Radial Currents Research Articles

A New Algorithm for Surface Currents Inversion With High-Frequency Over-the-Horizon Radar

The conventional method of extracting ocean surface currents by high-frequency over-the-horizon radar is based on the fixed first-order Bragg frequency formula and ignores the effects caused by the environment, especially in near-shore areas. In this letter, a current inversion model based on 2-D Fourier series expansion was developed. The first-order Bragg frequency and the Doppler offset induced by radial current are dealt with the bivariate functions of group distance and azimuth angle in the proposed method. By solving an overdetermined matrix equation with the least-square fitting method, the current at each detection grid can be estimated. As the Bragg frequency obtained by this new algorithm is adaptive to the environment, the accuracy of current measurement will be improved. The feasibility and effectiveness of the new method are verified with simulations and experimental results. The currents estimated by the traditional method and the new algorithm are compared with two in situ buoys. Results indicate that the new algorithm possesses comparable accuracy for far-shore areas and better accuracy for near-shore areas when compared with the conventional method.

Toroidal angular momentum balance during rotation changes induced by electron heating modulation in tokamak plasmas

An electron heating modulation numerical experiment based on a global full-f gyrokinetic model shows that transitions from ion temperature gradient driven (ITG) turbulence to trapped electron mode (TEM) turbulence induced by electron heating generate density peaking and rotation changes. Toroidal angular momentum balance during the rotation changes is revealed by direct observation of toroidal angular momentum conservation, in which in addition to ion turbulent stress, ion neoclassical stress, radial currents, and toroidal field stress of ions and electrons are important. Toroidal torque flipping between ITG and TEM phases is found to be related to reversal of the ion radial current, which indicates the coupling of particle and momentum transport channels. The ion and electron radial currents are balanced to satisfy the ambipolar condition, and the electron radial current is cancelled by the electron toroidal field stress, which indirectly affects toroidal torque.

Gravitational and electromagnetic signatures of accretion into a charged black hole

We present the derivation and the solutions to the coupled electromagnetic and gravitational perturbations with sources in a charged black hole background. We work in the so called ghost gauge and consider as source of the perturbations the infall of radial currents. In this way, we study a system in which it is provoked a response involving both, gravitational and electromagnetic waves, which allows us to analyze the dependence between them. We solve numerically the wave equations that describe both signals, characterize the waveforms and study the relation between the input parameters of the infalling matter with those of the gravitational and electromagnetic responses.

Experimental study of magnetized plasma rotation in crossed fields

Rotation of magnetized plasma between two coaxial electrodes in crossed electric and magnetic fields was studied experimentally. Three regimes of plasma rotation were observed. In the first regime, the radial electric field is created by a beam−plasma discharge due to the charging of the inner axial electrode by electrons, the outer electrode being grounded. Plasma rotation in this case is accompanied by strong high-frequency current oscillations detected by a Mach probe. When a negative voltage was applied to the coaxial electrodes, the second regime was observed, in which weakly perturbed quasi-stationary plasma rotation occurred at a relatively low radial current. The third regime of plasma rotation was observed upon a spontaneous disruption of the second regime. It is characterized by high currents of ~1 kA, sheared plasma rotation, and excitation of high-frequency perturbations.

MEG and EEG dipole clusters from extended cortical sources.

Data from magnetoencephalography (MEG) and electroencephalography (EEG) suffer from a rather limited signal-to-noise-ratio (SNR) due to cortical background activities and other artifacts. In order to study the effect of the SNR on the size and distribution of dipole clusters reconstructed from interictal epileptic spikes, we performed simulations using realistically shaped volume conductor models and extended cortical sources with different sensor configurations. Head models and cortical surfaces were derived from an averaged magnetic resonance image dataset (Montreal Neurological Institute). Extended sources were simulated by spherical patches with Gaussian current distributions on the folded cortical surface. Different patch sizes were used to investigate cancellation effects from opposing walls of sulcal foldings and to estimate corresponding changes in MEG and EEG sensitivity distributions. Finally, white noise was added to the simulated fields and equivalent current dipole reconstructions were performed to determine size and shape of the resulting dipole clusters. Neuronal currents are oriented perpendicular to the local cortical surface and show cancellation effects of source components on opposing sulcal walls. Since these mostly tangential aspects from large cortical patches cancel out, large extended sources exhibit more radial components in the head geometry. This effect has a larger impact on MEG data as compared to EEG, because in a spherical head model radial currents do not yield any magnetic field. Confidence volumes of single reconstructed dipoles from simulated data at different SNRs show a good correlation with the extension of clusters from repeated dipole reconstructions. Size and shape of dipole clusters reconstructed from extended cortical sources do not only depend on spike and timepoint selection, but also strongly on the SNR of the measured interictal MEG or EEG data. In a linear approximation the size of the clusters is proportional to the inverse SNR.

Comparison of predictive estimates of high‐latitude electrodynamics with observations of global‐scale Birkeland currents

AbstractTwo of the geomagnetic storms for the Space Weather Prediction Center Geospace Environment Modeling challenge occurred after data were first acquired by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). We compare Birkeland currents from AMPERE with predictions from four models for the 4–5 April 2010 and 5–6 August 2011 storms. The four models are the Weimer (2005b) field‐aligned current statistical model, the Lyon‐Fedder‐Mobarry magnetohydrodynamic (MHD) simulation, the Open Global Geospace Circulation Model MHD simulation, and the Space Weather Modeling Framework MHD simulation. The MHD simulations were run as described in Pulkkinen et al. (2013) and the results obtained from the Community Coordinated Modeling Center. The total radial Birkeland current, ITotal, and the distribution of radial current density, Jr, for all models are compared with AMPERE results. While the total currents are well correlated, the quantitative agreement varies considerably. The Jr distributions reveal discrepancies between the models and observations related to the latitude distribution, morphologies, and lack of nightside current systems in the models. The results motivate enhancing the simulations first by increasing the simulation resolution and then by examining the relative merits of implementing more sophisticated ionospheric conductance models, including ionospheric outflows or other omitted physical processes. Some aspects of the system, including substorm timing and location, may remain challenging to simulate, implying a continuing need for real‐time specification.

Calculation of the radial electric field from a modified Ohm's law

A modified Ohm's Law, derived from the conservation of deuterium and carbon ions and electron momentum and the requirement for charge neutrality, yields an expression for the radial electric field, Er, in the edge pedestal region in terms of the motional electric field due to the carbon and deuterium ion rotation velocities as well as pressure gradients and the radial plasma current. This analytical Ohm's Law model for Er is first shown to be consistent with the conventional “experimental” electric field calculated from the carbon radial momentum balance using experimental carbon rotation and pressure gradient measurements when experimental profiles are used to evaluate the Ohm's Law in three DIII-D [Luxon, Nucl. Fusion 42, 614 (2002)] representative discharges (for L-mode, H-mode, and Resonant Magnetic Perturbation operating regimes). In order to test the practical predictive ability of the modified Ohm's Law, the calculations were repeated using rotation velocities calculated with neoclassical rotation models instead of measured rotation velocities. The Ohm's Law predicted Er using theoretical rotation velocities did not agree with the “experimental” Er as well as the Ohm's Law prediction using experimental rotation velocities, indicating that more accurate models for predicting edge rotation velocity are needed in order to have a validated predictive model of Er in the plasma edge.

Analytic function expansion nodal (AFEN) method for solving multigroup neutron simplified P3 (SP3) equations in hexagonal-z geometry

In this paper, analytic function expansion nodal (AFEN) method was developed to solve multi-group and multi-dimensional neutron simplified P3 (SP3) equation in reactor cores with hexagonal fuel assembly. In this method, the intranodal fluxes are expanded into a set of analytic basis functions for each group and Legendre moment. Fourteen boundary conditions has been considered that constrain the intranodal flux distributions in the hexagonal-z node, which include twelve radial surface-averaged partial currents and two axial surface-averaged partial currents. The code takes few-groups cross sections produced by a lattice code and calculates the effective multiplication factor (keff), flux in multi-group energy, reactivity, and the relative power density at each fuel assembly. Finally, the solution accuracy is tested for the two and three dimensional IAEA benchmark problem. The 3D IAEA problem has been calculated for three different cases. The numerical results demonstrate that the SP3 AFEN method exhibits better accuracy compared to diffusion method especially when the control materials are inserted in the core.

The 2011 Tohoku tsunami south of Oahu: High‐frequency Doppler radio observations and model simulations of currents

AbstractA 16 MHz high‐frequency Doppler radio (HFDR) deployed on the south shore of Oahu (Hawaii) detected oscillatory radial currents following the arrival of the 2011 Tohoku tsunami. The observations over a two‐dimensional area provided an opportunity for intercomparison with the spatial patterns of currents and the resonant modes predicted by a nonhydrostatic model. Over the 50 m deep Penguin Bank, extending west from Molokai, the observed currents are intensified in two areas: 43 min period currents of 0.27 m s−1 lasting 6 h are observed on the south part of the bank, while 27 min period currents of 0.14 m s−1 lasting 2 h are observed on the north. The spatial EOFs suggest that standing full‐waves and 3/2 waves formed over the bank. Modeled currents over Penguin Bank are similar to the observations but their north‐south asymmetry is less pronounced than observed. Nearshore, observed alongshore currents showed long‐period oscillations of 43 min that stretched along the entire coastline, while modeled currents show strong evidence for edge waves. EOF analysis of the nearshore signal suggests that the HFDR and model reveal different processes. The discrepancy might be attributed to the fact that both the Penguin Bank and nearshore observations are limited by HFDR sensitivity to azimuthal sidelobe contamination and decreased angular resolution at high steering angles.

Radially polarized terahertz radiation in laser-induced linear plasma wake

The radially polarized terahertz radiation from laser-induced oscillating current is investigated by employing a two-dimensions plasma wakefield analytical model. It is shown that in linear plasma wake the currents have a periodical change along laser axis, and the current directions in potential well and potential barrier are opposite. The radial currents vanish at laser axis, which is different from longitudinal current, thus leading to a angle deviation of peak radiated power of radially polarized THz wave from laser axis. The deviation is more significant in thick plasma layer. The dependance of peak radiated power and total power of THz emission on thickness of plasma layer and laser intensity is discussed.

Bias Effects on the Reynolds Stress Using the Multi-Purpose Probe in IR-T1 Tokamak

The effect of the positive bias on Reynolds stress (RS) and its effect on the radial turbulent transport at the edge plasma (r/a = 0.9) and scrape-off layer (SOL) region of plasma in tokamak are investigated. The radial and poloidal electric fields (Er, Ep) and ion saturation current (Is) are measured by multi-purpose probe (MPP). This probe is fabricated and constructed for the first time in the IR-T1 tokamak. The most advantage of this probe is that the variations of Er and Ep can be measured in different radii at the single shot. Thus the information of different radii can be compared with high precision. The bias voltage is fixed at Vbias = 200 V and it has been applied with the limiter bias that is fixed in r/a = 0.9. Moreover, the phase difference between radial and poloidal electric fields, and temporal evolution of the RS spectrum detected by MPP are calculated. RS magnitude on the edge (r/a = 0.9) is more than its value in the SOL (r/a = 1.02). With the applied bias 200 V, RS and the magnitude of the phase difference between Er and Ep are increased, while the radial turbulent transport is decreased simultaneously. Thus it can be concluded that RS affects radial turbulence. Temporal evolution of the RS spectrum shows that the frequency of RS is increased and reaches its highest value at r/a = 0.9 in the presence of bias.

Development of a new analytic function expansion nodal code, HexDANM, for solving the neutron diffusion equation in hexagonal-Z geometry

Abstract In this paper, we developed a new approach of analytic function expansion nodal (AFEN) method to solve the multi-group and multi-dimensional neutron diffusion equation in reactor cores with hexagonal fuel assembly. This method represents a multidimensional intra nodal flux distribution in terms of analytic basis functions at any points in the node. New types of boundary conditions have been considered that constrain the intranodal flux distributions in the hexagonal-z node, which include twelve radial surface-averaged partial currents and two axial surface-averaged partial currents. We utilized the coarse group rebalancing (CGR) method to increase the speed of code calculations. The computer code takes a few-groups cross sections produced by a lattice code and calculates the effective multiplication factor (keff ), flux in multi-group energy, reactivity, and the relative power density at each fuel assembly. Finally, the solution accuracy is tested for two well-known benchmark problems. The numerical results demonstrate that the new AFEN method is an accurate method for calculating keff and power density distribution in hexagonal-z geometries.

Remote sensing of surface currents with single shipborne high-frequency surface wave radar

High-frequency surface wave radar (HFSWR) is a useful technology for remote sensing of surface currents. It usually requires two (or more) stations spaced apart to create a two-dimensional (2D) current vector field. However, this method can only obtain the measurements within the overlapping coverage, which wastes most of the data from only one radar observation. Furthermore, it increases observation’s costs significantly. To reduce the number of required radars and increase the ocean area that can be measured, this paper proposes an economical methodology for remote sensing of the 2D surface current vector field using single shipborne HFSWR. The methodology contains two parts: (1) a real space-time multiple signal classification (MUSIC) based on sparse representation and unitary transformation techniques is developed for measuring the radial currents from the spreading first-order spectra, and (2) the stream function method is introduced to obtain the 2D surface current vector field. Some important conclusions are drawn, and simulations are included to validate the correctness of them.

Numerical model for the development of magnetorotational instability in an annular channel

A laboratory experiment on studying magnetorotational instability (MRI) has been numerically simulated. The experiment involves driving liquid sodium placed in a vertical magnetic field in rotation by passing a radial electric current between the surfaces of coaxial cylinders. The numerical simulation yields the parameters of the future experimental device making it possible to develop MRI. These parameters include the size of the working chamber, the value of the radial current, and the strength of the initial magnetic field.

Spontaneous rotating vortex rings in a parametrically driven polariton fluid

We present the theoretical prediction of spontaneous rotating vortex rings in a parametrically driven quantum fluid of polaritons – coherent superpositions of coupled quantum well excitons and microcavity photons. These rings arise not only in the absence of any rotating drive, but also in the absence of a trapping potential, in a model known to map quantitatively to experiments. We begin by proposing a novel parametric pumping scheme for polaritons, with circular symmetry and radial currents, and characterize the resulting nonequilibrium condensate. We show that the system is unstable to spontaneous breaking of circular symmetry via a modulational instability, following which a vortex ring with large net angular momentum emerges, rotating in one of two topologically distinct states. Such rings are robust and carry distinctive experimental signatures, and so they could find applications in the new generation of polaritonic devices.

Evaluating Radial Current Measurement of Multifrequency HF Radar with Multidepth ADCP Data during a Small Storm

AbstractA multifrequency high-frequency (MHF) radar system was designed and developed by Wuhan University in 2007. This system can simultaneously operate at four frequencies mainly in the 7.5–25-MHz band. This paper focuses on discussing the performances of an MHF radar system deployed along the coast of the East China Sea based on comparisons with multidepth ADCP datasets, which were obtained from ADCPs deployed at different locations in August 2010 during a small storm. The comparisons illustrate that radar-derived radial currents are correlated with ADCP data at mainly a 2–4-m depth with correlation coefficients over 0.95 and RMS differences less than 0.12 m s−1 for both operating frequencies. Bearing offsets at points A, C, and D are computed for different operating frequencies with magnitudes of 0°–11°.The capability of MHF radar to measure currents at different depths is explored. The results indicate that the effective depth of current measurements by MHF radar increases with decreasing operating frequency. A linear regression (with a regression coefficient of 0.0576) of the responses in the mean effective depth on the predictors in radio wavelength is obtained. The dominant semidiurnal and diurnal constituents are also analyzed. The radial current amplitudes of the M2 and K1 constituents are strong in this area during this experiment. The residual currents vary with wind speed, with a correlation coefficient of 0.52. A correlation coefficient of 0.79 between nontidal currents and the radial wind speed after a clockwise rotation of the wind vector by about 50° was obtained.

Evaluating Two Array Autocalibration Methods with Multifrequency HF Radar Current Measurements

AbstractOne pivotal factor affecting the accuracy of HF radar current measurements is the direction of arrival (DOA) estimation performance of the current signal. The beamforming technology or superresolution algorithm cannot always perform best in practical applications because of the phase errors existing in array channels. These phase errors, which cause uncertain estimation of DOA, lead to confused values in radial current maps. To solve this problem, this paper is focused on discussing the performances of two autocalibration methods using sea echoes for multifrequency high-frequency (MHF) radar current measurements. These two array calibration methods, based on maximum likelihood (ML) and multiple signal classification (MU), first seek single-DOA sea echoes and then gather them for array calibration using different cost functions. The ML and MU methods provide approximate mean phases, while the standard phase errors of the MU method are smaller. After array calibration using these two methods, the results show significant improvements in current retrievals. Comparisons between the MHF radar and ADCPs reveal that array calibration using the ML and MU methods also improves the estimation of radial currents clearly, with correlation coefficients over 0.93 and rms differences of 0.09–0.18 m s−1 at different operating frequencies and sampling locations. The performance of the bearing offset is also improved. Only small bearing offsets less than 10° exist in radial current measurements. Therefore, this paper demonstrates that array calibration is a crucial part for current measurements, especially for direction-finding HF radar.

Magnetization in a Finite Superconducting Hollow Cylinder in the Presence of the Radial and Azimuthal Transport Currents

The main aim of this study is to investigate the magnetization of a finite superconducting hollow cylinder in an axial applied magnetic field and in presence of different values of the radial and azimuthal transport currents in the framework of the three critical states models, namely, Bean, Kim, and exponential models. A set of self-consistency coupled-integral equations for the magnetization hysteresis loops have been solved by using Newton’s numerical method. Throughout the calculations, it has been assumed that the flux penetration starts from the inner lateral surface of the sample. A comparison has been made between the magnetic responses of the sample in applying the radial and azimuthal transport currents. It was found that when the transport current increases, the saturation value, enclosed area of the hysteresis loops, and the initial slopes in the virgin curve decreases. However, given that the flux penetration and demagnetizing effect did not affect by the azimuthal angle, the rate of the mentioned changes as the transport current is in azimuthal direction was slower than when that is in the radial direction. In addition, the radial transport current destroyed the azimuthal symmetry of the shielding current distribution and induced the nonuniform magnetic field in the finite sample. The obtained results have shown the importance of the magnetic field dependence of the critical current density on the magnetic response of the finite superconducting hollow cylinder.

Origins of variation in conducted vasomotor responses.

Regulation of blood flow in the microcirculation depends on synchronized vasomotor responses. The vascular conducted response is a synchronous dilatation or constriction, elicited by a local electrical event that spreads along the vessel wall. Despite the underlying electrical nature, however, the efficacy of conducted responses varies significantly between different initiating stimuli within the same vascular bed as well as between different vascular beds following the same stimulus. The differences have stimulated proposals of different mechanisms to account for the experimentally observed variation. Using a computational approach that allows for introduction of structural and electrophysiological heterogeneity, we systematically tested variations in both arteriolar electrophysiology and modes of stimuli. Within the same vessel, our simulations show that conduction efficacy is influenced by the type of cell being stimulated and, in case of depolarization, by the stimulation strength. Particularly, simultaneous stimulation of both endothelial and vascular smooth muscle cells augments conduction. Between vessels, the specific electrophysiology determines membrane resistance and conduction efficiency-notably depolarization or radial currents reduce electrical spread. Random cell-cell variation, ubiquitous in biological systems, only cause small or no reduction in conduction efficiency. Collectively, our simulations can explain why CVRs from hyperpolarizing stimuli tend to conduct longer than CVRs from depolarizing stimuli and why agonists like acetylcholine induce CVRs that tend to conduct longer than electrical injections. The findings demonstrate that although substantial heterogeneity is observed in conducted responses, it can be largely ascribed to the origin of electrical stimulus combined with the specific electrophysiological properties of the arteriole. We conclude by outlining a set of "principles of electrical conduction" in the microcirculation.

Numerical Study on the Initial Formation of the Radial Sand Ridges

The radial sand ridges located in the southern Yellow Sea off the Jiangsu coast, are unique sand ridges. They are characterized by radical current field and radical arrangement. The formation and maintenance mechanism of the radial sand ridges have always been a focus of attention of scholars for a long time. The formation is related to the unique convergent-divergent tidal currents. In this paper, a numerical model is constructed to answer the two following questions. Firstly, can the radial sand ridges be formed with radical tidal current field? Secondly, can this be simulated by a numerical model? The radial tidal current can shape the radial sand ridges. Numerical simulations show that the radial sand ridges can be formed on present radical tidal current field and the formation and evolution of the radial sand ridges can be simulated by a numerical model.