Azimuthal Direction Research Articles

Characteristics of Multi-scale Current Sheets in the Solar Wind at 1 au Associated with Magnetic Reconnection and the Case for a Heliospheric Current Sheet Avalanche

Wind spacecraft measurements are analyzed to obtain a current sheet (CS) normal width d cs distribution of 3374 confirmed magnetic reconnection exhausts in the ecliptic plane of the solar wind at 1 au. The d cs distribution displays a nearly exponential decay from a peak at d cs = 25 d i to a median at d cs = 85 d i and a 95th percentile at d cs = 905 d i with a maximum exhaust width at d cs = 8077 d i . A magnetic field θ-rotation angle distribution increases linearly from a relatively few high-shear events toward a broad peak at 35° < θ < 65°. The azimuthal ϕ angles of the CS normal directions of 430 thick d cs ≥ 500 d i exhausts are consistent with a dominant Parker-spiral magnetic field and a CS normal along the ortho-Parker direction. The CS normal orientations of 370 kinetic-scale d cs < 25 d i exhausts are isotropic in contrast, and likely associated with Alfvénic solar wind turbulence. We propose that the alignment of exhaust normal directions from narrow d cs ∼ 15–25 d i widths to well beyond d cs ∼ 500 d i with an ortho-Parker azimuthal direction of a large-scale heliospheric current sheet (HCS) is a consequence of CS bifurcation and turbulence within the HCS exhaust that may trigger reconnection of the adjacent pair of bifurcated CSs. The proposed HCS-avalanche scenario suggests that the underlying large-scale parent HCS closer to the Sun evolves with heliocentric distance to fracture into many, more or less aligned, secondary CSs due to reconnection. A few wide exhaust-associated HCS-like CSs could represent a population of HCSs that failed to reconnect as frequently between the Sun and 1 au as other HCSs.

Relativistic high-order harmonic generation by a femto-second radially polarized laser pulse irradiating a ring plasma grating

A novel scheme for generating relativistic high-order harmonics by a relativistic radially polarized laser interacting with a plasma annular grating is proposed. The particle-in-cell results show that the radial laser field can drive the relativistic electron bunches to oscillate radially in all azimuth directions, resulting in the emission of strong harmonics. Firstly, the interference of the laser field on the plasma grating structure significantly enhances the radiated harmonics which match the phase conditions. Secondly, due to the common ring symmetry of the grating structure and laser polarization, the transverse distribution of harmonics presents a Bessel form, and there is a sharp bright spot in the center with relativistic intensity. Such high-intensity short-wave structured harmonics have broad applications in areas such as plasma diagnostics, high resolution imaging and detection.

Mitigation of atmospheric turbulence effect by light beams carrying self-rotating wavefront.

We propose an approach against the turbulence-induced degradation by using laser beam with self-rotating wavefront. Such laser beam, generated by the coherent combination of vortex beams with different helical charges and central angular frequencies, can introduce coupling of its wavefront in spatial and temporal domain, that is, periodic wavefront rotation. When the wavefront rotation is faster than the airflow, the laser beam can travel through the inhomogeneity and anisotropy of air in the azimuthal direction within the time interval of airflow. The wavefront distortion caused by the turbulent atmosphere is therefore rotated and gradually smoothed as the laser beam travels. After the laser propagating through the turbulent atmosphere, the total wavefront distortion becomes centrosymmetric with lower peak-to-valley (PV) value. Such smoothed wavefront distortion can dramatically eliminate the turbulence-induced degradation of laser beams, especially beam centroid drift. We believe that this approach can lead to new trend in remote sensing, free-space optical communication, lidar, etc.

Ultra-wideband bi-static RCS reduction with full angular stability using diffraction periodic symmetrical structures

In this paper, a planar, two-layer, and symmetrical structure is presented to obtain ultra-wideband bi-static RCS reduction with angular stability in azimuth and elevation directions. The proposed structure is composed of two layers. The bottom layer contains a periodic surface with periodicity larger than one wavelength to scatter multiple Floquet harmonic and thus bi-static RCS reduction. To achieve angular stability for the azimuth direction, the geometry of the unit cell is optimized with symmetry to its center. The top layer contains a substrate with high relative permittivity, which, according to refraction Snell’s law, increases the bandwidth of RCS reduction and contributes to angular stability regarding the elevation direction. The results show the RCS reduction by 10 dB over a wide frequency range from 4.2 GHz to 22.9 GHz (137%) at the normal incidence, and more than 80% for oblique incident waves within the elevation angle range.

Spaceborne SAR image formation enhancement using MOCO techniques

Synthetic aperture radar (SAR) is considered a prevailing tool for remote sensing. It benefits working with high efficiency in all weather and all-day circumstances, making SAR is very confident compared to other types of remote sensing. The SAR platform moves with constant velocity and height, with a linear path for ideal situations. However, this assumption is not realized in satellite movement, which is an elliptical orbiting that worsens the quality of the focused image. This paper introduces a methodology of motion compensation for motion errors due to satellite elliptical orbiting and perturbations in an orbital path. It represents two major contributions applied on a low earth orbit (LEO) spaceborne SAR. First, motion errors analysis in the range and azimuth directions. Second, an algorithm for motion error compensation (MOCO) combined with a chirp scaling algorithm (CSA) is performed. Moreover, a validation for the formulated algorithm is executed using sentinel-1 level-0 real raw data input, and the result is compared with the sentinel-1 level-1 single look complex (SLC) SAR image. The validation is performed using two different metrics. First, image quality measurement by sharpness, contrast, and entropy. Second, measuring the peak-sidelobe-ratio (PSLR), impulse-response-width (IRW), and integrated-sidelobe-ratio (ISLR) for five high power reflecting points in the scene area.

Three-Dimensional Flow Velocity Estimation of Mountain Glacier Based on SAR Interferometry and Offset-Tracking Technology: A Case of the Urumqi Glacier No.1

Remote sensing estimations of glacier flow velocity could provide effective methods for the long-term monitoring of glacier flow velocity. This paper calculated the velocity in the line-of-sight (LOS) direction by combining DInSAR and offset-tracking technology with ascending and descending Sentinel-1 images of the Urumqi Glacier No.1 from 2016 to 2017. Meanwhile, the velocity in the azimuthal direction was obtained by combining MAI and offset-tracking technology. Then, the eastward, northward, and upward flow velocities were retrieved using the Helmert variance component estimation method. Finally, the standard error of the mean and mean errors of surface velocity in non-glaciated areas of the Urumqi Glacier No.1 were calculated to evaluate the accuracy of the results generated by the proposed method. The results showed: (1) The ascending LOS velocity and the descending LOS velocity were 1.812 m/a and −1.558 m/a from 2016 to 2017. The ascending azimuthal and descending azimuthal velocities were 0.978 m/a and −2.542 m/a, respectively. (2) The glacier flow velocities were 2.571 m/a and 1.801 m/a, respectively, for the eastward and northward directions. In the vertical direction, the velocity was −0.554 m/a. (3) The accuracy of the results generated by the proposed method were 0.028 m/a, 0.085 m/a, and 0.063 m/a in the east, north, and vertical directions. Therefore, it is suitable to use ascending and descending Sentinel-1 images and the study method proposed in this paper to estimate the surface flow velocity of mountain glaciers.

3D On and Off-Grid Dynamic Channel Tracking for Multiple UAVs and Satellite Communications

The space-air-ground integrated network (SAGIN) has drawn increasing attention for its benefits, such as wide coverage, high throughput for 5G and 6G communications. As one of the links, space-air communications between multiple unmanned aerial vehicles (UAVs) and Ka-band orbiting low earth orbit (LEO) satellites face a crucial challenge in tracking the 3D dynamic channel information. This paper exploits a statistical dynamic channel model called the multi-dimensional Markov model (MD-MM), which investigates the more realistic spatial and temporal correlation in the sparse UAVs-satellite channel. Specifically, the spatial and temporal probabilistic relationships of multi-user (MU) hidden support vector, single-user (SU) joint hidden support vector, and SU hidden value vector are investigated. The specific transition probabilities that connect the SU and MU hidden support vector for both azimuth and elevation directions are defined. Moreover, based on the proposed MD-MM, we derive a novel multi-dimensional dynamic turbo approximate message passing (MD-DTAMP) algorithm for tracking the 3D dynamic channel in multiple UAVs systems. Furthermore, we also develop a gradient update scheme to recursively find the azimuth and elevation offset for 3D off-grid estimation. Numerical results verify that the proposed algorithm shows superior 3D channel tracking performance with smaller pilot overhead and comparable complexity.

On the flow of liquid crystals through 90° bends

During the processing of nematic soft solids through process flow elements (pipe bends, elbows, etc.), the constitutive behavior makes its presence felt via processing (with rheology driven effects increasing pressure drop) and the final product microstructure. This paper explores the flow and microstructure configurations of nematic liquid crystals in a pressure driven flow through 90° pipe bends with different types of wall anchoring. The governing equations of the Leslie–Ericksen theory are solved numerically in a newly developed OpenFOAM solver. We show that the bend curvature deforms the nematic axis distribution; the distortion can be driven either by elastic or hydrodynamic effects. The interaction between the nematic microstructure and flow field generates non-zero normal stresses (in the radial, azimuthal, and streamwise directions), which produce a secondary flow and increase pressure losses. The strength of the secondary flow depends on the type of wall anchoring and Ericksen number; in configurations with homeotropic anchoring, decreasing the Ericksen number increases the relative strength of the secondary flow (with respect to the mean flow velocity). Conversely, homogeneous (planar) anchoring reduces normal stresses, thus weakening the secondary flow strength. We show that as the fluid enters/leaves the bend, there is a perturbation in the transverse velocity caused by streamwise stress gradients. The perturbation magnitude depends on material properties and can be of different values at the bend exit and entrance. Finally, we show that the spatial development of the nematic field downstream of the bend exit is controlled by both material properties and the Ericksen number.

Acoustic signature of a propeller operating upstream of a hydrofoil

The Ffowcs-Williams and Hawkings acoustic analogy is utilized to analyze the signature of a system consisting of a propeller and a downstream hydrofoil, mimicking a rudder at 0° incidence. This study is carried out exploiting the database generated by Large-Eddy Simulations on a cylindrical mesh consisting of almost 2 × 109 grid points. Three rotational speeds of the propeller are considered. The analysis reveals that the major sources of sound are located at the leading edge of the hydrofoil, due to the impingement by the propeller wake. With the exception of small radial coordinates around the propeller wake, between two and four diameters from the propeller axis, where the non-linear sources of sound have the lead, most noise comes from the linear, loading sources on the surface of the hydrofoil, due to fluctuations of the hydrodynamic pressure. As a result, the azimuthal directivity of the sound pressure levels develops a dipole-like distribution, elongated in the direction of the span of the hydrofoil. The attenuation of the acoustic pressure along the radial direction is initially cubic, then quadratic, and eventually, within less than ten diameters away from the system, linear.

Fractional defect charges in liquid crystals with p-fold rotational symmetry on cones.

Conical surfaces, with a δ function of Gaussian curvature at the apex, are perhaps the simplest example of geometric frustration. We study two-dimensional liquid crystals with p-fold rotational symmetry (p-atics) on the surfaces of cones. For free boundary conditions at the base, we find both the ground state(s) and a discrete ladder of metastable states as a function of both the cone angle and the liquid crystal symmetry p. We find that these states are characterized by a set of fractional defect charges at the apex and that the ground states are in general frustrated due to effects of parallel transport along the azimuthal direction of the cone. We check our predictions for the ground-state energies numerically for a set of commensurate cone angles (corresponding to a set of commensurate Gaussian curvatures concentrated at the cone apex), whose surfaces can be polygonized as a perfect triangular or square mesh, and find excellent agreement with our theoretical predictions.

Unsteady MHD chemically reactive dissipative flow of nanofluid due to rotating cone

The current article is to classify advanced features of unsteady magnetohydrodynamics (MHD) flow by a rotary cone. Energy expression takes into account the radiation, dissipation and nanofluid features. Nonlinear differential system is based upon conservation laws of mass, linear momentum and energy. The resultant structure of solutions is associated with the Homotopy analysis method (HAM). To predict performance of fluid close to surface of cone, the wall shear stress, temperature change and mass frequency are obtainable in both graphical and tabular formats. The current work is also compared with the existing result in a limited sense. It is noticed that skin frictions in both azimuthal and tangential directions are increasing functions of buoyancy force parameter.

Reynolds number effects and outer similarity of pressure fluctuations in turbulent pipe flow

We study the structure of pressure fluctuations in turbulent pipe flow, up to friction Reynolds number Reτ=6000, using standard spectral decomposition and proper orthogonal decomposition (POD). The mean pressure distribution is found to be qualitatively different from the case of channel flow, with an additional mean pressure difference caused by combined centrifugal and swirling effects. The variance of the pressure fluctuations exhibits a wide region with negative logarithmic decay with the wall distance, due to the presence of a hierarchy of wall-attached eddies, which we clearly trace in spectral maps, and which are generally isotropic in nature. On the other hand, the largest eddies are strongly non-isotropic, and are mainly elongated in the azimuthal direction. POD is used to prove self-similarity of the attached eddies, whose typical scales are found to be linearly proportional to the wall distance of their center.

Comparison between the Mesospheric Winds Observed by Two Collocated Meteor Radars at Low Latitudes

This study compares the hourly mesospheric horizontal winds observed by two collocated and independent low-latitude meteor radars operating at 37.5 MHz and 53.1 MHz in Kunming, China (25.6°N, 103.8°E). Upon analyzing simultaneously detected meteor echoes, we find a fixed angular deviation between the baselines of the two meteor radar antenna arrays within the east–north–up coordinate system. Then, we correct the deviation in the antenna azimuth direction using a novel method and recalculate the horizontal zonal and meridional winds. A comparison of the results before and after the correction shows strong consistency between the winds observed by both meteor radars within the entire detection altitude range. Furthermore, we summarize the performance of different techniques for measuring mesospheric winds. Ultimately, our statistical analysis approach allows the uncertainties associated with meteor radar wind observations to be more precisely estimated.

A Novel Space-Borne High-Resolution SAR System with the Non-Uniform Hybrid Sampling Technology for Space Targets Imaging

In order to reduce the complexity of the receiving system for wideband signals, a novel space-borne high-resolution synthetic aperture radar (SAR) system with the non-uniform hybrid sampling technology is proposed in this paper. The non-uniform hybrid sampling technology is firstly applied in the SAR imaging system for the detection of space targets. The non-uniform hybrid sampling technology is able to optimize the transmitted and receiving timing of SAR signals, reducing the requirement of the sampling rate of the analog-to-digital converter (ADC) for the reception of the wideband echoes from space targets. Meanwhile, according to the oversampling requirement of SAR imaging in the azimuth direction, a theoretical model of non-uniform hybrid sampling parameters and relative velocity between the SAR system and the space targets is established. A series of simulation experiments with different targets and different non-uniform hybrid parameters are performed, and an X-band SAR imaging experimental system is constructed to verify the effectiveness of the proposed non-uniform hybrid sampling technology. The experimental results show that the imaging resolution is better than 8 cm. When the non-uniform hybrid sampling interval is 15 us, the imaging quality is consistent with imaging results of the Nyquist real-time sampling, and it is easier to implement in the high-resolution imaging for space targets.

Dipolar Bose-Hubbard model in finite-size real-space cylindrical lattices

Recent experimental progress in magnetic atoms and polar molecules has created the prospect of simulating dipolar Hubbard models with off-site interactions. When applied to real-space cylindrical optical lattices, these anisotropic dipole-dipole interactions acquire a tunable spatially-dependent component while they remain translationally-invariant in the axial direction, creating a sublattice structure in the azimuthal direction. We numerically study how the coexistence of these classes of interactions affects the ground state of hardcore dipolar bosons at half-filling in a finite-size cylindrical optical lattice with octagonal rings. When these two interaction classes cooperate, we find a solid state where the density order is determined by the azimuthal sublattice structure and builds smoothly as the interaction strength increases. For dipole polarisations where the axial interactions are sufficiently repulsive, the repulsion competes with the sublattice structure, significantly increasing entanglement and creating two distinct ordered density patterns. The spatially-varying interactions cause the emergence of these ordered states in small lattices as a function of interaction strength to be staggered according to the azimuthal sublattices.

Spectra and structure functions of the temperature and velocity fields in supergravitational thermal turbulence

We analyze the power spectra and structure functions (SFs) of the temperature and radial velocity fields, calculated in the radial and azimuthal directions, in annular centrifugal Rayleigh–Bénard convection (ACRBC) for Rayleigh number Ra ∈[108,1011], Prandtl number Pr = 10.7, and inverse Rossby number Ro−1=16 using the spatial data obtained by quasi-two-dimensional direct numerical simulation. Bolgiano and Obukhov-like (BO59-like) scalings for the energy spectrum in both the azimuthal and radial directions and thermal spectrum in the azimuthal direction are observed. The range of BO59-like scaling becomes wider as Ra increases. At Ra=1011, it is found that BO59-like scaling Eu(kr)∼kr−11/5 spans nearly two decades for the energy spectrum calculated in the radial direction. Power-law fittings in the range larger than the Bolgiano scales, the scaling exponents of transverse and longitudinal velocity SFs vs the order coincide with the theoretical prediction of BO59 scaling ζpu=3p/5 basically. The second-order temperature SFs exhibit a gradual transition from the Obukhov–Corrsin behavior at scales smaller than the Bolgiano scales to the BO59 behavior at scales larger than the Bolgiano scales. The slopes from the third to sixth-order temperature SFs are similar, which is similar to classical Rayleigh–Bénard convection and Rayleigh–Taylor turbulence. The probability density functions (p.d.f.) of temperature fluctuations δT/σT reveal the cold plumes are strong and the p.d.f. in different regions at high Ra are similar. The stronger turbulent-mixing and larger centrifugal buoyancy in ACRBC may result in the BO59-like scaling.

Direction Finding Using a Single Antenna With Blade Modulation

A single top-hat antenna using blade modulation is presented for direction finding (DF). Unlike conventional DF techniques requiring an antenna array with multiple elements, the fixed single antenna is utilized to receive incoming signals. To enhance radiation pattern diversity and perform direction of arrival estimation, four rotating blades are placed around four corners of the antenna. The antenna is designed to operate at the frequency where the incoming wave is in resonance with the metallic blades. The incoming signals are recorded as blades rotating with an angle from 0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula> to 180 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ $</tex-math></inline-formula> . To validate the radiation pattern diversity at the resonant frequency and out of the operational frequency band, correlation coefficients of amplitude and phase of complex radiation patterns between the antennas with different blade rotation angles are investigated. DF accuracy in azimuth directions of the proposed single antenna with blade modulation is demonstrated by Cramer–Rao bound using the complex radiation patterns at different frequencies.

Single-focus phase singularity generated by spiral zone plate with quasi-random distributed quantum dots

We report on a new vortex lens for producing a single-focus phase singularity which is termed as a quasi-random-dot-array binary spiral zone plates (QBSZPs). Differing from the abrupt transitions of the conventional spiral zone plates (SZPs), the key idea of the QBSZPs is to realize a sinusoidal transmittance by properly arranging lots of quantum dot arrays which take on the values of 0 and 1 in two dimensions. In this typical design, the number density of the selected primitives obey sinusoidal distribution along the radial direction and quasi-random in the azimuthal direction. Theoretical analysis indicates that the higher-order foci which inevitably exist in the SZPs have been indeed effectively suppressed with the QBSZPs. Moreover, the focusing performance of the QBSZPs is influenced by the ratio of circumcircle diameter of the primitives to the outermost zone width. These findings, which have been demonstrated by the performed experiment, may offer a new direction towards improving the performance of biomedical imaging, quantum computation and optical manipulation.

Counter-rotating Taylor-Couette flows with radial temperature gradient

In the present work, counter-rotating turbulent Taylor-Couette flows with radial temperature gradient have been studied as an extension of previously studied simple-rotating turbulent Taylor-Couette flows. The rotation axis is orthogonal to the gravity vector. Direct Numerical Simulations (DNS) have been carried out for Reynolds number ranging from 1000 to 5000 and Richardson number varying from 0 to 0.4. The effect of variation of Reynolds number and Richardson number on the flow statistics, flow dynamics, and near-wall coherent structures are studied. For neutrally buoyant cases, near-wall coherent structures exist near the inner and the outer wall, with the core being almost vortex-free. With an increase in Richardson number, more dense and finer vortical structures spread out in the core in half the circumferential domain only. This behavior is due to the spatial stable or unstable stratification in the azimuthal direction, leading to the generation of turbulence in the lower half of the domain and mitigation of turbulence in the upper half of the domain. Further, the turbulent kinetic energy (TKE) budget analysis reveals an increase in turbulence on heating the outer cylinder wall due to an increase in the production term. Heating the outer cylinder wall leads to a slight decrease in skin friction for the inner cylinder.

Characteristics of Tropical‐Cyclone Turbulence and Intensity Predictability

AbstractThis study examines the characteristics of tropical‐cyclone (T‐C) turbulence and its related predictability implications. Using the Fourier‐Bessel spectral decomposition for convection‐permitting simulations, it is shown that T‐C turbulence possesses different spectral properties in the azimuthal and radial directions, with a steeper power law in the radial‐wavenumber than that in the azimuthal‐wavenumber direction. This spectral difference between the azimuthal and radial directions prevents one from using a single wavenumber to interpret T‐C intensity predictability as for classical homogeneous isotropic turbulence. Analyses of spectral error growth for a high‐wavenumber perturbation further confirm that the spectral growth is more rapid for high azimuthal wavenumbers than for the radial wavenumbers, reaching saturation after ∼9 hr and ∼18 hr for the azimuthal and radial directions, respectively. This result highlights the key difficulty in quantifying T‐C intensity predictability based on spectral upscale error growth for future applications.