Maximum Spectral Amplitude Research Articles

Polarization of gravitational waves from helical MHD turbulent sources

We use direct numerical simulations of decaying primordial hydromagnetic turbulence with helicity to compute the resulting gravitational wave (GW) production and its degree of circular polarization. The turbulence is sourced by magnetic fields that are either initially present or driven by an electromotive force applied for a short duration, given as a fraction of one Hubble time. In both types of simulations, we find a clear dependence of the polarization of the resulting GWs on the fractional helicity of the turbulent source. We find a low frequency tail below the spectral peak shallower than the f 3 scaling expected at super-horizon scales, in agreement with similar recent numerical simulations. This type of spectrum facilitates its observational detection with the planned Laser Interferometer Space Antenna (LISA). We show that driven magnetic fields produce GWs more efficiently than magnetic fields that are initially present, leading to larger spectral amplitudes, and to modifications of the spectral shape. In particular, we observe a sharp drop of GW energy above the spectral peak that is in agreement with the previously obtained results. The helicity does not have a huge impact on the maximum spectral amplitude in any of the two types of turbulence considered. However, the GW spectrum at wave numbers away from the peak becomes smaller for larger values of the magnetic fractional helicity. Such variations of the spectrum are most noticeable when magnetic fields are driven. The degree of circular polarization approaches zero at frequencies below the peak, and reaches its maximum at the peak. At higher frequencies, it stays finite if the magnetic field is initially present, and it approaches zero if it is driven. We predict that the spectral peak of the GW signal can be detected by LISA if the turbulent energy density is at least ∼3% of the radiation energy density, and the characteristic scale is a hundredth of the horizon at the electroweak scale. We show that the resulting GW polarization is unlikely to be detectable by the anisotropies induced by our proper motion in the dipole response function of LISA. Such signals can, however, be detectable by cross-correlating data from the LISA-Taiji network for turbulent energy densities of ∼5%, and fractional helicity of 0.5 to 1. Second-generation space-base GW detectors, such as BBO and DECIGO, would allow for the detection of a larger range of the GW spectrum and smaller amplitudes of the magnetic field.

GNSS-R Snow Depth Inversion Based on Variational Mode Decomposition With Multi-GNSS Constellations

Snow depth monitoring is meaningful for climate analysis, hydrological research and snow disaster prevention. Global Navigation Satellite System-Reflectometry (GNSS-R) technology uses the relationship between the modulation frequency of the signal-to-noise ratio (SNR) and reflector height to monitor snow depth. Existing research on single constellation has made good progress and is gradually developing towards multi-constellation combined inversion. Aiming at the accuracy of snow depth inversion, this paper introduces the variational mode decomposition (VMD) algorithm with the characteristics of an adaptive high-pass filter to detrend the SNR data. The experimental results of KIRU station and P351 station show that VMD algorithm is suitable for different constellations and has better signal separation effect. The snow depth inversion results for both stations are in high agreement with the in-situ snow depths provided by the Swedish Meteorological and Hydrological Institute (SMHI) and the SNOTEL network, respectively. The root mean square error (RMSE) of the inversion results is reduced by 20-40% compared to the least squares fitting (LSF) algorithm, and the correlation coefficients are also greatly improved. Moreover, considering that there is no overlap between the climate station and the inversion area, this paper introduces the maximum spectral amplitude as another reference data source and obtains basically consistent experimental conclusions. On this basis, the maximum spectral amplitude is used as the input variable of the entropy method, and the feasibility of the combination strategy is studied. The results show that the combined strategy reduces a little inversion error and improves the temporal resolution of snow depth monitoring. It is of great significance for more accurate and rapid monitoring of snow depth changes and disaster warnings, and provides an important reference for further research on GNSS-R technology.

An experimental investigation of the interface behavior of slug flow in horizontal pipe

The simultaneous gas-liquid slug flow phenomena often occur in the industrial production such as the gas or oil transportation system. The research on the gas-liquid interface is essential to the safety design of industrial piping systems and the comprehension of flow mechanism. However, most of the researches on slug flow are only concentrated on the spatial domain parameters of slug bubble and seldom consider the frequency domain characteristics. In this paper, a series of experiments were performed to investigate the influence of gas and liquid phases on the interface shape of slug flow. The results show that the increase of gas or liquid superficial velocity has an opposite effect on the interface shape of slug flow. Moreover, the breakup of slug bubble tail was observed in the slug flow, and the breakup mechanism was developed based on the analysis of the frequency distribution of gas-liquid interface. Finally, the maximum spectral amplitude was presented as a new interface parameter to characterize the interface distribution of slug flow, and a correlation between the gas-liquid superficial velocity and the maximum spectral amplitude was established, which can further reveal the relationship between the interface behavior and the flow velocity.

Frequency offset estimation in the intermediate frequency for satellite-based AIS signals

In this paper, an accurate frequency offset estimator is investigated in the intermediate frequency for the satellite-based automatic identification system (AIS) signals. Using Gaussian minimum shift keying (GMSK) modulation for transmission, the AIS signal is shown to be a plane wave with the modulated phase information and carrier frequency resulting from the Doppler effects. Hence, the phase information can be eliminated with a re-modulated signal, and the frequency offset can be estimated by the ratio of the maximum spectral amplitude and its neighbor spectral amplitude based on the fast Fourier transformation (FFT) interpolation. The estimator has low complexity, and it is easy to implement. Computer simulations are used to assess the performance of the estimator.

Tsunami-driven ionospheric perturbations associated with the 2011 Tohoku earthquake as detected by subionospheric VLF signals

The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiving station at Moshiri, Hokkaido, are used to detect the response of the lower ionosphere to the tsunami triggered by the 2011 Tohoku earthquake. Disturbances in the phase and amplitude of VLF signals propagating from the transmitter in Hawaiian Islands are observed during the tsunami wave passage, and these effects in the ionosphere are compared to the in situ sea-level global positioning system (GPS) measurements near Japan. The frequency of the maximum spectral amplitude both for the VLF and GPS data is found to be in the range of periods of 8–50 min, which is likely to correspond to the period of the internal gravity waves generated by the tsunami.

Ionospheric effects of the Mt. Kirishima volcanic eruption as seen from subionospheric VLF observations

Data from the Pacific network of VLF receivers have been used to study the response of the lower ionosphere to the January 2011 Mt. Kirishima (South Japan) volcanic eruption. A major explosive eruption occurred on January 27, which was preceded by several small eruptions. Perturbations of nighttime subionospheric VLF signals have been detected on the day of the first small eruption on January 18 (UT) with the maximum observed about 1.5h after the eruption. The nighttime signal remained disturbed during the subsequent pre-eruptive and eruptive activity of Mt. Kirishima. The daytime perturbations were not observed. The frequency of the maximum spectral amplitude was found to be in the range of periods of 6–30min, which corresponds to the periods of internal gravity waves. These results suggest that the observed VLF ionospheric effects can possibly be produced by the penetration of gravity waves caused by the volcanic activity into the ionosphere.

Development of an Educational Environment for Simulating Multistatic UWB Radars for Locating an Intruder Inside Indoor Environment

This work describes a educational tool developed in Java language for simulating a multi-static radar. This radar is used to identify the presence of an intruder inside a residence. In order to obtain high resolution, the radar operates with ultra-wideband pulses (Gaussian monocycle), which have maximum spectral amplitude at 1 GHz and half power bandwidth of approximately 1 GHz. For the estimation of the intruder's position, the optimization technique particle swarm optimization (PSO) is employed. The simulator allows that the users define the parameters to be used by the radar and obtain the estimation of the intruder's position. Besides that, the software automatically generates animations of the wave propagation inside the residence, graphics of the signal at the receiving points and a final report with all the information related to the simulations.

Excitation of Resonant Modes along the Japanese Coast by the 1993 and 1983 Tsunamis in the Japan Sea

We observed seiches at 55 ports in Japan facing the Japan Sea and obtained dominant periods from their maximum spectral amplitudes. These periods were mostly determined ranging from 10 to 40 minutes. They were compared with dominant periods of the 1993 Hokkaido Nansei-oki tsunami and the 1983 Nihonkai Chubu-oki tsunami at the same ports. As a result, relations of dominant periods between seiches and tsunamis are classified into three types. The first one is fundamental mode excitation, the second is higher mode excitation and the third is no excitation. Plotting the maximum spectral amplitude normalized at an epicentral distance of 50 km versus the ratio of the tsunami dominant period to the seiche dominant period, we obtained resonance curves having maxima at one. This fact shows a contribution of resonance to the amplification. Thus it is recognized that the dominant period of seiching is an important factor in interpreting amplification and resonance of tsunami.

Physical analysis of the transition to turbulence in the wake of a circular cylinder by three-dimensional Navier–Stokes simulation

The transition to turbulence of the flow around a circular cylinder is studied by a three-dimensional numerical simulation of the Navier–Stokes equations system in the Reynolds number range 100–300. The numerical method is second-order accurate in space and time and Neumann boundary conditions are used at the two boundaries in the spanwise direction; non-reflecting boundary conditions are specified for the outlet downstream boundary. This study predicts the frequency modulation and the formation of a discontinuity region delimited by two frequency steps within the present Reynolds number range. These features are related to the birth of streamwise vorticity and to the kinetic energy distribution in the near wake. The development of the mean dynamic quantities, the Reynolds stress correlations and the variation of their maximum values are provided in this region, where the similarity laws do not hold. The spatial evolution of the von Karman mode and of its spectral amplitude are quantified and the variation laws of the maximum spectral amplitude and of its location as a function of Reynolds number are established. The critical Reynolds number for the appearance of the first discontinuity in the present flow system is evaluated by the fully nonlinear approach.

Site Response on Seismic Motion in the Granada Basin (Southern Spain) Based on Microtremor Measurements.

We investigate the feasibility of using microtremor measurements in short and long period ranges for the prediction of frequency-dependent amplification effects due to local site geology in the Granada basin and Granada city (Southern Spain). Following a seismic reflection profile crossing the Granada basin, microtremor measurements were recorded at 12 different sites. Peaks centered at 0.7 s (1.5 Hz) and 3-4 s are observed in the microtremor spectra. The 0.7 s peak is interpreted in terms of microtremors and the second one in terms of ocean-generated microseisms. The former is present at all sites on Pliocene and Quaternary sedimens while the 3-4 s peak is present at all sites regardless of the surface geology, although with differences in the spectral amplitude. In order to investigate the possible influence of geologic parameters on long period motion, the maximum spectral amplitude value in the range 1 to 5 s was estimated at all sites in Granada basin. A good linear relation between the maximum spectral amplitude value and surface geology and depth to underground basement was found. However, for short period range (T<1 s), only the surface geology has any relation with the maximum spectral amplitude value. For Granada city, differences in the microtremor spectra were found to depend on the local geology. In the downtown area, situated on alluvial clay and sand from the Quaternary age, differences of up to 10 times with respect to the hill zones (located on compact conglomerates of Pliocene age) are observed. Results from coda-Q analysis from local earthquakes in the Granada basin show overestimations of Qc, for lapse times of 70 s, of up to 4 times at stations situated on sediment when compared to those obtained on bedrock for the 1.5 Hz frequency and to a lesser extent for 3 Hz. This overestimation is explained in terms of seismic-wave amplification in the Granada basin.

Book reviews

Summary. The paper presents a method of investigating seismic surface waves in (two-dimensional) layered media involving lateral heterogeneities. The method combines forward modelling by computation of synthetic seismograms for the disturbed layering and subsequent inversion of the seismogram spectra with respect to the wave modes. In terms of ‘L region’ and ‘R region', denoting the laterally homogeneous regions of the waves before and after propagation across the disturbed region, the method is described as follows. (1) Analytical definition of the wave in the L region. A time-dependent wavelet is assumed at some surface point, and the spatial displacement field corresponding to a free plane surface wave is computed from the eigen-functions of the waveguide. (2) Numerical propagation of the wave across the laterally heterogeneous region. The general heterogeneous wave equation is approximated by a finite difference time marching scheme, where the analytical wave motion enters as initial value. (3) Inversion of seismograms ‘observed’ at a number of locations in the R region. The seismogram Fourier spectra are modelled by a linear combination of modes, contaminated by random noise. Least-squares fitting between the modelled and the observed seismogram spectra yields estimates of the ‘mode participation factors'. The method is applied to Love waves of an elastic upper crustal model composed of a layer (shear velocity β=2.5 km s-1) over a homogeneous substratum (β=3.5 km s-1). Embedded in the surface layer, of 10 km thickness, is an idealized rectangular shaped sediment basin with lateral and vertical dimensions of 15 and 5 km, and a velocity of 0.6 or 0.4 of that of the layer. The initial pulse in the form of a Ricker wavelet is defined to represent a pure fundamental mode, the wavelengths of which vary between 60 and 6 km in the frequency interval 0.05-0.42 Hz ('analysis interval'), the wavelength of maximum spectral amplitude being 15 km. Observations at a distance of 60 km beyond the sediment basin yield the following results. (1) Propagation across the basin gives rise to significant scattering of the fundamental mode energy into the higher modes. The fraction of the Love wave energy transmitted is about 50 per cent for both models (slightly higher for the model of lower velocity contrast). The transmitted Love wave energy density exhibits different behaviour between the two models. For the model of lower velocity contrast, there is an increase with frequency from about 30 to 80 per cent; for the model of higher velocity contrast, there are variations between corresponding values, without a significant frequency trend. (2) Owing to the multimodal character of the Love wave transmitted, the phase velocity estimated from the horizontal phase gradient, differs from the fundamental mode velocity by amounts of up to about 10 per cent. Moreover, phase velocity as well as the seismogram spectral amplitudes are dependent upon the horizontal coordinate. This means that it is generally inadequate to interpret phase velocity (from phase differences) or attenuation (from amplitude ratios) in corresponding subsurface structures in terms of one single mode.

Deep ocean current and its correlation with the ocean tide off the coast of northern California

Tidal heights and currents were measured at a depth of 3903 meters off the coast of northern California for 46 days in 1966. From spectral analysis of the data, the amplitudes of various harmonics of tidal heights and current speeds and their corresponding phase differences were estimated. Good correlation between water current amplitudes and tidal heights was observed. Significant spectral peaks were found for both currents and tides corresponding to the O1, K1, N2, and M2 components. It is concluded that the measured ocean-bottom currents at this location are produced primarily by tides. The maximum spectral amplitude, 2.4 cm/sec, was obtained for the M2 component of current. The M2 component of tidal height was found to lead the M2 component of current by 45.4 degrees. Assuming two opposed traveling waves for ocean tides, theoretical amplitude relations and phase differences between tidal heights and current speeds were derived. The estimated total dissipation of energy for all the components was found to be 7.31 × 10−2 erg cm−2 sec−1.