Apparent Frequency Research Articles

Effect of fluids and frequencies on Poisson’s ratio of sandstone samples

Poisson’s ratio [Formula: see text] is an important parameter when interpreting measured geophysical and seismic data. For an isotropic medium, it directly relates to the ratio of P- and S-wave velocities. We have measured [Formula: see text] as a function of pressure and frequency in fluid-saturated sandstones. The method of measuring [Formula: see text] was first tested as a function of pressure and frequency using standard samples. The phase shift [Formula: see text] between radial and axial strains was also measured. For all standard samples, such as the linear viscoelastic Plexiglas, the data indicated that [Formula: see text] correlated with [Formula: see text] and related to a dissipation on [Formula: see text]. Then, [Formula: see text] and [Formula: see text] were measured as a function of pressure and frequency for two dry and fluid-saturated Fontainebleau sandstone samples. Under dry conditions, no frequency dependence and very small pressure dependence were observed. Unusual behaviors were observed under fluid-saturated conditions. In particular, [Formula: see text] of one sample indicated a frequency-dependent bell-shaped dispersion under water and glycerin saturation that correlated with peaks in [Formula: see text]. Plotting the measurements as a function of apparent frequency (i.e., normalizing by the fluid viscosity) indicated a good fit between the water- and glycerin-saturated measurements. The bell-shaped dispersion in [Formula: see text] that was observed for one particular sandstone held for all effective pressures. These variations fully correlated with the peaks of [Formula: see text] observed. Our results can be interpreted using fluid flow and effective medium theories in the case of a porous microcracked rock. Drained/undrained and relaxed/unrelaxed transitions have frequency and magnitude of variations that are consistent with the measurements. The rock sample microcrack density strongly affects this frequency dependence. The inferred [Formula: see text] ratio at low effective pressures also indicates a large frequency-dependent bell-shaped dispersion. The parameter [Formula: see text] is a clear indicator of the frequency-dependent dissipation of [Formula: see text] and relates to the attenuation of P- and S-waves.

Corrigendum to “Do successor effects in reading reflect lexical parafoveal processing? Evidence from corpus-based and experimental eye movement data” [J. Mem. Lang. 79–80 (2015) 76–96

In the past, most research on eye movements during reading involved a limited number of subjects reading sentences with specific experimental manipulations on target words. Such experiments usually only analyzed eye-movements measures on and around the target word. Recently, some researchers have started collecting larger data sets involving large and diverse groups of subjects reading large numbers of sentences, enabling them to consider a larger number of influences and study larger and more representative subject groups. In such corpus studies, most of the words in a sentence are analyzed. The complexity of the design of corpus studies and the many potentially uncontrolled influences in such studies pose new issues concerning the analysis methods and interpretability of the data. In particular, several corpus studies of reading have found an effect of successor word (n + 1) frequency on current word (n) fixation times, while studies employing experimental manipulations tend not to. The general interpretation of corpus studies suggests that readers obtain parafoveal lexical information from the upcoming word before they have finished identifying the current word, while the experimental manipulations shed doubt on this claim. In the present study, we combined a corpus analysis approach with an experimental manipulation (i.e., a parafoveal modification of the moving mask technique, Rayner & Bertera, 1979), so that, either (a) word n + 1, (b) word n + 2, (c) both words, or (d) neither word was masked. We found that denying preview for either or both parafoveal words increased average fixation times. Furthermore, we found successor effects similar to those reported in the corpus studies. Importantly, these successor effects were found even when the parafoveal word was masked, suggesting that apparent successor frequency effects may be due to causes that are unrelated to lexical parafoveal preprocessing. We discuss the implications of this finding both for parallel and serial accounts of word identification and for the interpretability of large correlational studies of word identification in reading in general.

Rheological Characterization of Ethanolamine Gel Propellants

Ethanolamine is considered to be an environmentally friendly propellant system because it has low toxicity and is noncarcinogenic in nature. In this article, efforts are made to formulate and prepare ethanolamine gel systems, using pure agarose and hybrids of paired gelling agents (agarose + polyvinylpyrrolidine (PVP), agarose + SiO2, and PVP + SiO2), that exhibit a measurable yield stress, thixotropic behavior under shear rate ranges of 1–1,000 s−1 and a viscoelastic nature. To achieve these goals, multiple rheological experiments (including flow and dynamic studies) are performed. In this article, results are presented from experiments measuring the apparent viscosity, yield stress, thixotropy, dynamic strain, frequency sweep, and tan δ behaviors, as well as the effects of the test temperature, in the gel systems. The results show that the formulated ethanolamine gels are thixotropic in nature with yield stress between 30 and 60 Pa. The apparent viscosity of the gel decreases as the test temperature increases, and the apparent activation energy is the lowest for the ethanolamine-(PVP + SiO2) gel system. The dynamic rheology study shows that the type of gellant, choice of hybrid gelling materials and their concentration, applied frequencies, and strain all vitally affect the viscoelastic properties of the ethanolamine gel systems. In the frequency sweep experiment, the ethanolamine gels to which agarose, agarose + PVP, and agarose + SiO2 were added behave like linear frequency-dependent viscoelastic liquids, whereas the ethanolamine gel to which PVP + SiO2 was added behaves like a nearly frequency-independent viscoelastic solid. The variation in the tan δ of these gelled propellants as a function of frequency is also discussed.

Vibroacoustics of orthotropic plates ribbed in both directions: Application to stiffened rectangular wood panels.

This paper is focused on the vibroacoustic behavior of a rectangular ribbed wood panel. This is done by developing an analytical model based on a variational approach, taking into account the kinetic and strain energies of a special orthotropic plate, 11 ribs oriented in a first direction and 1 other strong stiffener oriented in the perpendicular direction, which are considered as beams tied to the plate. A modal decomposition is adopted on the basis of the simply supported orthotropic plate. This allows calculating the modes of the wood panel (ribbed modes) in the frequency range 0-5000 Hz. The acoustical radiation of the baffled panel is also calculated. The radiation coefficients of the ribbed modes are presented and compared, when possible, to similar unribbed plate modes. Finally, the vibroacoustic analysis of the structure shows that an excitation placed on the hard point makes the panel particularly radiative and decreases the apparent critical frequency.

Angiographic Characteristics of Catheter-Induced Spasm of the Left Main Coronary Artery

Spasm of the left main coronary artery (LM) is considered to be rare. We investigated the angiographic characteristics of the LM in patients with combined LM and disease involving additional vessels, who underwent coronary artery bypass grafting (CABG) and underwent repeat coronary angiography within 5 years of the CABG, to examine the apparent frequency of spasm of the LM on initial angiography and its possible predictors. A retrospective analysis was performed of patients who underwent coronary angiography in our institute, who were found to have significant LM stenosis and disease involving additional vessels, underwent CABG and repeat coronary angiography within 5 years of the CABG. Data on angiographic characteristics of the LM on the initial angiogram were investigated. Of 84 patients, 17 (20%) were found to have a normal LM on repeat angiography (group A), and 67 (80%) demonstrated significant stenosis (group B). The degree of LM stenosis was milder in the initial angiogram in group A than in group B (64 ± 15% vs 72 ± 14%, p = 0.047). Most patients in group A demonstrated tubular LM stenosis at initial catheterization in comparison to group B (71% vs 18%). Using multivariate analysis, the only predictor for a normal LM at repeat catheterization was found to be tubular stenosis at initial catheterization (odds ratio 123, 95% confidence interval 4.0 to 3696). In conclusion, LM coronary spasm is a common finding, particularly in those with the appearance of tubular stenosis, and it should be excluded even in patients with additional coronary disease in certain instances to prevent unnecessary CABG.

A Simple Kinetic Model of Oil Generation, Vaporization, Coking, and Cracking

A simple model is presented that accounts for the effects of temperature, pressure, and externally supplied gas flow rate on the kinetics of oil evolution during oil shale pyrolysis. The primary transport mechanism for generated oil from retorting oil shale particles is, by far, vaporization in co-generated gases. The simple model is based on the concept that an equivalent volatility of the oil must be achieved in order to attain the same extent of oil evolution from the particle. Oil generated at lower temperature (hence, lower volatility) must undergo additional coking and cracking reactions to reach equivalent volatility, and that effect can be modeled by adding a simple term to the effective activation energy. This simple approach explains much of the compensation effect observed in the literature between apparent activation energies and frequency factors derived under different pyrolysis conditions. A similar approach can model the effect of pressure on oil evolution. The effect of pyrolysis temperat...

Genome-wide analysis of myxobacterial two-component systems: genome relatedness and evolutionary changes.

BackgroundTwo-component systems (TCSs) are abundant prokaryotic signaling pathways, whose evolution is of particular importance because of their role in bacterial pathogenicity. Comparative genomics can provide important insights into the evolution of these genes, but inferences are dependent on the relatedness of the compared genomes. This study investigated the relationship between evolutionary distance and TCS evolution in myxobacterial genomes, of which there are several sequenced examples, of varying relatedness, and which encode large numbers of TCSs.MethodsMyxobacterial TCS gene sets were compared, orthologues defined, and changes in TCS properties such as gene organisation, domain architecture and size identified. Results Genome relatedness/evolutionary distance was found to have a large effect on the apparent frequency of evolutionary events affecting TCS genes, but not on the relative dominance of different types of mutations. Large (≥1 gene) indels were the most common changes, often giving rise to gene organisation changes. Smaller indels were also common, sometimes changing domain architecture, and/or leading to pseudogene formation. Individuality of myxobacterial TCS gene sets seems primarily due to lineage specific gene loss. However, there is also evidence of extensive acquisition of genes by lateral transfer, with gene duplication also creating new TCS genes.ConclusionsThis study provides catalogues of myxobacterial TCS gene sets and their orthology relationships, benchmarked against genome relatedness. It also provides insights into the relationship between evolutionary distance and the inference of TCS estudies of TCS evolution beyond the myxobacteriavolution, which may be important for studies of TCS evolutiThe online version of this articleon beyond the myxobacteria.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2018-y) contains supplementary material, which is available to authorized users.

Bacterial Cellulose Supported Gold Nanoparticles with Excellent Catalytic Properties

Amidoxime surface functionalized bacterial cellulose (AOBC) has been successfully prepared by a simple two-step method without obviously changing the morphology of bacterial cellulose. AOBC has been used as the reducing agent and carrier for the synthesis of gold nanoparticles (AuNPs) that distributed homogeneously on bacterial cellulose surface. Higher content in amidoxime groups in AOBC is beneficial for the synthesis of AuNPs with smaller and more uniform size. The AuNPs/AOBC nanohybrids have excellent catalytic activity for reduction of 4-nitrophenol (4-NP) by using NaBH4. It was found that catalytic activity of AuNPs/AOBC first increases with increasing NaBH4 concentration and temperature, and then leveled off at NaBH4 concentration above 238 mM and temperature above 50 °C. Moreover, AuNPs with smaller size have higher catalytic activity. The highest apparent turnover frequency of AuNPs/AOBC is 1190 h(-1). The high catalytic activity is due to the high affinity of 4-NP with AuNPs/AOBC and the reduced product 4-aminophenol has good solubility in water in the presence of AuNPs/AOBC. The catalytic stability of the AuNPs/AOBC was estimated by filling a fluid column contained AuNPs/AOBC and used for continuously catalysis of the reduction of 4-NP by using NaBH4. The column works well without detection of 4-NP in the eluent after running for more than two months, and it is still running. This work provides an excellent catalyst based on bacterial cellulose stabilized AuNPs and has promising applications in industry.

Statistical Properties of Strong Ground Motions from the Generalized Spectral Inversion of Data Observed by K‐NET, KiK‐net, and the JMA Shindokei Network in Japan

Abstract The objective of this study is to investigate in detail the characteristics of strong ground motions separated from acceleration Fourier spectra of ground motions observed by K‐NET, KiK‐net, and the Japan Meteorological Agency (JMA) Shindokei network in Japan using the generalized spectral inversion method. The separation method used here is the same as that proposed by Kawase and Matsuo (2004), with the same reference bedrock site at YMGH01. We include events with magnitude equal to or larger than 4.5 observed from 1996 to 2011. Our results are in good agreement with previous results regarding source, path, and site characteristics, but with higher stability thanks to the increased amount of data. We obtain site amplification factors common to all source types but with different Q ‐values for three source categories: crustal earthquakes (type C), subducting plate‐boundary earthquakes (type B), and intraplate earthquakes (type I). We find that our frequency‐dependent Q ‐values are comparable to those of previous studies, and that the obtained Q ‐values depend on the traveling regions. As for the geometric spreading factor n , we find that for type B and type I earthquakes, the value is close to 1.0 as expected, but type C earthquakes show apparent frequency dependence. From the corner frequencies of source spectra, we calculate Brune’s stress parameters and find a clear magnitude dependence, in which smaller events tend to spread over a wider range while maintaining the same maximum value. This means that stress drops for larger events would be maximal. We confirm that this is exactly the case for several mainshock–aftershock sequences. The average stress parameters are 5.1 and 6.0 MPa for types B and I, and 0.8 MPa for type C. This large difference between types B and I and type C can be explained primarily by the depth dependence of stress parameters.

Experimental study of Young’s modulus dispersion and attenuation in fully saturated sandstones

Although seismic wave dispersion and attenuation have been found to occur in sedimentary rocks, it remains challenging to experimentally observe these effects. A new experimental setup has been developed to measure the Young’s modulus and Poisson’s ratio of rocks over a wide range in pressure ([Formula: see text]) and frequency ([Formula: see text]). Calibration with standard samples determined the following: (1) no dependence of the apparatus to pressure and frequency and (2) a good fit between published data and the measured and inferred elastic properties. The measured Young’s modulus dispersion and attenuation of Plexiglas were also consistent with the published data. The Young’s modulus and the attenuation of Fontainebleau sandstone samples saturated by water and glycerin were then measured. Although small variations were observed for one sample, the second one exhibited strong pressure- and frequency-dependent variations of Young’s modulus and attenuation. A frequency-dependent fluid flow was simultaneously measured. The characteristic frequency for these variations was highly fluid dependent. Accounting for the in situ fluids’ viscosity using an apparent frequency parameter, we determined the Young’s modulus and attenuation of a fluid-saturated Fontainebleau sandstone over an apparent frequency band of [Formula: see text]. The measurements under water and glycerin saturation compared favorably, and two frequency-dependent phenomena were observed that were interpreted as the drained/undrained and undrained/unrelaxed transitions. The undrained/unrelaxed transition occurred in a large frequency range, which was attributed to a distribution in aspect ratio of the rock’s microcracks.

Fast bubble dynamics and sizing

Single bubble sizing is usually performed by measuring the resonant bubble response using the Dual Frequency Ultrasound Method. However, in practice, the use of millisecond-duration chirp-like waves yields nonlinear distortions of the bubble oscillations. In comparison with the resonant curve obtained under harmonic excitation, it was observed that the bubble dynamic response shifted by up to 20 percent of the resonant frequency with bubble radii of less than 100μm. In the case of low pressure waves (P<5kPa), an approximate formula for the apparent frequency shift is derived. Simulated and experimental bubble responses are analyzed in the time–frequency domain using an enhanced concentrated (reassigned) spectrogram. The difference in the resonant frequency resulted from the persistence of the resonant mode in the bubble response. Numerical simulations in which these findings are extended to pairs of coupled bubbles and to bubble clouds are also presented.

Critical Depths for Higher Modes by Minimally-invasive Multimodal Surface Wave (MMSW) Method: Simulations and Field Test

A Minimally-invasive Multimodal Surface Wave (MMSW) geophysical testing method, which is a hybrid of surface and borehole seismic methods, was developed recently by the authors to measure more extensive multi-mode dispersion data and thus improve the accuracy of inversion profiles. The new MMSW method employs a borehole geophone at selected depths to record seismic waves from different source offsets on the soil surface. Presented in this paper is a procedure for estimating a range of optimum geophone depths to capture a given higher mode by the MMSW method. Stiffness matrix and finite-element-based numerical simulations of the MMSW method are performed to identify the relationships between critical geophone depths and apparent cutoff frequencies of higher modes. Specifically, it is shown for increasing velocity profiles that 1) at a given borehole sensor measurement depth, the apparent cutoff frequencies of higher modes increase with mode number, 2) at a given frequency, the critical geophone depth at which a higher mode will first become dominant increases with mode number, and 3) for a given higher mode, the apparent cutoff frequency decreases as measurement depth increases. A preliminary field test is conducted using a vertical geophone placed at five different depths while impacts are applied to the soil surface from 3.66 to 43.89 m from the borehole, with an impact spacing of 3.66 m. Dispersion images from the five geophone depths are superimposed to produce a dispersion image having three modes with improved clarity relative to the surface-only Multichannel Analysis of Surface Waves (MASW) method. A comparison of the experimental and theoretical apparent cutoff frequencies for higher modes is used to validate the theoretical prediction of critical depths by the stiffness matrix method. Matching of experimental and theoretical apparent cutoff frequencies could provide additional optimization constraints to reduce the uncertainty of final inversion profiles.

Correlation between Structural and Catalytic Properties of Copper Supported on Porous Alumina for the Ethanol Dehydrogenation Reaction

AbstractStructural and surface features of highly dispersed copper catalysts supported on hierarchical porous alumina were evaluated for the ethanol dehydrogenation reaction. The catalysts were prepared by incipient wetness impregnation of alumina obtained by a dual template sol–gel procedure. Structural characterizations provided evidence that the copper was highly dispersed on the alumina in the form of monomeric octahedral Cu2+ species adsorbed into an incomplete layer. A two‐step reduction was observed for the catalysts in a temperature range of 50–250 °C under H2/He flow. The temperature for onset of reduction and the percentage of Cu+ species formed upon activation were dependent on the Cu loading. At lower Cu loadings, the amount of Cu+ increased, leading to a slower and less complete reduction. Based on the apparent activation energy and turnover frequency, it could be concluded that Cu+ containing a mixture of Cu+/Cu0 results in a more efficient catalyst than Cu0 for the ethanol dehydrogenation reaction.

Plants, fungi and oomycetes: a 400‐million year affair that shapes the biosphere

Plants, fungi and oomycetes: a 400‐million year affair that shapes the biosphere

Structural health monitoring of suspension bridges with features affected by changing wind speed

Tools based on multivariate statistical analysis are becoming very popular for automatically revealing the existence of damage in structures using vibration data under changing environmental and operational conditions (typically temperature, humidity and traffic intensity). In this paper, methods of multivariate statistical analysis are newly applied for monitoring the structural health state of the main cables of suspension bridges, with the main contribution of removing non-linear aeroelastic correlations between dynamic features and wind speed.Considering natural frequencies as damage-sensitive features, an analytical parametric model of suspension bridge with damage in one main cable and subjected to wind loading is formulated, at first, as an extension of previous work. Model predictions demonstrate that apparent frequency variations caused by changes in incoming wind speed can likely be more significant than those produced by a small damage. A technique based on principal component analysis and novelty detection is adopted to cope with this issue and its application to long-term pseudo-experimental buffeting response data, generated by means of the analytical model, is presented. The results demonstrate the feasibility of permanent monitoring systems to reveal the existence of damages in wind-excited long-span bridges producing relative variations in the most sensitive natural frequency smaller than 0.1%.

Application of resonant steel tuning forks with circular and rectangular cross sections for precise mass density and viscosity measurements

The feasibility of using commercially available steel tuning forks for viscosity and mass density sensing is investigated. For this task, the tuning forks are electromagnetically driven and read out to record their frequency responses containing the fundamental resonant mode upon immersion in a sample liquid. Evaluated resonance frequencies and quality factors are then related to the liquids’ mass density and viscosity. The used electromagnetic actuation and readout principle allows that only the tuning fork which is placed in the center of a glass tube gets wetted with the liquid to be examined. All excitation and read out related structures and electronics are placed outside the glass tube and thus, are not affected or influenced by the liquid. A generalized model relating evaluated quality factors and resonance frequencies to viscosity and mass density is used to describe the tuning forks’ sensitivities and furthermore to estimate required stabilities of apparent quality factors and resonance frequencies to achieve measurement accuracies similar to those of laboratory instruments. It is shown that relative accuracies in the order of 1% in viscosity and 0.1mg/cm3 in mass density are achievable.

Thickness dependent effective viscosity of a polymer solution near an interface probed by a quartz crystal microbalance with dissipation method.

The solution viscosity near an interface, which affects the solution behavior and the molecular dynamics in the solution, differs from the bulk. This paper measured the effective viscosity of a dilute poly (ethylene glycol) (PEG) solution adjacent to a Au electrode using the quartz crystal microbalance with dissipation (QCM-D) technique. We evidenced that the effect of an adsorbed PEG layer can be ignored, and calculated the zero shear rate effective viscosity to remove attenuation of high shear frequency oscillations. By increasing the overtone n from 3 to 13, the thickness of the sensed polymer solution decreased from ~70 to 30 nm. The zero shear rate effective viscosity of the polymer solution and longest relaxation time of PEG chains within it decrease with increasing solution thickness. The change trends are independent of the relation between the apparent viscosity and shear frequency and the values of the involved parameter, suggesting that the polymer solution and polymer chains closer to a solid substrate have a greater effective viscosity and slower relaxation mode, respectively. This method can study the effect of an interface presence on behavior and phenomena relating to the effective viscosity of polymer solutions, including the dynamics of discrete polymer chains.

Flicker adaptation or superimposition raises the apparent spatial frequency of coarse test gratings

Independent channels respond to both the spatial and temporal characteristics of visual stimuli. Gratings <3cycles per degree (cpd) are sensed by transient channels that prefer intermittent stimulation, while gratings >3cpd are sensed by sustained channels that prefer steady stimulation. From this we predict that adaptation to a spatially uniform flickering field will selectively adapt the transient channels and raise the apparent spatial frequency of coarse sinusoidal gratings. Observers adapted to a spatially uniform field whose upper or lower half was steady and whose other half was flickering. They then adjusted the spatial frequency of a stationary test (matching) grating on the previously unmodulated half field until it matched the apparent spatial frequency of a grating falling on the previously flickering half field. The adapting field flickered at 8Hz and the spatial frequency of the gratings was varied in octave steps from 0.25 to 16cpd. As predicted, adapting to flicker raised the apparent spatial frequency of the test gratings. The aftereffect reached a peak of 11% between 0.5 and 1cpd and disappeared above 4cpd. We also observed that superimposed 10Hz luminance flicker raised the apparent spatial frequency of 0.5cpd test gratings. The effect was not seen with slower flicker or finer test gratings. Altogether, our study suggests that apparent spatial frequency is determined by the balance between transient and sustained channels and that an imbalance between the channels caused by flicker can alter spatial frequency perception.

Structural Health Monitoring of a 54-Story Steel-Frame Building Using a Wave Method and Earthquake Records

The variations of identified wave velocities of vertically propagating waves through the structure are investigated for a 54-story steel-frame building in downtown Los Angeles, California, over a period of 19 years since construction (1992–2010), using records of six earthquakes. The set includes all significant earthquakes that shook this building, which produced maximum transient drift ∼0.3% and caused no reported damage. Wave velocity profiles β( z) are identified for the NS, EW, and torsional responses by fitting layered shear beam/torsional shaft models in the recorded responses, by waveform inversion of pulses in impulse response functions. The results suggest variations larger than the estimation error, with a coefficient of variation about 2–4.4%. About 10% permanent reduction of the building stiffness is detected, caused mainly by the Landers and Big Bear earthquake sequence of 28 June 1992, and the Northridge earthquake of 17 January 1994. Permanent changes of comparable magnitude were identified also in the first two apparent modal frequencies, f1; app, and f2; app, which were identified from the peaks of the transfer-function amplitudes.

Pc2-3 geomagnetic pulsations on the ground, in the ionosphere, and in the magnetosphere: MM100, CHAMP, and THEMIS observations

Abstract. We analyze Pc2-3 pulsations recorded by the CHAMP (CHAllenging Minisatellite Payload) satellite in the F layer of the Earth's ionosphere, on the ground, and in the magnetosphere during quiet geomagnetic conditions. The spectra of Pc2-3 pulsations recorded in the F layer are enriched with frequencies above 50 mHz in comparison to the ground Pc2-3 spectra. These frequencies are higher than the fundamental harmonics of the field line resonances in the magnetosphere. High quality signals with dominant frequencies 70–200 mHz are a regular phenomenon in the F layer and in the magnetosphere. The mean latitude of the maximum Pc2-3 occurrence rate lies at L &amp;amp;approx; 3.5 in the F layer, i.e., inside the plasmasphere. Day-to-day variations of the L value of the CHAMP Pc2-3 occurrence rate maximum follow the plasmapause day-to-day variations. Polarization and amplitude of Pc2-3s in the magnetosphere, in the ionosphere, and on the ground allow us to suggest that they are generated as fast magnetosonic (FMS) waves in the outer magnetosphere and are partly converted into shear Alfven waves near the plasmapause. The observed ground-to-ionosphere amplitude ratio during the night is interpreted as a result of the Alfven wave transmission through the ionosphere. The problem of wave transmission through the ionosphere is solved theoretically by means of a numerical solution of the full-wave equation for the Alfven wave reflection from and transmission through a horizontally stratified ionosphere. The best agreement between the calculated and measured values of the ground-to-ionosphere amplitude ratio is found for k = 5 × 10−3 km−1, i.e., the observed ground-to-ionosphere amplitude ratio corresponds to a wave spatial scale which could provide a Doppler shift within a few percent of the apparent frequency of the Pc2-3 pulsations as recorded by a low-orbiting spacecraft.