Field-aligned Currents Research Articles

Cross-scale Dynamics Driven by Plasma Jet Braking in Space

Abstract Plasma jets are ubiquitous in space. In geospace, jets can be generated by magnetic reconnection. These reconnection jets, typically at fluid scale, brake in the near-Earth region, dissipate their energies, and drive plasma dynamics at kinetic scales, generating field-aligned currents that are crucial to magnetospheric dynamics. Understanding of the cross-scale dynamics is fundamentally important, but observation of coupling among phenomena at various scales is highly challenging. Here we report, using unprecedentedly high-cadence data from NASA's Magnetospheric Multiscale Mission, the first observation of cross-scale dynamics driven by jet braking in geospace. We find that jet braking causes MHD-scale distortion of magnetic field lines and development of an ion-scale jet front that hosts strong Hall electric fields. Parallel electric fields arising from the ion-scale Hall potential generate intense electron-scale field-aligned currents, which drive strong Debye-scale turbulence. Debye-scale waves conversely limit intensity of the field-aligned currents, thereby coupling back to the large-scale dynamics. Our study can help in understanding how energy deposited in large-scale structures is transferred into small-scale structures in space.

Research on Adaptive Design and Optimization of High-Speed Coil Launcher

With the increase of coil stages of the high-speed electromagnetic coil launcher, the mutual coupling between the parameters and the increase of the variables to be designed make the design of the multistage high-speed electromagnetic coil launcher more difficult. In this article, an adaptive design method of electromagnetic coil launcher based on the current filament model is proposed. Its feasibility is verified by a 25-stage coil launcher design scheme calculation example. However, the coil temperature rise cannot be restricted with the traditional step-by-step trial method and adaptive design method alone. The use of traditional optimization algorithms will consume a lot of time to get a result. Therefore, this article adopts the genetic algorithm (GA) based on the predictive model to obtain constrained optimization results. With the predictive model of support vector regression (SVR), the average prediction error of the armature peak speed can be controlled below 5%. Using the established predictive model, GA is used to optimize the adjustable parameters in the adaptive design. The peak velocity of the predictive model output is selected as the fitness function, and the coil temperature rise and acceleration stability are constrained. Sixteen sets of optimization results are obtained by repeated optimization calculation. The peak speed of the armature can reach 1083 m/s, the launch efficiency of the launcher is 25.78%, and the maximum temperature rise of the coil is 76.88°C. Compared with ordinary optimization algorithms, the calculation time is greatly reduced.

Current Filamentation and Switching Effect in Chalcogenide Glassy Semiconductors: A Review

A review on current filamentation and switching effect in chalcogenide glassy semiconductors (CGSs), which are promising materials for the development of phase change memory devices, is presented. First, a history of the research on CGSs and their properties is considered. Next, formation of a current filament in CGSs is discussed and the scale of heat release in the material as well as the geometric shape of the filament is analyzed. Finally, various hypotheses developed for the explanation of the switching effect in CGSs are reviewed. It is shown that the most relevant model of the switching effect in CGS is the model of multi-phonon tunneling ionization of the so-called ‘negative-U centers’. This model is based on the assumption that an avalanche-like increase in current at a certain point in time is associated with mass tunneling of electrons located on atoms, occuring due to thermal vibrations of atoms.

High‐Latitude Electrodynamics Specified in SAMI3 Using AMPERE Field‐Aligned Currents

AbstractA new technique has been developed to determine the high‐latitude electric potential from observed field‐aligned currents (FACs) and modeled ionospheric conductances. FACs are observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), while the conductances are modeled by Sami3 is Also a Model of the Ionosphere (SAMI3). This is a development of the Magnetosphere‐Ionosphere Coupling approach first demonstrated by Merkin and Lyon (2010), https://doi.org/10.1029/2010ja015461. An advantage of using SAMI3 is that the model can be used to predict total electron content (TEC), based on the AMPERE‐derived potential solutions. 23 May 2014 is chosen as a case study to assess the new technique for a moderately disturbed case (min Dst: −36 nT, max AE: 909 nT) with good GPS data coverage. The new AMPERE/SAMI3 solutions are compared against independent GPS‐based TEC observations from the Multi‐Instrument Data Analysis Software (MIDAS) by Mitchell and Spencer (2003), and against Defense Meteorological Satellite Program (DMSP) ion drift data. The comparison shows excellent agreement between the location of the tongue of ionization in the MIDAS GPS data and the AMPERE/SAMI3 potential pattern, and good overall agreement with DMSP drifts. SAMI3 predictions of high‐latitude TEC are much improved when using the AMPERE‐derived potential as compared to Weimer's (2005), https://doi.org/10.1029/2005ja011270 model. The two potential models have substantial differences, with Weimer producing an average 77 kV cross‐cap potential versus 60 kV for the AMPERE‐derived potential. The results indicate that the 66‐satellite Iridium constellation provides sufficient resolution of FACs to estimate large‐scale ionospheric convection as it impacts TEC.

Computing Eigenvalues of Dielectric Waveguides by a Method of Auxiliary Sources With Two Excitation Sources

The Method of Auxiliary Sources with an Excitation Source (MAS-ES) has been successfully employed to compute the eigenvalues of arbitrarily-shaped, simply and multiply-connected hollow waveguides with perfectly electric conducting (PEC) walls. The main advantages of this method are its simplicity, and that it is free of spurious eigenvalues, in contrast to the standard MAS approach. In this paper, we demonstrate that the MAS-ES is also effective in computing the propagation constants (eigenvalues) <inline-formula><tex-math notation="LaTeX">$\beta$</tex-math></inline-formula> of a cylindrical dielectric waveguide with core of arbitrary cross section. It is emphasized that two excitation sources (an electric and a magnetic current filament lying within the core) are required to excite hybrid modes of the dielectric waveguide; a hollow PEC waveguide requires only one source. The modified method, thus obtained, is named MAS with Two Excitation Sources (MAS-TES). The fact that the propagating modes are localized in the vicinity of the core allows us to determine the eigenvalues by measuring the response of the core to the excitation sources. This is performed by employing a response function <inline-formula><tex-math notation="LaTeX">$F(\beta)$</tex-math></inline-formula> which is maximized when a standing wave is formed in the core. Plotting <inline-formula><tex-math notation="LaTeX">$F(\beta)$</tex-math></inline-formula> for a dense set of <inline-formula><tex-math notation="LaTeX">$\beta$</tex-math></inline-formula> results in a response curve the peaks of which correspond to the waveguide&#x2019;s eigenvalues. The method is tested for several dielectric waveguides&#x2019; geometries, including two multimode cases, and it is shown to be free from discrete and continuous spurious solutions. All the MAS-TES results are compared with those obtained by an FEM-based commercial software and an excellent agreement is exhibited.

Temperature response features of ferroelectric ceramics in electrocaloric effect study

Temperature response of a material to an external electric field is the main method for electrocaloric effect study in ferroelectrics. In this work, for 0.65PbFe2/3W1/3O3-0.35PbTiO3 solid solution as a model object, it is shown that with an increase in the electric field strength, current filamentation effect can occur. It leads to formation of local regions of increased conductivity in the sample. The associated thermal effect have short characteristic times, due to the small volume of the filament. They are comparable to the times of the electrocaloric response of the material, and can lead to significant errors in the detection of the electrocaloric effect. Keywords: electrocaloric effect, temperature response, ferroelectric ceramics, current filamentation effect.

Localization of Relativistic Electrons in Stationary Current Filaments of the Plasma Focus

Localization of Relativistic Electrons in Stationary Current Filaments of the Plasma Focus

Temperature response features of ferroelectric ceramics in electrocaloric effect study

Temperature response of a material to an external electric field is the main method for electrocaloric effect study in ferroelectrics. In this work, for 0.65PbFe2/3W1/3O3-0.35PbTiO3 solid solution as a model object, it is shown that with an increase in the electric field strength, current filamentation effect can occur. It leads to formation of local regions of increased conductivity in the sample. The associated thermal effect have short characteristic times, due to the small volume of the filament. They are comparable to the times of the electrocaloric response of the material, and can lead to significant errors in the detection of the electrocaloric effect. Keywords: electrocaloric effect, temperature response, ferroelectric ceramics, current filamentation effect.

Intermittent Dissipation at Kinetic Scales in the Turbulent Reconnection Outflow

AbstractBased on the unprecedented high‐resolution data from the Magnetospheric Multiscale mission, we investigate the energy dissipation in turbulent reconnection outflow during turbulent magnetic reconnection in the terrestrial magnetotail. It is found that this reconnection outflow has plenty of intermittent structures at kinetic scales accompanied by abundant current filaments. Strong energy dissipation occurs in the intermittent structures with a high partial variance of increments index, and the regions with strong current filaments. Further analyses reveal that electron heating with the presence of increase in electron temperature occurs in the intermittent structures and the regions with strong currents. This electron heating is more significant in the component parallel to the magnetic field. Our observations demonstrate that intermittent dissipation at kinetic scales occurs in the turbulent reconnection outflow.

An adjustable low frequency vibration isolation with high-static-stiffness low-dynamic-stiffness property using a novel negative stiffness element

A vibration isolator with a novel negative stiffness element (NSE) configured with two electromagnetic springs in parallel was proposed in this study. The electromagnetic force and stiffness model of NSE was built based on the Ampere current model and filament method, the effect of configuration parameters on the negative stiffness was investigated in detail. The linear negative stiffness principle was proposed based on the impact of height difference on the nonlinear component sign of the negative stiffness, and the linear negative stiffness parameter design strategy for NSE was also developed. The isolator prototype was fabricated with the parameter design result, and the correctness of modeling and principle for linear negative stiffness were validated by magnetic force measurement. A series of experiments under different coil currents and vibration amplitude was conducted. The results show that high-static-stiffness and low-dynamic-stiffness property of the isolator is realized, the resonant frequency is 3.85 Hz with the coil current is 1A is reduced by up to 58.5% compared with the resonant frequency when coil without energized, the isolation bandwidth has been stretched and can be adjusted by coil current and performance has been significantly improved also. In addition, the isolator is stable and the performance is not deteriorator under different vibration amplitudes.

Nanoscale Electrostatic Modulation of Mega-Ampere Electron Current in Solid-Density Plasmas.

Transport of high-current relativistic electron beams in dense plasmas is of interest in many areas of research. However, so far the mechanism of such beam-plasma interaction is still not well understood due to the appearance of small time- and space-scale effects. Here we identify a new regime of electron beam transport in solid-density plasma, where kinetic effects that develop on small time and space scales play a dominant role. Our three-dimensional particle-in-cell simulations show that in this regime the electron beam can evolve into layered short microelectron bunches when collisions are relatively weak. The phenomenon is attributed to a secondary instability, on the space- and timescales of the electron skin depth (tens of nanometers) and few femtoseconds of strong electrostatic modulation of the microelectron current filaments formed by Weibel-like instability of the original electron beam. Analytical analysis on the amplitude, scale length, and excitation condition of the self-generated electrostatic fields is clearly validated by the simulations.

Impact of the Solar Wind Dynamic Pressure on the Field‐Aligned Currents in the Magnetotail: Cluster Observation

AbstractWe statistically investigate the influence of the solar wind dynamic pressure (SW Pdyn) on the field‐aligned currents (FACs) in the magnetotail with 1,492 FAC cases from July to October in 2001 and 2004, which covers 74 Cluster crossings of the plasma sheet boundary layer (PSBL) in both storm time and non‐storm time. The FAC density in the magnetotail is derived from the magnetic field data with the four‐point measurement of Cluster, and the SW Pdyn is taken from ACE data. The results indicate the FAC density becomes stronger with increasing SW Pdyn. The statistics show that the FAC occurrence increased monotonically with SW Pdyn in the three levels (Weak: SW Pdyn &lt; 2 nPa; Medium: 2 nPa ≤ SW Pdyn ≤ 5 nPa; Strong: SW Pdyn &gt; 5 nPa). The FAC density increased with increasing SW Pdyn, while its footprint (invariant latitude, ILAT) in the polar region decreased with increasing SW Pdyn. The response of the FAC to SW Pdyn in the magnetotail had a north‐south hemispheric asymmetry. The FAC density had a better correlation with SW Pdyn in the Northern hemisphere, while the footprint had a better correlation with SW Pdyn in the Southern hemisphere. Possible underlying mechanisms for our results are analyzed and discussed. However, it requires more observations and simulation studies to find out the mechanism of north‐south asymmetry.

The Role of Diffuse Electron Precipitation in the Formation of Subauroral Polarization Streams

AbstractThe role of diffuse electron precipitation in the formation of subauroral polarization streams (SAPS) is investigated with the Multiscale Atmosphere‐Geospace Environment (MAGE) model. Diffuse precipitation is derived from the distribution of drifting electrons. SAPS manifest themselves as a separate mesoscale flow channel in the duskside ionosphere, which gradually merges with the primary auroral convection toward dayside as the equatorward auroral boundary approaches the poleward Region‐2 field‐aligned currents (FACs) boundary. SAPS expand to lower latitudes and toward the nightside during the main phase of a geomagnetic storm, associated with magnetotail earthward plasma flows building up the ring current and intensifying Region‐2 FACs and electron precipitation. SAPS shrink poleward and sunward as the interplanetary magnetic field turns northward. When diffuse precipitation is turned off in a controlled MAGE simulation, ring current and duskside Region‐2 FACs become weaker, but subauroral zonal ion drifts are still comparable to auroral convection. However, subauroral and auroral convection manifest as a single broad flow channel without showing any mesoscale structure. SAPS overlap with the downward Region‐2 FACs equatorward of diffuse precipitation, where poleward electric fields are strong due to a low conductance in the subauroral ionosphere. The Region‐2 FACs extend to latitudes lower than the diffuse precipitation because the ring current protons penetrate closer to the Earth than the electrons do. This study reproduces the key physics of SAPS formation and their evolution in the coupled magnetosphere‐ionosphere during a geomagnetic storm. Diffuse electron precipitation is demonstrated to play a critical role in determining SAPS location and structure.

Roles of the M‐I Coupling and Plasma Sheet Dissipation on the Growth‐Phase Thinning and Subsequent Transition to the Onset

AbstractThe plasma‐sheet dynamics leading to the substorm onset is analyzed from a global simulation. We calculated three cases, the normal (case 1), reduced magnetosphere‐ionosphere (M‐I) coupling (case 2) and reduced tail dissipation (case 3). The normal solution reproduces the thinning, reconnection, tail flow, and the dipolarization in the magnetosphere, and the AU/AL, the initial brightening, the beads, the pre‐onset arc, and the poleward expansion in the ionosphere. Case 2 shows that the M‐I coupling promotes the thinning by evacuating magnetic field from the plasma sheet. Case 3 shows that it is implausible to reproduce the AU/AL of the expansion phase only with numerical dissipation. The projection area of the growth phase plasma sheet is so narrow that the quiet arcs on the high‐latitude side are projected to the lobe. The key point to understand such narrow projection is to consider the origin of the field‐aligned current (FAC). The FAC occurs to transmit motion, and particle precipitation is the consequence. The upward FACs for the quiet arc and initial brightening, which produce the substorm sequence, are generated to transmit enhanced convection from the magnetosphere to the ionosphere. The substorm onset is the transmission of the convection transient. The onset FAC is generated by the near‐earth dynamo. Before the onset, a two‐stage reconnection occurs at the medium‐tail (MTNL) and the near‐earth (NENL) neutral lines through retreating nulls. The accompanying flow explains the model that assumes two causal areas for the onset in the near‐earth and middle tails.

Field‐Aligned Current During an Interval of BY‐Dominated Interplanetary‐Field; Modeled‐to‐Observed Comparisons

AbstractWe model an interval of remarkable interplanetary magnetic field (IMF), for which we have a comprehensive set of observational data. This interval is associated with the arrival of an interplanetary coronal mass ejection. The solar wind densities at the time are particularly high and the IMF is primarily northward over many hours. This results in strong auroral emissions within the polar cap in a cusp spot, which we associate with lobe reconnection at the high‐latitude magnetopause. We also observe areas of upwards field‐aligned current (FAC) within the summer Northern Hemisphere polar cap that exhibit large current magnitudes. The model can reproduce the spatial distribution of the FACs well, even under changing conditions in the incoming IMF. Discrepancies exist between the modeled and observed current magnitudes. Notably, the winter Southern Hemisphere exhibits much lower current magnitudes overall. We also model a sharp transition of the location of magnetopause reconnection at the beginning of the interval, before the IMF remained northward for many hours. The reconnection location changed rapidly from a subsolar location at the low‐latitude magnetopause under southward IMF conditions, to a high‐latitude lobe reconnection location when the field is northward. This occurs during a fast rotation of the IMF at the shock front of a magnetic cloud.

On the Deviation of MAS Fields in the Far-Field Region of Elementary Sources Close to Infinite Boundaries

For the simple problem of an electric current filament close to a perfectly conducting infinite plane, the method of auxiliary sources (MAS) yields numerical solutions that are systematically deviating from the exact one beyond a certain horizontal distance (i.e., outside the region demarcated by the finite set of auxiliary sources). In this article, in order to shed light on the edge-effect nature of the aforementioned discrepancy, also arising in more complex problems, this issue is investigated both numerically and asymptotically through an infinite version of MAS, which assumes an infinite number of auxiliary sources. The analysis is followed by discussions on the uniqueness of the exact solution and the adequacy/suitability of certain error metrics as quality and reliability indicators of MAS results.

Evolution of the conductive filament system in HfO2-based memristors observed by direct atomic-scale imaging

The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO2 has been accepted as one of the most promising resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO2-based memristors. Here, the conductive filament system in the amorphous HfO2-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf6O and its crystalline surroundings (monoclinic or tetragonal HfOx). The phase of the HfOx shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO2 is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO2-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.

Reconstructing Mercury's magnetic field in magnetosphere using radial basis functions

Based on the Magnetometer observations of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit around Mercury, we construct a global magnetic field model of Mercury's magnetosphere using radial basis functions (RBFs). This model is parameterized by magnetic disturbance level and the fields are a sum of internal and external sources in the magnetosphere, including magnetopause and magnetospheric currents. The RBF-based model does not impose preconceived assumptions on the fields and constructs the system of tail currents and field-aligned currents (FACs) simultaneously. Compared with the previous models, the residuals between the observed and modeled magnetic field are significantly reduced at different magnetic activity levels and this indicates that our model is reliable and robust. Some features in Mercury's magnetosphere are also constructed based on the RBF model. The magnetic field is compressed in the boundary layer inside the dayside magnetosphere and stretched at the nightside due to the strong dawn-dusk cross-tail currents beyond 1.5 RM(RM is Mercury's radius). The X-line location at the nightside is reduced from 4.6 RM in the magnetic quiet period to 2.9 RM in the magnetic active period and fits the magnetic disturbance indices in linear form. The FACs are downward (upward) in the dawn (dusk) sector and are increased with magnetic disturbance levels. In summary, the RBF-based model could reveal all principal features in the magnetosphere and their changes associated with magnetic disturbance levels.

Magnetospheric Source and Electric Current System Associated With Intense SAIDs

AbstractSubauroral Ion Drifts (SAIDs) are fast azimuthal plasma flows in the ionosphere. In this study, we use the Rice Convection Model to simulate an extremely intense SAID event which may trigger atmospheric emission phenomenon by specifying consecutive low‐entropy bubble injections. The results show strong SAID flow faster than ∼5,000 m/s that lasts longer than 100 min. Our model indicates that SAIDs map to the sharp inner edge of a strongly enhanced partial ring current, which connects to a Region‐2 downward field‐aligned current (FAC). The simulation predicts that the peak of the SAIDs, the center of the downward FAC, the subauroral boundary, the plasmapause, the inner edge of the proton and electron partial ring current, and the maximum azimuthal drift in the magnetosphere are pressed tightly within 0.5° latitude or 0.2 Re in the equatorial plane.

Correlation analysis of field-aligned currents from the magnetic measurements of GRACE follow-on mission

In this study we performed a detailed analysis on the scale-size of field-aligned currents (FACs) at auroral latitudes, using the well-calibrated magnetic data from the non-dedicated magnetic field mission, Gravity Recovery and Climate Experiment Follow-On (GRACE-FO). With two spacecraft following each other, the GRACE-FO provides a good opportunity to identify the variation of FACs with different scale lengths. The results show that the auroral FACs can be classified into two groups: the small-scale ones, shorter than some tens of kilometers, dominated by kinetic Alfvén waves, are quite dynamic; and the large-scale ones, typically larger than 150 km, can be considered as quasi-static and persist longer than 1 min. The GRACE-FO observations also reveal that the small-scale FACs at the same location sometimes can persist over 25 s, e.g., around dusk and dawn hours, which is longer than the typical persistent period (10 s) of kinetic Alfvén waves as earlier reported. The FAC structures show clear magnetic local time dependence, with higher correlations between the spacecraft around dusk and dawn hours; lower correlations are found around midnight and lowest correlations around noon, implying that the small-scale FACs most frequently appear at the noon cusp region. Slightly better correlations of FACs between two spacecraft are found during local summer, and such seasonal dependence is dominated by the correlations of small-scale FACs at noon. However, further analysis shows that the small-scale FACs at noon have largest occurrence and intensity during local summer, which reveals that when interpreting the cross-correlation analysis the intensity of FACs needs to be taken into account.Graphical