Low-frequency Sources Research Articles

Ga2O3-based p-i-n solar blind deep ultraviolet photodetectors

This study applied a vapor cooling condensation system for depositing p-ZnO:LiNO3, i-Ga2O3, and n-Ga2O3:Si layers on sapphire substrates to fabricate solar-blind p-i-n deep-ultraviolet photodetectors (DUV-PDs). The deposited i-Ga2O3 absorption layer was measured to exhibit high performance levels. Therefore, when the resulting solar-blind p-i-n DUV-PDs were operated at a reverse bias voltage of − 5 V, the dark current and the DUV (250 nm)/visible (500 nm) rejection ratio were 1.45 pA and 2.5 × 104, respectively. Moreover, when the solar-blind p-i-n DUV-PDs were operated at a reverse bias voltage of − 5 V, the photoresponsivity, noise power density, noise equivalent power, and specific detectivity were 0.73 A/W, 2.45 × 10−26 A2/Hz, 4.80 × 10−13 W, and 1.97 × 1012 cmHz0.5W−1, respectively. Furthermore, flicker noise was the dominant low-frequency noise source in the photodetectors. These results demonstrated the effectiveness of the proposed solar-blind p-i-n DUV-PDs.

Passive Source Depth Discrimination in Deep-Water

This paper addresses the problem of passive source depth discrimination in ocean acoustics using a horizontal line array (HLA). The scope is restricted to low-frequency sources (frequency $f Hz), broadband signals (bandwidth $B$ of a few Hz), deep-water environment (water depth $D>\text{1000}$ m), and distant sources (range $r$ greater than several kilometers) at the endfire position. In this context, the environment acts as a dispersive waveguide, and one should not use classical source localization methods based on plane waves or any other simplistic wave model. Instead, a method based on the modal behavior driving the propagation is proposed. It notably uses the concept of the waveguide invariant, a scalar that summarizes the waveguide dispersion. In deep water, the waveguide invariant largely depends on source depth, and thus is an interesting input for source depth discrimination. An algorithm is proposed to compute energy ratio in groups of modal interferences. The input data for the algorithm is a range-frequency intensity, as measured on a HLA. The modal interference groups are defined based on their respective waveguide invariant values which in turns depend on source depth. This idea is formalized to propose a source depth discrimination method, which is performed as a binary classification problem. As long as the sound speed profile features a surface thermocline, the algorithm does not require detailed knowledge about the environment and it allows the classification of sources under two hypotheses, above or under a user-chosen threshold depth.

Multi-branch sandwich piezoelectric energy harvester: mathematical modeling and validation

Energy harvesting from ambient environment has attracted intensive attention over the years for powering low-power autonomous electronic devices. A conventional piezoelectric energy harvester (CPEH) can hardly meet the requirements of effective energy harvesting from wideband, low frequency and low amplitude vibration sources due to its simple substrate and single resonant peak response. Using sandwich structure comprised of a soft-core layer and two thin skins can drastically reduce the natural frequency and increase voltage output of the harvester. The stiffness of sandwich harvester and consequently its resonant frequency can be tuned more flexibly than the CPEH by choosing different materials and adjusting the geometric dimensions of the core and skins. In this paper, a novel multi-branch sandwich piezoelectric energy harvester (MSPEH) is proposed to harvest energy from wideband, low frequency and low amplitude vibration sources. The proposed harvester comprises of a main sandwich beam as substrate with a patch of piezoelectric layer bonded over it. Multiple inner single branches with tip masses are connected to the main substrate to generate multiple resonant peaks and tune the range of the interested frequency. Firstly, a mathematical model of the proposed harvester is presented, and the electromechanical coupling equations are obtained. Subsequently, a prototype of MSPEH with two inner single branches is fabricated and tested under harmonic base excitation to study its performance. When tested at a low harmonic excitation of 0.02 g, the harvester generates 2.48 V, 6.21 V and 1.55 V at three resonant frequencies of 18.18 Hz, 24.74 Hz and 28.12 Hz, respectively. Finally, the accuracy of derived mathematical model is verified by comparing with the predictions from finite element simulation and experimental results. The novel MSPEH offers excellent design flexibility to adjust resonant frequencies by choosing inner single branches and tip masses and thus it has potential to generate sufficient power output from wideband, low frequency and low amplitude vibration sources.

Comparison of high- and low-frequency signal sources for very-long-period seismic events at Asama volcano, Japan

Very-long-period (VLP) seismic events at Asama volcano in central Japan are characterized by a transient signal of 10–20 s duration. Associated with the transient motion, a high-frequency (HF) oscillation within the 5–10 Hz band is observed. We investigated the location and size of the oscillation source in the HF band (HF source) using an amplitude source location method and compared our results with those for the VLP band (VLP source) deduced in our previous waveform inversion study. We analysed 1437 VLP events recorded during an intense observation campaign during 2008–2009, and additional 571 events surrounding an eruptive activity in 2015 (including a VLP event that immediately preceded an eruption). The HF source locations of most events were deeper than the VLP source locations by ∼150 m, although there was almost no time lag between the signals. This suggests a strong connection between the two sources, with one source immediately responding to the other. The eruptive VLP event had an HF source location close to normal VLP events but had a greater event size. We surmise that the HF signal is caused by an inflow of volcanic gas from depth into a semi-vertical crack-like cavity (as imaged in our previous waveform inversion study), and the VLP signal is caused by resultant inflation of the same cavity. The centroid of inflation (VLP source) is near the roof of the cavity, where gas accumulates, whereas the HF oscillation is emitted more intensely from lower and narrower portions of the cavity. This results in source depth differences between the two signal bands. In this model, the inflation rate of the cavity is controlled by the volume flux of the gas inflow, producing similar temporal variations between the VLP and HF signals. According to this model, the eruptive VLP is associated with a greater amount of gas inflow and accumulation, which likely played a key role in the eruption.

Travel-time correction and preliminary results for ocean acoustic tomography in South China Sea

An acoustic tomography trial experiment was conducted in South China Sea during May to August in 2016. Two moorings are installed apart from about 56.94 km, while each consists of one low frequency source, 20 hydrophones deployed from the depth of about 400 m to 1600 m, total 32 depth sensors and 3 compass and tilt sensors. Due to internal waves and currents in this area, as a typical value, horizontal drift of a mooring can reach 300 m, thus moorings drift need to be considered to correct ray travel-time. In this paper, the shape of a mooring is estimated firstly and locations of all hydrophone array elements are then calculated and finally used to determine travel-time perturbation of acoustic arrivals. The mooring is modelled as 2 curves, while the end of the mooring is fixed at the cement anchor on the sea floor. Optimization is used to acquire hydrophone location inferential solution. The inferred shape of hydrophone array and element locations are used to correct the travel-times measured in the experiment. We find that corrected travel-times match the trend of the change of sound speed profile better in the sea. Finally, the corrected travel-times are used to tomography of sound speed profile. AR (Autoregressive) process is used to describe the dynamic evolution of sound speed profile and Kalman filter is applied in the sequential estimation. The performances of the time-independent method and the method using AR process and Kalman filter are compared, reasonably the latter is better than the former in particular with abundant measured data.

Влияние ионосферы и неоднородной структуры Земли на поляризационные характеристики магнитного поля в диапазоне частот 0.2-200 Hz в ближней зоне горизонтальной заземленной антенны

AbstractPolarization characteristics of the field of an on-Earth emitter located at the Kola Peninsula are experimentally measured at a distance that is no greater than the height of an effective ionospheric waveguide in the FENICS-2014 experiment. Variations in the field amplitude and orientation of the major axis of polarization ellipse are observed at lower frequencies upon significant changes of the K index of geomagnetic activity. Polarization characteristics of the horizontal component of magnetic field calculated with allowance for the ionosphere and two-layer Earth structure prove the observed sensitivity of the ultralow- and lower-frequency filed in the near-field zone to the state of ionosphere at lower conductivity of underlying medium. Theoretical results are compared with the experimental data. The results are important for deep sounding of the Earth and monitoring of ionosphere with the aid of controlled low-frequency ground sources.

Computational Low-Frequency Electromagnetic Dosimetry Based on Magnetic Field Measurements

This paper compares different experimental-computational strategies for the estimation of electric fields induced in human bodies by low-frequency magnetic sources characterized by a set of magnetic field measurements. The analysis is carried out by considering three alternative procedures, which use, as the first input, the distribution of the magnetic flux density in a volume containing the studied body or on a surface surrounding the sources. The comparison is performed on a realistic model problem, related to transcranial magnetic stimulation (TMS), in which numerically simulated “virtual measurements” are employed. The comparative analysis is developed in terms of both result accuracy and robustness against noisy input due to unavoidable experimental uncertainties. It results that by performing the measurements on a surface surrounding the sources, a significant reduction of the experimental burden is found with respect to the case of volume measurements, affecting neither the accuracy nor the robustness of the procedure. In particular, when whole body electric field evaluation must be carried out, the advantage of surface measurements with respect to volume ones becomes significant. Moreover, a preferable scheme obtained as hybridization of previously proposed strategies is identified. Besides the adoption of a TMS model problem in the comparison procedure, the achieved result can be extended to any low-frequency dosimetric assessments, where the magnetic sources are difficult to model or not completely known.

Coseismic Radiation of the 2008 Mw 7.9 Wenchuan Earthquake and Its Relationship to Fault Complexities

The long rupture length and complex fault geometries of the 2008 Mw 7.9 Wenchuan earthquake offer a great opportunity to investigate the relationship between coseismic radiation and fault complexities. In this study, we first improved the compressive-sensing back-projection method (CS-BP) with window time adjustment and parallel computation to enhance its spatial–temporal resolution and computational efficiency. Then we applied the CS-BP to invert for the coseismic radiation of the Wenchuan earthquake in multiple frequency bands. Our results show that the coseismic radiation of the Wenchuan earthquake is frequency-dependent and strongly associated with the fault geometries: low-frequency radiation sources are concentrated on smooth fault segments with large coseismic slip, while higher-frequency sources are mainly related to fault complexities (e.g., Xiaoyudong fault, Gaochuan fault-bend, Leigu fault-bend, and Nanba fault step-over). An acceleration of rupture speed from ~ 1.5 to ~ 3.0 km/s is also observed the near Xiaoyudong fault, where strong high-frequency energies were radiated. Finally, we discuss the role of rupture speed in mediating the relationship between coseismic radiation and fault complexities. On one hand, sudden changes of rupture speeds contribute to strong high-frequency radiation. On the other hand, fault complexities can also strongly affect the rupture speed: a sudden break of a large asperity is able to propel the acceleration, while rupture front may decelerate when it jumps across geometric barriers.

Optimizing Pulsar Timing Array Observational Cadences for Sensitivity to Low-frequency Gravitational-wave Sources

Abstract Observations of low-frequency gravitational waves (GWs) will require the highest possible timing precision from an array of the most spin-stable pulsars. We can improve the sensitivity of a pulsar timing array (PTA) to different GW sources by observing pulsars with low timing noise over years to decades and distributed across the sky. We discuss observing strategies for a PTA focused on a stochastic GW background such as from unresolved supermassive black hole binaries as well as focused on single continuous-wave sources. First, we describe the method to calculate a PTA’s sensitivity to different GW-source classes. We then apply our method to the 45 pulsars presented in the North American Nanohertz Observatory for the GW 11 year data set. For expected amplitudes of the stochastic background, we find that all pulsars contribute significantly over the timescale of decades; the exception is for very pessimistic values of the stochastic-background amplitude. For individual single sources, we find that a number of pulsars contribute to the sensitivity of a given source, but that which pulsars contribute is different depending on the source, or versus an all-sky metric. Our results seem robust to the presence of red noise in pulsar arrival times. It is critical to obtain more robust pulsar-noise parameters as they heavily affect our results. Our results show that it is also imperative to locate and time as many high-precision pulsars as possible, as quickly as possible, to maximize the sensitivity of next-generation PTA detectors.

Interplanetary scintillation studies with the Murchison Wide-field Array – IV. The hosts of sub-arcsecond compact sources at low radio frequencies

Around 10% of bright low-frequency radio sources observed with the Murchison Widefield Array (MWA) show strong Interplanetary Scintillation (IPS) on timescales of a few seconds, implying that almost all their low-frequency radio emission comes from a compact component less than 0.5 arcsec in angular size. Most of these objects are compact steep-spectrum (CSS) or MHz-peaked spectrum (MPS) radio sources. We have used mid-infrared data from the Wide-field Infrared Survey Explorer (WISE) catalogue to search for the host galaxies of 65 strongly-scintillating MWA sources and compare their properties with those of the overall population of bright low-frequency radio sources. We identified WISE mid-infrared counterparts for 91% of the bright sources in a single 900 square degree MWA field, and found that the hosts of the strongly-scintillating sources were typically at least 1 mag fainter in the WISE W1 (3.4 micron) band than the hosts of weakly-scintillating MWA sources of similar radio flux density. This difference arises mainly because the strongly-scintillating sources are more distant. We estimate that strongly-scintillating MWA sources have a median redshift of z ~ 1.5, and that at least 30% of them are likely to lie at z > 2. The recently-developed wide-field IPS technique therefore has the potential to provide a powerful new tool for identifying high-redshift radio galaxies without the need for radio spectral-index selection.

Depth Discrimination for Low-Frequency Sources Using a Horizontal Line Array of Acoustic Vector Sensors Based on Mode Extraction.

Depth discrimination is a key procedure in acoustic detection or target classification for low-frequency underwater sources. Conventional depth-discrimination methods use a vertical line array, which has disadvantage of poor mobility due to the size of the sensor array. In this paper, we propose a depth-discrimination method for low-frequency sources using a horizontal line array (HLA) of acoustic vector sensors based on mode extraction. First, we establish linear equations related to the modal amplitudes based on modal beamforming in the vector mode space. Second, we solve the linear equations by introducing the total least square algorithm and estimate modal amplitudes. Third, we select the power percentage of the low-order modes as the decision metric and construct testing hypotheses based on the modal amplitude estimation. Compared with a scalar sensor, a vector sensor improves the depth discrimination, because the mode weights are more appropriate for doing so. The presented linear equations and the solution algorithm allow the method to maintain good performance even using a relatively short HLA. The constructed testing hypotheses are highly robust against mismatched environments. Note that the method is not appropriate for the winter typical sound speed waveguide, because the characteristics of the modes differ from those in downward-refracting sound speed waveguide. Robustness analysis and simulation results validate the effectiveness of the proposed method.

The low-frequency source of Saturn’s kilometric radiation

Understanding how auroral radio emissions are produced by magnetized bodies requires in situ measurements within their source region. Saturn's kilometric radiation (SKR) has been widely used as a remote proxy of Saturn's magnetosphere. We present wave and plasma measurements from the Cassini spacecraft during its ring-grazing high-inclination orbits, which passed three times through the high-altitude SKR emission region. Northern dawn-side, narrow-banded radio sources were encountered at frequencies of 10 to 20 kilohertz, within regions of upward currents mapping to the ultraviolet auroral oval. The kilometric waves were produced on the extraordinary mode by the cyclotron maser instability from 6- to 12-kilo-electron volt electron beams and radiated quasi-perpendicularly to the auroral magnetic field lines. The SKR low-frequency sources appear to be strongly controlled by time-variable magnetospheric electron densities.

Characterization of bottom parameters in the New England Mud Patch using geophone and hydrophone measurements

Measurements of acoustic pressure and particle velocity were made during the Seabed Characterization Experiment (SCEx) in the New England Mud Patch south of Cape Cod in about 70 meters of water. The University of Rhode Island and Woods Hole Oceanographic Institution deployed the “geosled” with a four-element geophone array, a tetrahedral array of four hydrophones, and several hydrophone receive units (SHRUs). In addition, a new low frequency source, interface Wave Sediment Profiler (iWaSP) was deployed to excite interface waves (Scholte waves). The iWaSP system consists of a source to generate the interface wave and a four-element accelerometer receive array. Modal arrivals from broadband sources (SUS and CSS) on geophones and hydrophones will be presented and compared. The individual arrivals on the sensors will be identified though seismo-acoustic modeling. Based on the propagation paths of the various waves (compressional, shear and Scholte waves), geoacoustic parameters will be estimated. Seismic data collected at the location will be used to constrain the model parameters in the inversion. Sediment data from cores, other in-situ measurements and inversions using other types of data will be used to compare and validate our inversions. [Work supported by Office of Naval Research.]

Reliable detection and characterization of low-frequency polarized sources in the LOFAR M51 field

Context.The new generation of broad-band radio continuum surveys will provide large data sets with polarization information. New algorithms need to be developed to extract reliable catalogs of linearly polarized sources that can be used to characterize those sources and produce a dense rotation measure (RM) grid to probe magneto-ionized structures along the line of sight via Faraday rotation.Aims.The aim of the paper is to develop a computationally efficient and rigorously defined source-finding algorithm for linearly polarized sources.Methods.We used a calibrated data set from the LOw Frequency ARray (LOFAR) at 150 MHz centered on the nearby galaxy M 51 to search for polarized background sources. With a new imaging software, we re-imaged the field at a resolution of 18″ × 15″ and cataloged a total of about 3000 continuum sources within 2.5° of the center of M 51. We made small StokesQandUimages centered on each source brighter than 100 mJy in total intensity (201 sources) and used RM synthesis to create corresponding Faraday cubes that were analyzed individually. For each source, the noise distribution function was determined from a subset of the measurements at high Faraday depths where no polarization is expected; the peaks in polarized intensity in the Faraday spectrum were identified and thep-value of each source was calculated. Finally, the false discovery rate method was applied to the list ofp-values to produce a list of polarized sources and quantify the reliability of the detections. We also analyzed sources fainter than 100 mJy but that were reported as polarized in the literature at at least another radio frequency.Results.Of the 201 sources that were searched for polarization, six polarized sources were detected confidently (with a false discovery rate of 5%). This corresponds to a number density of one polarized source per 3.3 square degrees, or 0.3 source per square degree. Increasing the false discovery rate to 50% yields 19 sources. A majority of the sources have a morphology that is indicative of them being double-lobed radio galaxies, and the ones with literature redshift measurements have 0.5 < z < 1.0.Conclusions.We find that this method is effective in identifying polarized sources, and is well suited for LOFAR observations. In the future, we intend to develop it further and apply it to larger data sets such as the LOFAR Two-meter Survey of the whole northern sky, LOTSS, and the ongoing deep LOFAR observations of the GOODS-North field.

Underwater bubble low frequency source for Arctic acoustic tomography, navigation, and communication

Arctic acoustic tomography requires a broadband source in the frequency range of 5–100 Hz. To meet this requirement, Teledyne Marine suggests applying a seismic source, recently developed for the Marine Vibrator Joint Industry Project. The source is considered to be less harmful for marine inhabitants and gives more precise and higher resolution imaging of the subsurface formations and structures. The coherent source is based on the application of an underwater, gas filled bubble resonator covered by an elastic membrane. The membrane supports high volume displacement. The sources are not sensitive to cavitation and to coupling effects. The fluid dynamics and acoustics of a spherical resonator are defined by the Rayleigh-Plesset equation. The buoyancy deforms the shape of a real bubble from spherical. The 3D simulation and experiments have shown that a cylindrical form is a practical engineering solution. It performances similar to a spherical bubble, keeps its shape and can be towed with a high speed. The Q-factor of a practical bubble resonator is ~10. Two systems are considered to cover a wide frequency band: a tunable resonator and a broadband multi-resonance system. The research proved that the bubble sound source is a practical solution for a frequency below 100 Hz.

The feasibility of a multipurpose acoustic network in Baffin Bay

Operating autonomous underwater vehicles at high latitudes is a challenge because ice cover prevents the use of GPS or data communications. As a result, our scientific observations are biased towards late spring, summer, and early autumn when ships can navigate and autonomous platforms can safely surface. To address this problem, we studied the feasibility of a basin-scale multipurpose acoustic network called the “Baffin Bay Acoustic Navigation and Communication System” (BBANC). BBANC would deploy broadband low frequency sources and receivers, offering one-way communication, acoustic positioning, and acoustic thermometry services. Passive acoustic listening elements would support the study of marine mammal communication and ambient noise from ships, ocean-based resource exploitation, and ice dynamics, as well as gate acoustic source operation in the presence of marine mammals. We describe the challenges and design parameters for such a system, as well as define additional acoustic and remote sensing measurements required to complete a system design. Drawing from a large database of Baffin Bay hydrography, we present simulations of under-ice sound speed conditions, ice properties derived from satellite remote sensing and upward looking sonar data, and modelled acoustic propagation paths in an ice-covered Baffin Bay. We also assess the feasibility of non-coherent and coherent communication.

Passive source depth discrimination in deep-water

In this work, we address the problem of passive source depth discrimination using a horizontal line array (HLA). The scope is restricted to low-frequency sources (f<500 Hz), broadband signals (a few Hertz bandwidth), deep-water environment (D>1000 m), and distant sources (at least several kilometers). The proposed method is based on the underlying physics driving the propagation. It notably uses the concept of mode trapping and waveguide invariant. In deep water, the waveguide invariant largely depends on source depth, and is thus an interesting input for source depth discrimination. An algorithm is proposed to compute energy ratio in modal groups. The input data for the algorithm are a range-frequency intensity, as measured on a HLA. The modal groups are thus defined based on their respective waveguide invariant values, which is a main difference with existing depth discrimination methods adapted to shallow water context. This idea is explored, and extended to propose a source depth discrimination which is performed as a binary classification problem. As long as the sound speed profile features a surface thermocline, the algorithm does not require detailed knowledge about the environment and it allows one to classify sources under two hypotheses, above or under a threshold depth.

Experimental study and analytical modelling of the effect of the driving frequencies on dual frequency capacitively coupled plasmas

A capacitively coupled radio frequency discharge driven by two harmonics substantially different from each other allows some degrees of independent control of the ion energy and ion flux. The low frequency (ωLF) source controls the ion energy, while the ion flux is controlled by the high frequency (ωHF) source. The choices of the driving frequencies can influence the properties of the dual frequency capacitively coupled plasmas (2f CCP). Here, we study the effect of the driving frequencies on the properties of the 2f CCPs, i.e., on the generation of a DC self-bias, the excitation of the non-linear Plasma Series Resonance (PSR) effect as well as the independent control over the mean ion energy and the flux in three different 2f CCP experiments: namely (2.26 + 13.56) MHz, (2.26 + 27.12) MHz, and (13.56 + 27.12) MHz. We also use a non-liner global model that consists of a description of the plasma bulk based on a fluid dynamic approach coupled to a separate model of the sheath. We use argon and cover a wide range of operating conditions. We find the choices of different ωLF and ωHF's result in substantial changes of the decoupling of the mean ion energy from the ion flux. We also observe that increasing ωHF decreases the DC self-bias at a fixed value of ωLF. The PSR effect and, therefore, the electron power deposition strongly depend on the high frequency source. The experimental results are consistent with the predictions of the non-linear global model which proves the usefulness of such a rather simplistic model to study of 2f CCP discharges.

Analysis of array gain degradation of near-field passive synthetic aperture method

A previous study established a method for localizing low-frequency acoustic sources in the near field using a passive synthetic aperture method [Lei et al., 2015]. Sound fields sampled at different positions (sub-apertures) and times are synthesized to improve the spatial resolution of conventional beamforming. However, in practice, the position errors of the sub-apertures could significantly degrade the performance of the method. In this work, the analysis is extended to measure the sensitivity of the method to the position errors of the sub-apertures using array gain degradation (AGD). First, the position errors in three directions are assumed to be independent and obey the same Gaussian distribution to present a simple expression for AGD. Then, the analysis is generalized to the case of different Gaussian distributions to obtain an expression with explicit physical meaning. The two expressions are verified by simulations and partially validated using the experimental data sampled by a planar array in a semi-anechoic chamber. Finally, the results of localizing the sources on a transformer in a 750 kV substation are revealed.

An Enhanced Sharpness on Acoustic Focus Via Artificial Iterative Phase Conjugation Processing

For a low-frequency source in a homogenous medium, when the source is focused by phase conjugation (also referred to as the time reversal in the time domain), the focal spot is large because of the half-wavelength limit. In this paper, to reduce the focal spot size, artificial iterative phase-conjugated processing is proposed based on passive phase conjugation to focus a point-like source. As AIPCP operates in iteration mode using a computer, the iteration loops of each transducer are calculated independently, and then, AIPCP is achieved by summing the iteration outputs of all transducers together. Numerical simulations and experiments on an audible sound field are employed to illustrate that AIPCP generates a narrower focal spot than passive phase conjugation. Further analyses considered the theoretical change in focal spot size with the iteration number using Fraunhofer far-field approximation, and the result shows that the focal spot size decreases with increase in the iteration number. This finding is predicted in the near field and validated in the far field by simulations and experiments. Moreover, the half-wavelength limit is overcome at a distance from the sound source equal to the wavelength.