatory, Antarctica. As a result of the examination, three groups of impulses were identified: (1) impulses accompanied by impulsive bursts of intervals of pulsations with rising periods (IPRPs)-type geomagnetic pulsations—16% of all events, (2) impulses accompanied by impulsive bursts of the Pi1B type (bursts of irregular magnetic pulsations)—48% of all events, and (3) impulses which were not accompanied by geomagnetic pulsations within a high-frequency band—36% of all events.It was found that the maximum frequency of occurrence of the impulses accompanied by impulsive bursts of the IPRP and Pi1B types was observed between 1200 and 1300 MLT. The events of the first two groups were observed predominantly when Bz>0. It was shown that the filling frequency of impulsive bursts that accompany the occurrence of impulses depends on the amplitude of the bursts. The maximum frequency of the occurrence of impulses which were not accompanied by impulsive bursts is between 1000 and 1100 MLT. The events of the third group were observed predominantly when Bz<0. In most cases, the occurrence of impulsive bursts coincided with the leading edge of the MIE.It is supposed that the MIE generation is stimulated by intensification of the plasma turbulence level at the dayside magnetopause in consequence of modulation instability development or reconnection processes.     

The Frequency Modulation of Serpentine Emission as Compared to the Set of the Known Periodicities of Solar Oscillations

Based on the data from the Vostok station, Antarctica, for the period from 1966 to 1971, the frequency modulation of the serpentine-emission-type (SE) geomagnetic pulsations is studied. The spectral analysis of the selected emission events revealed a number of modulation periods in the interval from 1 to 60 min. The 5-min SE modulation period is most common. The obtained results are compared to the observations of solar atmosphere oscillations. The correspondence of the periods of the SE frequency modulation to the periods of oscillations of the solar photosphere is established. This confirms the previously suggested hypothesis of the existence of a genetic connection between the frequency modulation of the emission and oscillations of the solar surface.     

Spectral analysis of geomagneticPc3 pulsations recorded at the observatories of Budkov and Belsk

Summary The numerical method of Fourier's spectral analysis is used to treat 29 pairs of samples of pc3 pulsations recorded at the observatories of Budkov (Czechoslovakia) and Belsk (Poland). The amplitude, frequency and phase characteristics of their spectra are compared. The polarization characteristics of the spectral frequency components of pulsations recorded at both observatories are compared in a quasi-stationary approximation of a plane oscillation.     

Global Pi3 geomagnetic pulsations as a response of large variations in the solar wind and IMF during the magnetic storm of August 5, 2011

Spatial-temporal and spectral features of ground geomagnetic pulsations in the frequency range of 1–5 mHz at the initial phase of a strong magnetic storm of the 24th cycle of solar activity (August 5–6, 2011, with a Dst-variation in the storm maximum of ?110 nT) are analyzed. Large opposite in sign amplitudes of variations in IMF parameters (from ?20 to +20 nT) at a high velocity of the solar wind (～650 km/s) accompanied by intense bursts in solar-wind density (up to ～50 cm^?3) were distinctive feature of interplanetary medium conditions causing the storm. Geomagnetic Pi3 pulsations global in longitude and latitude and in-phase in the middle and equatorial latitudes were found. The onset of pulsation generation was caused by a pulse of dynamic pressure of the solar wind (～20 nPa), i.e., by a considerable compression of the magnetosphere. The maximum (2–3 mHz) in the amplitude spectrum of near-equatorial pulsations coincided with the maximum of pulsations in the daytime polar cap. After the next jump of the dynamic pressure of the solar wind (～35 nPa), an additional maximum appeared in the pulsation spectrum in the frequency band of ～3.5–4.5 mHz. Global pulsations suddenly stopped after a sharp decrease in the solar-wind dynamic pressure and corresponding extension of the magnetosphere. The obtained results are compared with the time dynamics of the position and shape of the plasmapause.     

Two-component spectral analysis of geomagneticpi2 pulsations

Summary The method of two-component spectral analysis is described in two versions: as the so-called static analysis, in which the pulsation sample being studied is treated as a whole, and as the dynamic analysis, in which the given sample is processed piecewise in order to determine the time variations of various parameters. Both versions were applied to geomagnetic pi2 pulsations. The pi2 oscillations are resolved into several frequency components. The properties of these components are studied in dependence on the frequency and local time. The time variation of the amplitude (attenuation) and the polarization characteristics (precession) of pi2's are also studied.     

Disturbances of the ionospheric conductivity owing to the generation of magnetic pulsations in the Pc1 frequency range by means of the heating of electrons by a ground-based high-power high-frequency transmitter

Experiments on the generation of artificial electromagnetic pulsations constitute an important part of investigations of the magnetosphere-ionosphere system with the use of an active action. The investigation of the generation of magnetic pulsations in the Pc1 frequency range has shown that the response of the ionosphere to heating is detected only in a few experiments. Although the primary perturbed parameter is the electron temperature, the efficiency of the generation of pulsations is determined by the perturbations of the ionospheric conductivity. The magnitude of these hertz perturbations depends complexly on the electron density profile and the parameters of a pump wave. The numerical experiment demonstrates the determining effect of the electron density in the D region on the magnitude of perturbations of the ionospheric conductivity. Under conditions of a low electron density, it is impossible to create a large perturbation of the conductivity in the Pc1 frequency range, although perturbations of the electron temperature can be large in this case. In view of a large number of electrons at altitudes of 70–90 km, which absorb a considerable fraction of the energy of a high-frequency wave, the electron temperature in the E region of the ionosphere cannot be sharply increased, but the amplitude of the variations of the ionospheric conductivity in this case is larger than that for the profiles with a low electron density. In the presence of the developed D region, the efficiency of the modification of the conductivity in the indicated frequency range can be increased by choosing the optimal frequency and polarization of the pump wave. A low efficiency of the experiments on the generation of artificial magnetic pulsations in the Pc1 frequency range is apparently explained by the fact that they were performed in winter in the absence of a well-developed D region of the ionosphere.     

Quantification of fundamental frequency drop for unreinforced masonry buildings from dynamic tests

The knowledge of fundamental frequency and damping ratio of structures is of uppermost importance in earthquake engineering, especially to estimate the seismic demand. However, elastic and plastic frequency drops and damping variations make their estimation complex. This study quantifies and models the relative frequency drop affecting low‐rise modern masonry buildings and discusses the damping variations based on two experimental data sets: Pseudo‐dynamic tests at ELSA laboratory in the frame of the ESECMaSE project and in situ forced vibration tests by EMPA and EPFL. The relative structural frequency drop is shown to depend mainly on shaking amplitude, whereas the damping ratio variations could not be explained by the shaking amplitude only. Therefore, the absolute frequency value depends mostly on the frequency at low amplitude level, the amplitude of shaking and the construction material. The decrease in shape does not vary significantly with increasing damage. Hence, this study makes a link between structural dynamic properties, either under ambient vibrations or under strong motions, for low‐rise modern masonry buildings. A value of 2/3 of the ambient vibration frequency is found to be relevant for the earthquake engineering assessment for this building type. However, the effect of soil–structure interaction that is shown to also affect these parameters has to be taken into account. Therefore, an analytical methodology is proposed to derive first the fixed‐base frequency before using these results. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Observations of Pc 3–4 and Pi 2 geomagnetic pulsations in the low-latitude ionosphere

Day-time Pc 3–4 (≃5–60 mHz) and night-time Pi 2 (≃5–20 mHz) ULF waves propagating down through the ionosphere can cause oscillations in the Doppler shift of HF radio transmissions that are correlated with the magnetic pulsations recorded on the ground. In order to examine properties of these correlated signals, we conducted a joint HF Doppler/magnetometer experiment for two six-month intervals at a location near L = 1.8. The magnetic pulsations were best correlated with ionospheric oscillations from near the F region peak. The Doppler oscillations were in phase at two different altitudes, and their amplitude increased in proportion to the radio sounding frequency. The same results were obtained for the O- and X-mode radio signals. A surprising finding was a constant phase difference between the pulsations in the ionosphere and on the ground for all frequencies below the local field line resonance frequency, independent of season or local time. These observations have been compared with theoretical predictions of the amplitude and phase of ionospheric Doppler oscillations driven by downgoing Alfvén mode waves. Our results agree with these predictions at or very near the field line resonance frequency but not at other frequencies. We conclude that the majority of the observations, which are for pulsations below the resonant frequency, are associated with downgoing fast mode waves, and models of the wave-ionosphere interaction need to be modified accordingly.     

Ulf-wave diagnostics of a mid geomagnetic latitude model of the ionosphere

Summary Wave diagnostics of several alternatives of a constructed model of the daytime and night ionosphere of the mid geomagnetic latitudes were carried out in the ULF range of electromagnetic waves. The altitude profiles of local wave parameters of the ionosphere, the wavelengths and wave attenuation, were analysed at a fixed frequency. The frequency dependence of the amplitude and polarization characteristics of the total wave at the Earth's surface were also analysed in dependence on the frequency-amplitude spectrum of geomagnetic pulsations. The effect of the low regions of the ionosphere (h<100 km) was tested on the damping of the propagating wave and on the frequency-amplitude and polarization characteristics of the wave at the Earth's surface.     

Periodic modulation of Pc3 and Pc4 pulsations in the polar cap by interplanetary and atmospheric processes

The variations in the daily average energy of geomagnetic pulsations and noise in the Pc3 (20–60 mHz) and Pc4 (10–19 mHz) frequency bands in the polar cap have been studied based on the data from P5 Antarctic station (corrected geomagnetic latitude ?87°) from November 1998 to November 1999. The daily average pulsation energy has been calculated using the method for detecting the wave packets, the spectral amplitude of which is higher than the threshold level, from the dynamic spectrum. A spectral analysis of the energy of pulsations and noise in the Pc3 and Pc4 bands, performed using the maximal entropy method, has revealed periodicities of 18 days in the local winter and 26, 13, and 7–9 days during the local summer. The simultaneous and coherent variations with periods of 26, 13, and 7–9 days in the solar wind velocity and IMF orientation indicate that the variations in the Pc3–4 wave energy in the polar cap at a sunlit ionosphere are mainly controlled by the parameters of the interplanetary medium. The variations in the Pc3–4 wave energy with a period of 18 days are observed only during the local winter and are supposedly related to the variations in the ionospheric conductivity modulated by planetary waves.     

Statistical characteristics of the spatial distribution of Pc3-4 geomagnetic pulsations at high latitudes in the antarctic regions

The diumal variations in the parameters of Pc3 (20–60 mHz) and Pc4 (10–19 mHz) pulsations at latitudes of the dayside cusp and polar cap have been studied using data of the magnetic stations of the trans-Antarctic meridional profile for the time interval from January to March 1997 (local summer) under weakly disturbed geomagnetic conditions (AE ≤ 250 nT). The technique for estimating pulsation parameters is based on the separation of the wave packets and noise. The diumal variations in the hourly average parameters of the wave packets in the Pc3 and Pc4 bands and noise in the Pc3-4 band (10–60 mHz)—the average number of wave packets, energy of wave packets and noise, and energy of a single wave packet—turned out to be different for the stations located deep in the polar cap (Φ ～ 87°) and at the latitudes of the dayside polar cusp (Φ ～ 70°) and auroral oval (Φ ～ 66°). Several sources of pulsations caused by different channels of wave energy penetration into the magnetosphere through the dayside cusp, dayside magnetopause, and dawn flank of the magnetotail apparently exist at high latitudes.     

Daily variations of the characteristics of beating-typePc3 (Bpc3) pulsations

Summary The spectra of nearly 100 samples of Bpc3 pulsations were computed in the X and Y components of data from the Budkov Observatory. These spectra were used to study the daily variations of the fundamental characteristics of the pulsations, the frequency f ₀ and amplitude A ₀ of the main spectral peaks. The daily variation of the ellipticity of the polarization ellipses of oscillations in frequency f ₀ was also studied in the XY-plane.Part of these results was reported at the XVth General Assembly of the IUGG, Moscow, August 1971.     

Properties of daytime long-period pulsations during magnetospheric storm commencement

Long-period geomagnetic pulsations during the SSC of July 14, 2012, are studied. The prenoon longitudinal sector (09:20–11:30) MLT, from the boundaries of which pulsations propagate azimuthally onto the dawn and dusk sides with an opposite polarization direction and increased amplitude, has been distinguished. The position of this sector relative to noon (a shift to the dawn side) depends on the front azimuthal inclination. It has been found that the polarization direction reverses in going from low (<30°) to middle/subauroral (≥50°) latitudes on the entire dayside. The geomagnetic pulsations mainly fluctuate near the f₁ = 2.9 and f₂ = 4.4 mHz frequencies. Fluctuations with frequency f₁, which coincide with the fluctuation frequency of the IMF х component, predominate at the polar cap latitudes (the open field line region) in the form of rapidly attenuating impulses and at low latitudes with a much smaller amplitude. Fluctuations with frequency f₂ are globally registered at all latitudes in the dayside sector below the magnetopause projection as a train of several fluctuations. It is assumed that fluctuations with frequency f₁ penetrate from the solar wind, and fluctuations with frequency f₂ are radial magnetopause oscillations.     

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.     

首都圈地区爆破、矿塌和天然地震的识别研究

选取首都圈地区2008年8月——2009年9月ML在2.0——2.2范围内的爆破、矿塌和天然地震数据资料,从时间域和频率域进行分析对比,总结出识别爆破、矿塌和天然地震的依据．在时域方面,爆破的初动方向向上,矿塌向下,天然地震的初动方向依赖于台站的分布情况;爆破和矿塌的面波比较发育;天然地震的S波与P波最大振幅比(AS/AP)大于爆破和矿塌,同时,爆破和矿塌的能量衰减比天然地震快．在频域方面,高频成分的能量衰减快于低频;天然地震的拐角频率较高,爆破次之,矿塌的最小;在震中距200 km范围内,爆破的顶峰频率主要分布在5——7 Hz,矿塌分布在2——4 Hz,天然地震的顶峰频率较大,在10——18 Hz范围内.另外,天然地震的频率域较宽,其次为爆破、矿塌．      

Ponderomotive Force of Ion Cyclotron Waves in the <Emphasis Type="Italic">Pc</Emphasis>1 Frequency Range with Magnetosonic Dispersion

Nonlinear properties of the Pc1 geomagnetic pulsations with anomalous (magnetosonic) dynamic spectrum are studied. The nonlinear properties of the waves are reflected in the emergence of ponderomotive force proportional to the squared amplitude of the waves. Just as in the case of the Alfven waves, at small values of parameter ν₀ = ω/ω_ci0 < 0.4 (ratio of the carrier frequency to proton gyrofrequency in the equatorial plane), the ponderomotive force leads to the modification of the background plasma through increasing its density towards the equator. At ν₀ > 0.4, plasma is expelled from the equator towards the higher latitudes. The dependence of the nonlinear modification of background plasma for the different parameters of the magnetosphere is studied.     

Generation of VLF emissions during the substorm preliminary phase and the auroral dynamics at the auroral oval northern edge

About 100 breakups of different types and intensities are studied on the basis of Lovozero Observatory data. Magnetic pulsations in different frequency ranges, VLF emissions, and auroral activity are analyzed using the TV data. It is found that magnetic pulsations in all frequency ranges lag behind the moment of breakup by 0.5–2.0 min, and bursts of low-intensity broadband VLF emission hiss are observed 3–10 min before breakup. Hiss leading breakup corresponds to feeble auroras located northward of a pre-breakup arc.