Coherent Scatter Research Articles

Excitation threshold and gyroharmonic suppression of artificialEregion field-aligned plasma density irregularities

[1] Ionospheric modification experiments have been carried out using the HAARP facility along with a 30 MHz coherent scatter radar imager in Alaska to examine properties of artificial E region field-aligned plasma density irregularities (FAIs). In one set of experiments, the RF emission power was varied gradually in order to determine the threshold electric field for irregularity generation. A threshold O mode peak electric field amplitude of 170–195 mV/m at an altitude of 99 km and a heating frequency of 2.7 MHz was identified based on the full-wave formalism of Thide and Lundborg (1986). In another, the pump frequency was varied gradually to investigate the suppression of the FAIs at frequencies near the second electron gyroharmonic frequency (2Ωe). Coherent echoes were found to be suppressed for pump frequencies in an asymmetric band 40–50 kHz wide around 2Ωe but only for irregularities driven marginally above threshold. Theoretical context for these results is provided.

Ground settlement monitoring based on temporarily coherent points between two SAR acquisitions

An InSAR analysis approach for identifying and extracting the temporarily coherent points (TCP) that exist between two SAR acquisitions and for determining motions of the TCP is presented for applications such as ground settlement monitoring. TCP are identified based on the spatial characteristics of the range and azimuth offsets of coherent radar scatterers. A method for coregistering TCP based on the offsets of TCP is given to reduce the coregistration errors at TCP. An improved phase unwrapping method based on the minimum cost flow (MCF) algorithm and local Delaunay triangulation is also proposed for sparse TCP data. The proposed algorithms are validated using a test site in Hong Kong. The test results show that the algorithms work satisfactorily for various ground features.

Three‐dimensional numerical simulation of equatorial F region plasma irregularities with bottomside shear flow

A three‐dimensional numerical simulation of plasma density irregularities in the postsunset equatorial F region ionosphere leading to equatorial spread F (ESF) is described. The simulation advances the plasma number density and electrostatic potential forward in time by enforcing the constraints of quasi‐neutrality and momentum conservation. The magnetic field lines are not modeled as equipotentials. Simulations are performed for cases with no background winds, with no background electric field or gravity, and with winds, a background electric field, and gravity all working in concert. The first run produced generalized Rayleigh Taylor (GRT) instability, and the second produced collisional shear instability (CSI). The combined run produced an instability which developed into an intense ESF event more quickly and with more realistic characteristics than the other two. Simulation results are compared with incoherent and coherent scatter radar data from the magnetic equator. A number of signature ESF characteristics are shown to be reproduced by the simulation.

Coordinated sporadic E layer observations made with Chung-Li 30 MHz radar, ionosonde and FORMOSAT-3/COSMIC satellites

A nighttime sporadic E (Es) layer responsible for range spread Es trace in ionogram and quasi-periodic echoes scattered from 5-m field-aligned electron density irregularities (FAIs) is investigated by using a ground-based 30 MHz coherent scatter radar, ionosonde and FORMOSAT-3/COSMIC satellite. A comparison shows a positive correlation between the 30 MHz radar backscatter and critical frequency foEs of the Es trace, which are consistent with earlier observational results. Interferometry measurement indicates that the spatial patterns of the radar echoes from the FAIs were in patchy form with vertical extent of about a few kilometers and horizontal dimensions of about 10–20 km. The echo structures experienced quasi-periodic oscillations not only in vertical but also in horizontal directions with periods of approximately 5–6 min. Observed Doppler velocities of the FAIs also exhibited quasi-periodic oscillations with the same periods. From their downward phase movement, these oscillations were very likely attributed to the modulations of upward propagating gravity waves with vertical wavelengths of about 10–20 km. In addition, vertical shears of the FAI Doppler velocity were observed, which are believed to result from the change in the phase of gravity wave with height. A physical process is proposed in this article to account for the relation between gravity wave-perturbed neutral wind and height variation of the FAI Doppler velocity. It is believed that the oscillations in the Doppler velocity profiles are associated with the wave-induced polarization electric fields in a finite plasma structure in the azimuth direction.

Low-frequency sound scattering from microbubble distributions.

Backscatter from the sea surface is governed by the roughness of the surface and subsurface micro bubble distributions. At low frequencies, due to the paucity of large bubbles, scattering results primarily from coherent and/or collective scatter from bubbles entrained by the subsurface vorticity or carried to depth by the Langmuir circulation and thermal convection. Theoretical treatments using the work of Mallock and Wood show that scattering from compact regions is a function of the volume fraction of air and to first order can be described by a Minnaert formula modified with the volume fraction. Experiments to test this theory were conducted in a lake and large tank facility. Back scattering from a submerged bubble cloud produced results consistent with our theory when parameters independently measured, such as bubble size distribution, volume fraction, and the cloud size, were used to calculate the resonant frequency with the modified Minnaert formula. This theory was further tested by backscattering experiments on bubbly liquid filled tubes, bubbly gel filled cylinders, and voided polyurethane spheres. Again the resonant frequency and backscatter measurements were found consistent with theoretical expectations; however, the effective dampening was found to depend on the nature of the material containing the microbubbles.

On the Temporal Characteristics of Radar Coherent Structures in Snow and Rain

Abstract Previous work showed that the magnitudes of the radar-backscattered amplitudes have statistically significant periodic components of frequencies ( f ) in excess of those arising from the Doppler velocity fluctuations of incoherent scatter. Analyses in both rain and snow in the earlier work revealed what is interpreted as pervasive coherent scatter. This coherency is thought to come from precipitation structures acting like gratings in resonance with the radar wavelength that, when they move with a velocity component transverse to the beam, induce the observed f. The purpose of this article is to characterize briefly the temporal structure of f and, thereby indirectly, the temporal character of the structures producing the radar coherent backscatter. It is found that these structures last considerably longer than the decorrelation times of a few to 10 milliseconds, characteristic of Doppler velocity fluctuations associated with incoherent scatter. For the data analyzed, though, most last no more than a significant fraction of 1 s. Hence, for the observed transverse velocity of 2 ms−1, the dimensions of the gratings producing the radar coherent backscatter are only on the order of tens of centimeters to a few meters. Therefore, the typically large sampling volumes of most radars will contain many of these grids at any given time. Consequently, during 1 s of observations, one can envision the coherent scatter as coming from many individual grids twinkling on and off, much like the transient spectral reflections off ice crystals falling in sunlight.

Direct Observations of Coherent Backscatter of Radar Waves in Precipitation

Abstract In previous work, it was argued that a source of radar coherent scatter occurs in the direction perpendicular to the direction of wave propagation because of the presence of grids of enhanced particle concentrations with spatial periodicities in resonance with the radar wavelength. While convincing, the evidence thus far has been indirect. In this work the authors now present direct observations of radar coherent backscattered signals in precipitation in the direction of wave propagation. The theory is developed for the cross-correlation function of the complex amplitudes in the direction of propagation calculated for nearest neighbor range bins. Data are analyzed in snow and in rain. The results agree with the earlier conclusions in the previous work, namely that coherent scatter occurs in both rain and snow, that it is larger in snow than it is in rain, and that it can be significant at times.

Quantitative x-ray dark-field computed tomography

The basic principles of x-ray image formation in radiology have remained essentially unchanged since Röntgen first discovered x-rays over a hundred years ago. The conventional approach relies on x-ray attenuation as the sole source of contrast and draws exclusively on ray or geometrical optics to describe and interpret image formation. Phase-contrast or coherent scatter imaging techniques, which can be understood using wave optics rather than ray optics, offer ways to augment or complement the conventional approach by incorporating the wave-optical interaction of x-rays with the specimen. With a recently developed approach based on x-ray optical gratings, advanced phase-contrast and dark-field scatter imaging modalities are now in reach for routine medical imaging and non-destructive testing applications. To quantitatively assess the new potential of particularly the grating-based dark-field imaging modality, we here introduce a mathematical formalism together with a material-dependent parameter, the so-called linear diffusion coefficient and show that this description can yield quantitative dark-field computed tomography (QDFCT) images of experimental test phantoms.

Electrical Modulation of Coherent Scattering in Dual Frequency Liquid Crystal Tissue Models

Molecular orientation clusters are created in a cell of non aligned liquid crystal to study the light propagation in electrically controllable anisotropic non uniform media. Coherent scattering phenomenon and predominant role of large orientation clusters in such scattering are demonstrated. Fast relaxation of molecular orientation and modulation of light scattering in those clusters is obtained by using “dual frequency” liquid crystal mixtures and frequency modulation of the drive signal. The obtained results show the good potential for the creation of electrically tunable tissue models by using liquid crystalline materials.

On the Enhanced Temporal Coherency of Radar Observations in Precipitation

Abstract In this work, the authors present observations of enhanced temporal coherency beyond that expected using the observations of the standard deviation of the Doppler velocities and the assumption of a family of exponentially decaying autocorrelation functions. The purpose of this paper is to interpret these observations by developing the complex amplitude autocorrelation function when both incoherent and coherent backscatter are present. Using this expression, it is then shown that when coherent scatter is present, the temporal coherency increases as observed. Data are analyzed in snow and in rain. The results agree with the theoretical expectations, and the authors interpret this agreement as an indication that coherent scatter is the likely explanation for the observed enhanced temporal coherency. This finding does not affect decorrelation times measured using time series. However, when the time series is not available (as in theoretical studies), the times to decorrelation are often computed based upon the assumptions that the autocorrelation function is a member of the family of exponentially decaying autocorrelation functions and that the signal decorrelation is due solely to the Doppler velocity fluctuations associated with incoherent scatter. Such an approach, at times, may significantly underestimate the true required times to decorrelation thus leading to overestimates of statistical reliability of parameters.

Poster — Thur Eve — 21: Monte Carlo Simulation of X‐Ray Scatter in Diagnostic Range with EGSnrc and GEANT4

In diagnostic x‐ray imaging, the attenuation of matter to x‐rays is substantially due to Compton and Rayleigh scattering in low Z materials. Consequently, the contrast of the x‐ray images is degraded by scattered photons. Because the total coherent scatter cross‐section is only about 10% of total Compton scatter cross‐section, Compton scatter plays a crucial role in x‐ray image quality. The x‐ray scatter effect can be corrected with experimentally measured or simulated data to improve image quality. It is necessary to acquire accurate x‐ray scatter information. Monte Carlo simulation is an important way of acquiring the x‐ray scatter information and the only practical way to analyze the Compton and Rayleigh scatter separately. However, due to the different ways of modeling the physics and various approximations in different Monte Carlo simulation packages, inconsistent simulation results have been observed. Using the Monte Carlo simulation packages of EGSnrc and GEANT4 we investigated the effects of the electron‐binding effects on the simulated x‐ray scattering in the diagnostic range. It is found that the Compton scatter and Rayleigh scatter are significantly different when simulated with different simulation packages and different physics models, e.g. with or without electron binding effect simulated. These differences are probably due to the ways the scattering process is modeled in the Monte Carlo simulation packages. Because the crucial role of simulated x‐ray scatter in scatter correcting in diagnostic imaging, improved modeling to the x‐ray scatter process in the Monte Carlo simulation package is desired.

Propagating Librations in Ice XI: Model Analysis and Coherent Inelastic Neutron Scattering Experiment

We report on propagating librational (rotational) modes intrinsic to ice XI. Contrary to the theoretical picture presumed so far, namely, that all librational modes are localized in each water molecule, our calculations identify large dispersions for some of the modes, which basically originate from intermolecular dipole–dipole interactions. On the basis of this finding, the known incoherent inelastic neutron scattering spectrum and infrared absorption spectrum are newly analyzed to give the interpretation that the spectra are mixtures of contributions of localized and propagating modes. Furthermore, we perform an experiment on coherent inelastic neutron scattering using the MAPS spectrometer in order to search ω– Q space more directly than in a previous work and find that the observation also strongly suggests the existence of large dispersions.

SU‐HH‐BRB‐03: Correction for Multiple and Coherent Scatter in First Generation Incoherent Scatter CT for In‐Vivo Breast Imaging

Purpose: We are currently working on a bench top system to investigate the benefits of reconstructing electron density (ED) images of the breast. Single incoherently scattered photons can be directly related to ED but multiple and coherent scatter cause additional scattering which lead to image artifacts. This work presents a correction for multiple and coherent scatter in first generation incoherent scatter CT of the breast. Method and Materials: Monte Carlo (MC) simulations were used to quantify the increase of scatter for breasts with different sizes and compositions. Scatter increase correction ratios (SICR) as a function of radiological path length were fitted to an empirical equation. The equation allows us to analytically calculate SICR using transmission CT. The accuracy of the correction was tested by reconstructing ED images of MC simulated scans for a homogeneous and an inhomogeneous breast. Results: Coherent scatter contributes ∼9% to the total scatter signal for a typical breast, demonstrating that multiple scatter is the major source of image artifacts. The fitted equation corrects for multiple and coherent scatter with errors of 11% ± 2% for the homogeneous breast and 21% ± 10% for the inhomogeneous breast. The errors in ED images are of − 9% ± 4% for the homogeneous breast and −4% ± 3% for the inhomogeneous breast. The corrected ED image is twice as accurate as the uncorrected image and twice as precise as the image based on the linear attenuation coefficient. Conclusion: The scatter increase due to multiple and coherent scatter is weakly dependent on breast size and composition. This allows an empirical equation to be used to correct multiple and coherent scatter artifacts. Our study shows the feasibility of reconstructing ED breast images using our approach and future efforts will be devoted to its practical implementation.

Partially Coherent Backscatter in Radar Observations of Precipitation

Abstract Classical radar theory only considers incoherent backscatter from precipitation. Can precipitation generate coherent scatter as well? Until now, the accepted answer has been no, because hydrometeors are distributed sparsely in space (relative to radar wavelength) so that the continuum assumption used to explain coherent scatter in clear air and clouds does not hold. In this work, a theory for a different mechanism is presented. The apparent existence of the proposed mechanism is then illustrated in both rain and snow. A new power spectrum Z( f ), the Fourier transform of the time series of the radar backscattered reflectivities, reveals statistically significant frequencies f of periodic components that cannot be ascribed to incoherent scatter. It is shown that removing those significant fs from Z( f ) at lower frequencies greatly reduces the temporal coherency. These lower frequencies, then, are associated with the increased temporal coherency. It is also shown that these fs are also directly linked to the Doppler spectral peaks through integer multiples of one-half the radar wavelength, characteristic of Bragg scatter. Thus, the enhanced temporal coherency is directly related to the presence of coherent scatter in agreement with theory. Moreover, the normalized backscattered power spectrum Z( f ) permits the estimation of the fractional coherent power contribution to the total power, even for an incoherent radar. Analyses of approximately 26 000 one-second Z( f ) in both rain and snow reveal that the coherent scatter is pervasive in these data. These findings present a challenge to the usual assumption that the scatter of radar waves from precipitation is always incoherent and to interpretations of backscattered power based on this assumption.

WE-B-201B-05: Monte Carlo Simulation of Coherent Scatter Computed Tomography (CSCT)

Purpose: To determine the design characteristics of a new tissue‐specific x‐ray computed tomography(CT)system that may potentially improved tumor specificity in CTimaging of similar Z‐material through detection of coherently scatteredphotons. Methods & Materials: Utilizing the open source Monte Carlo code system Electron Gamma Shower 5 (EGS5), we simulated coherently scatted photons for the purpose of investigating coherent scattercomputed tomography (CSCT). Unlike other simulation systems, ESG5 allows for correlation effects between molecules, which properly simulate coherent scatter. The simulated geometry mimics a “3rd generation” fan beam acquisition with a post‐scatter collimator (multi‐slit linear grid), which only permits scatter from a specific vertical segment of the phantom to be detected. The simulation consisted of a 10cm diameter cylindrical phantom of fat enclosing two 3.75cm diameter off‐center cylinders of muscle and air, centered midway between a monoenergetic (70kV) fan beam source and a bank of detection elements, which were located at a distance of 60 cm. The simulated detection elements were 1×1mm2 in area and covered a horizontal distance equivalent to a 55 degree arc centered on the source and a vertical distance of 175 mm. Results: Reconstruction of coherently scattered data was performed using filtered back projection and is comparable to previously published empirical results. Several material enhanced contrast images were generated from a single data acquisition (one tube rotation), with image intensity proportional to the amount of coherent scatter originating from a particular material at a particular location. Conclusion: These results demonstrate the utility of EGS5 to generate coherent scatter data for CSCT image investigation. Simulating scatter allows for a more feasible means to optimize acquisition parameters (photon spectrum, setup geometry, object attenuation) to create a CSCT system capable of exploiting the forward peaked nature of coherently scatteredphotons for discriminating similar Z‐material in topographic images.

Correlation of in situ measurements of plasma irregularities with ground‐based scintillation observations

We present here the first comparison of in situ ion density fluctuations measured by the Planar Langmuir Probe on Communication\Navigation Outage Forecasting System (C/NOFS) with coincident ground‐based measurements of scintillation from the Scintillation Decision Aid (SCINDA) network and coherent scatter radar located on Christmas Island (3°N, 157°W) over a 15 month period from May 2008 through June 2009. The Planar Langmuir Probe on C/NOFS measures absolute ion densities from 10 to 108 cm−3 at a rate up to 1024 Hz. The instrument is conceptually similar to ion traps and retarding potential analyzers that have flown on many past satellite and rocket missions. However, the present design includes advances in electronic capability compared with past designs. Initial results from this study show that in situ density fluctuations observed on magnetic flux tubes that pass over Christmas Island can be used as an indicator of ionospheric radio wave scintillation at that site. This is true even when the measurements are made at horizontal ranges of over 1000 km away from the ground site as long as the field line apex altitudes are less than ∼600 km.

Physical Meaning of Bistatic Polarimetric Parameters

A generalization of the polarization theory to bistatic scattering had shown that coherent scattering mechanisms are characterized by six bistatic fork parameters. The purpose of this paper is to try again to assign physical properties to these parameters as it was done in the monostatic case by Huynen. Our conclusions are validated with COSMO (Coherent Scattering Model) model and BABI (Base BIstatique) measurements.

A statistical study of the spatial distribution of Co‐operative UK Twin Located Auroral Sounding System (CUTLASS) backscatter power during EISCAT heater beam‐sweeping experiments

High‐power electromagnetic waves can excite a variety of plasma instabilities in Earth's ionosphere. These lead to the growth of plasma waves and plasma density irregularities within the heated volume, including patches of small‐scale field‐aligned electron density irregularities. This paper reports a statistical study of intensity distributions in patches of these irregularities excited by the European Incoherent Scatter (EISCAT) heater during beam‐sweeping experiments. The irregularities were detected by the Co‐operative UK Twin Located Auroral Sounding System (CUTLASS) coherent scatter radar located in Finland. During these experiments the heater beam direction is steadily changed from northward to southward pointing. Comparisons are made between statistical parameters of CUTLASS backscatter power distributions and modeled heater beam power distributions provided by the EZNEC version 4 software. In general, good agreement between the statistical parameters and the modeled beam is observed, clearly indicating the direct causal connection between the heater beam and the irregularities, despite the sometimes seemingly unpredictable nature of unaveraged results. The results also give compelling evidence in support of the upper hybrid theory of irregularity excitation.

Local determination of ionospheric plasma convection from coherent scatter radar data using the SECS technique

A new technique for merging line‐of‐sight (LOS) data of the ionospheric plasma convection velocity, as obtained from coherent scatter radars, into a full velocity vector field on a sphere is presented. This technique is based on the expansion into Spherical Elementary Current Systems (SECS) which have been successfully applied to many other problems in ionosphere‐magnetosphere physics. Despite their name mentioning currents for historical reasons, SECS can be used as basis functions for any continuously differentiable vector field on a sphere. In contrast to the traditional modeling of the radar data with spherical harmonics over the whole auroral zone, the new technique does not require any “a priori” model input but relies solely on the measured data, nor does it need any explicit boundary conditions to be specified. The new technique is designed to be applied locally to areas where sufficient radar backscatter exists. The analysis area that satisfies this condition may have any shape and is not limited to, e.g., spherical caps. A test with synthetic data shows that the method performs excellently (less than 5% relative error) if 25% or more of the optimal coverage of input data are actually available as backscatter data, with respect to the scale on which the results for the velocity vector field are desired to be obtained. Still if only 10% of the optimal coverage of input data are available, the technique performs fairly well with a relative error of ∼12%. A second test with real LOS input data from the SuperDARN radars shows that on such a local area with sufficient backscatter, our new technique is able to reproduce mesoscale details of the LOS data significantly better than the current standard analysis based on the technique of Ruohoniemi and Baker (1998) which processes the radar data on the whole auroral zone. While the new technique is presented here for the application with LOS radar data, it can be applied for merging any kind of vector component data on a sphere to a vector field.

Coherent Scattering of a Multiphoton Quantum Superposition by a Mirror BEC

We present the proposition of an experiment in which the multiphoton quantum superposition consisting of N approximately 10{5} particles generated by a quantum-injected optical parametric amplifier, seeded by a single-photon belonging to an Einstein-Podolsky-Rosen entangled pair, is made to interact with a mirror-Bose-Einstein condensate (BEC) shaped as a Bragg interference structure. The overall process will realize a macroscopic quantum superposition involving a microscopic single-photon state of polarization entangled with the coherent macroscopic transfer of momentum to the BEC structure, acting in spacelike separated distant places.