Current Sheet Model Research Articles

Long‐Term Variability of Jupiter's Magnetodisk and Implications for the Aurora

AbstractObservations of Jupiter's UV auroral emissions collected over several years show that the ionospheric positions of the main emission and the Ganymede footprint can vary by as much as 3° in latitude. One explanation for this shift is a change of Jupiter's current sheet current density, which would alter the amount of field line stretching and displace the ionospheric mapping of field lines from a given radial distance in the magnetosphere. In this study we measure the long‐term variability of Jupiter's magnetodisk using Galileo magnetometer data collected from 1996 to 2003. Using the Connerney et al. (1981) current sheet model, we calculate the current sheet density parameter that gives the best fit to the data from each orbit and find that the current density parameter varies by about 15% of its average value during the Galileo era. We investigate possible relationships between the observed current sheet variability and quantities such as Io's plasma torus production rate inferred from volcanic activity and external solar wind conditions extrapolated from data at 1 AU but find only a weak correlation. Finally, we trace Khurana (1997) model field lines to show that the observed changes in Jupiter's current sheet are sufficient to shift the ionospheric footprint of Ganymede and main auroral emission by a few degrees of latitude, consistent with the magnitude of auroral variability observed by Hubble Space Telescope (HST). However, we find that the measured auroral shifts in HST images are not consistent with concurrent changes in the current density parameter measured by Galileo.

Force-free collisionless current sheet models with non-uniform temperature and density profiles

We present a class of one-dimensional, strictly neutral, Vlasov-Maxwell equilibrium distribution functions for force-free current sheets, with magnetic fields defined in terms of Jacobian elliptic functions, extending the results of Abraham-Shrauner [Phys. Plasmas 20, 102117 (2013)] to allow for non-uniform density and temperature profiles. To achieve this, we use an approach previously applied to the force-free Harris sheet by Kolotkov et al. [Phys. Plasmas 22, 112902 (2015)]. In one limit of the parameters, we recover the model of Kolotkov et al. [Phys. Plasmas 22, 112902 (2015)], while another limit gives a linear force-free field. We discuss conditions on the parameters such that the distribution functions are always positive and give expressions for the pressure, density, temperature, and bulk-flow velocities of the equilibrium, discussing the differences from previous models. We also present some illustrative plots of the distribution function in velocity space.

The ion temperature gradient: An intrinsic property of Earth's magnetotail

AbstractAlthough the ion temperature gradient along (XGSM) and across (ZGSM) the Earth's magnetotail, which plays a key role in generating the cross‐tail current and establishing pressure balance with the lobes, has been extensively observed by spacecraft, the mechanism responsible for its formation is still unknown. We use multispacecraft observations and three‐dimensional (3‐D) global hybrid simulations to reveal this mechanism. Using THEMIS (Time History of Events and Macroscale Interactions during Substorms), Geotail, and ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun) observations during individual, near‐simultaneous plasma sheet crossings from 10 to 60 RE, we demonstrate that the ion temperature ZGSM profile is bell‐shaped at different geocentric distances. This ZGSM profile is also prevalent in statistics of ~200 THEMIS current sheet crossings in the near‐Earth region. Using 3‐D global hybrid simulations, we show that mapping of the XGSM gradient of ion temperature along magnetic field lines produces such a bell‐shaped profile. The ion temperature mapping along magnetic field lines in the magnetotail enables construction of two‐dimensional distributions of these quantities from vertical (north‐south) spacecraft crossings. Our findings suggest that the ion temperature gradient is an intrinsic property of the magnetotail that should be considered in kinetic descriptions of the magnetotail current sheet. Toward this goal, we use theoretical approaches to incorporate the temperature gradient into kinetic current sheet models, making them more realistic.

Cassini observations of aperiodic waves on Saturn's magnetodisc

AbstractThe location and motion of Saturn's equatorial current sheet is the result of an interplay between a quasi‐static deformation that varies in radial distance and local time, impulsive perturbations that produce large‐scale displacements, quasiperiodic perturbations near the planetary rotation period, and wave‐like structures on shorter timescales. This study focuses on the latter, aperiodic wave pulses with periods from 1 to 30 min, which are unrelated to the quasiperiodic “flapping” with a period near that of Saturn's rotation. Cassini magnetometer data were surveyed for these aperiodic structures and then fitted to a simple model in order to estimate the properties of the waves. The model consists of a modified Harris current sheet model deformed by a Gaussian pulse wave function. This then allows for the extraction of wave parameters and current sheet properties. In particular, we show an increase in current sheet scale height with radial distance from Saturn, an increase in the wave amplitude with radial distance, and the resolution of propagation directions using the wave vector fitted by the model. The dominant propagation direction is found to be radially outward from Saturn.

Contrasting evidence of Holocene ice margin retreat, south‐western Greenland

ABSTRACTConstraining the Greenland Ice Sheet's (GrIS) response to Holocene climate change provides calibrations for ice sheet models that hindcast past ice margin fluctuations. Ice sheet models predict enhanced ice retreat in south‐western Greenland during the middle Holocene; however, few geological observations corroborating the extensive retreat are available. We present new data from lake sediment cores from the Isua region, south‐western Greenland, which provide constraints on Holocene fluctuations of the GrIS margins. Our data indicate that the main GrIS margin was 30 km west of its present‐day extent by ∼5.7k cal a BP and had retreated to within 3 km of its present‐day margin by ∼4.6k cal a BP; with subsequent re‐advances by ∼1.8 and at ∼1.7, ∼1.3 and ∼0.5k cal a BP. Times of ice retreat are coeval with increasing temperatures in western Greenland, suggesting that temperature was a dominant factor in ice retreat. The late retreat at Isua is in contrast to the early retreat observed in the Godthåbsfjord area and is probably related to the lack of fjords extending to the present Isua ice margin. Our data are not consistent with current ice sheet models that overestimate the middle Holocene ice retreat in the Isua region of south‐western Greenland.

The model of a collisionless current sheet in a homogeneous gravity field

The self-consistent 1D kinetic Harris-like model of a collisionless current sheet is developed for the case of the current sheet experiencing the impact of an external uniform gravity field. The ambipolar Pannekoek-Rosseland electric field appears in the system as a result of the additional drift motion of ions and electrons. This produces separation of charges, which is responsible for corresponding changes of the current sheet form. The presence of gravitation leads to formation of asymmetric distributions of the magnetic field as well as the plasma and the current density changes. Our estimations show that gravity-forced disruptions of the current sheet profile may occur in the Mercurial magnetosphere and, most probable, in the Io plasma torus near the Jupiter. Also, the model can be applied to magnetospheres of exoplanets.

Estimation of electromagnetic field in air by a magnetic dipole in the sea based on a current sheet model

Considerable efforts have been made to investigate the electromagnetic (EM) field in air produced by a magnetic dipole in sea. The conventional methods for this investigation mostly involve complicated analysis of Sommerfeld-type integrals. To conveniently provide a meaningful physical explanation of the EM field radiation in a two-layered media, this study considers a current sheet at the sea-to-air interface created by an underwater magnetic dipole. This current behaves like an antenna located on the surface and creates a wave that propagates through the atmosphere. Analysis of this current sheet can take advantage of the Huygens–Fresnel principle. The results using this approach can be verified by previous calculation and experiments on the sea, and show good agreement with experimental data. The present approach, which involves simple algebra, can easily explain the physical aspect of the EM wave propagation across the sea-to-air interface.

Seguridad quirúrgica y cumplimentación del registro de información intraquirúrgica en España: Un análisis comparativo de dos instrumentos de registro

El objetivo del presente estudio es describir y comparar los porcentajes de no cumplimentación de dos instrumentos de registro: hoja circulante (HC) y lista de verificación quirúrgica (LVQ), en un mismo entorno quirúrgico para una muestra de pacientes de características similares. <br /><br />Metodología: Estudio descriptivo realizado sobre registros intraquirúrgicos de 3024 pacientes de Cirugía de Ortopedia y Traumatología. 1732 pacientes intervenidos en 2009 con modelo de hoja circulante, cumplimentada al finalizar la intervención y 1292 en 2010 intervenidos con modelo de registro lista de verificación quirúrgica (checklist) cumplimentado durante la intervención en tres tiempos. Se han calculado características descriptivas (media, desviación típica, mínimo y máximo) del porcentaje de no cumplimentación global en ambos registros y el porcentaje de no cumplimentación (intervalo de confianza al 95%) de cada ítem de los registros estudiados. <br /><br />Resultados: Se observa mayor porcentaje de cumplimentación global y, en general, también individual, en la hoja circulante que en la lista de verificación quirúrgica. <br />Conclusiones: El registro intraquirúrgico que mayor porcentaje de cumplimentación ha tenido de manera global ha sido la hoja de circulante y se evidencia la necesidad de implantar estrategias para mejorar el grado de cumplimentación de la LVQ por su relación con la seguridad de pacientes.<br /><br />

Radiation Characteristics Analysis of Tri-Polarized Dipole Antenna

Dual-polarized antenna involves polarization purity degradation and power gain loss when scanning at wide angles. Aiming at mitigating the above limitations, we exploit a tri-polarized antenna for better radiation characteristics in this letter. We formulate the beampattern synthesis problem of a single tri-polarized dipole antenna in a convex form, and solve it with the Lagrange multiplier method. When circular polarization is synthesized, we show that degree-of-freedom increase improves the polarization purity and power gain of the multipolarized antenna, especially at wide scan angles. We further find that, using the infinite current sheet model, the radiation pattern synthesized by tri-polarized antenna is the same with the optimally oriented dual-polarized antenna - the dual-polarized antenna is mechanically rotated to be oriented normally to the scanning direction. We discuss the situation when linear polarization is synthesized. We show that the power gain is increased corresponding to scan angle, while the polarization purity is not improved obviously.

Kinetic model of force-free current sheets with non-uniform temperature

The kinetic model of a one-dimensional force-free current sheet (CS) developed recently by Harrison and Neukirch [Phys. Rev. Lett. 102(13), 135003 (2009)] predicts uniform distributions of the plasma temperature and density across the CS. However, in realistic physical systems, inhomogeneities of these plasma parameters may arise quite naturally due to the boundary conditions or local plasma heating. Moreover, as the CS spatial scale becomes larger than the characteristic kinetic scales (the regime often referred to as the MHD limit), it should be possible to set arbitrary density and temperature profiles. Thus, an advanced model has to allow for inhomogeneities of the macroscopic plasma parameters across the CS, to be consistent with the MHD limit. In this paper, we generalise the kinetic model of a force-free current sheet, taking into account the inhomogeneity of the density and temperature across the CS. In the developed model, the density may either be enhanced or depleted in the CS central region. The temperature profile is prescribed by the density profile, keeping the plasma pressure uniform across the CS. All macroscopic parameters, as well as the distribution functions for the protons and electrons, are determined analytically. Applications of the developed model to current sheets observed in space plasmas are discussed.

Tri‐polarised antenna – a virtual rotatable dual‐polarised antenna used in polarised beampattern synthesis

Tri-polarised antennas are exploited to synthesise the optimal polarised beampattern in this study. First, the characteristics of a single tri-polarised antenna are studied with an infinite current sheet model. Using the cross-product algorithm, the authors find that three co-located orthogonal current sheets are equivalent to a pair of orthogonal current sheets in a particular orientation. It implies that the tri-polarised antenna is just virtually equivalent to a rotatable dual-polarised antenna. Then by formulating the problem in a convex form, they adopt the convex optimisation algorithm to synthesise optimal polarised beampattern of the tri-polarised array. The simulation results show that due to the increased orientation degree of freedom, the sidelobe and cross-polarisation level of tri-polarised array are significantly lower than the dual-polarised array.

North–south component of galactic cosmic ray anisotropy at 1 AU

The galactic cosmic ray (GCR) solar diurnal anisotropy (SDA) may be represented by a vector (A; Ar, Aφ, Aθ) in a spherical polar coordinate system centered on the sun. We reported (elsewhere) the results of a detailed study of time variations of yearly radial (Ar) and east–west (Aφ) components recorded by the global network of the neutron monitors (NMs) with a long track record for 1963–2013, for four sunspot number (SSN) cycles (20–23) and the rising phase of cycle 24. A powerful new technique is used to compute and study time variations of the transverse component (Aθ) due to off-ecliptic GCR contributions, with the same NM data; GCR radial particle density gradient (Gr) drives all three components. The north–south anisotropy (Aθ) is computed from yearly NM data (Gϕ=0), a flat heliospheric current sheet (HCS) model and the concept of GCR isotropic hard sphere scattering in the solar wind plasma. Relationships to SSN, rigidity and solar polarity intervals are studied. For a positive (p-) polarity the solar magnetic field in the northern hemisphere points outward and GCRs drift from polar regions toward equatorial plane and out along HCS, setting up a symmetric gradient (Gθs) pointing away from HCS (there is a local GCR density minimum on HCS); for n-polarity interval Gθs points towards HCS (there is a local GCR density maximum on HCS). Also, there exists a heliospheric asymmetric density gradient (Gθa) perpendicular to the ecliptic plane, it is the main contributor to Aθ for the period of our analysis. This is the most interesting and significant insight.

A new stationary analytical model of the heliospheric current sheet and the plasma sheet

AbstractWe develop a single‐fluid 2‐D analytical model of the axially symmetric thin heliospheric current sheet (HCS) embedded into the heliospheric plasma sheet (HPS). A HCS‐HPS system has a shape of a relatively thin plasma disk limited by separatrices that also represent current sheets, which is in agreement with Ulysses observations in the aphelion, when it crossed the HCS perpendicular to its plane. Our model employs a differential rotation of the solar photosphere that leads to unipolar induction in the corona. Three components of the interplanetary magnetic field (IMF), the solar wind speed, and the thermal pressure are taken into account. Solar corona conditions and a HCS‐HPS system state are tied by boundary conditions and the “frozen‐in” equation. The model allows finding spatial distributions of the magnetic field, the speed within the HPS, and electric currents within the HCS. An angular plasma speed is low within the HPS due to the angular momentum conservation (there is no significant corotation with the Sun), which is consistent with observations. We found that the HPS thickness L decreases with distance r, becoming a constant far from the Sun (L ~2.5 solar radii (R0) at 1 AU). Above the separatrices and at large heliocentric distances, the solar wind behavior obeys Parker's model, but the magnetic field spiral form may be different from Parker's one inside the HPS. At r ≤ 245 R0, the IMF spiral may undergo a turn simultaneously with a change of the poloidal current direction (from sunward to antisunward).

Influence of Non-Potential Coronal Magnetic Topology on Solar-Wind Models

By comparing a magneto-frictional model of the low coronal magnetic field to a potential-field source-surface model, we investigate the possible impact of non-potential magnetic structure on empirical solar-wind models. These empirical models (such as Wang-Sheeley-Arge) estimate the distribution of solar-wind speed solely from the magnetic-field structure in the low corona. Our models are computed in a domain between the solar surface and 2.5 solar radii, and are extended to 0.1 AU using a Schatten current-sheet model. The non-potential field has a more complex magnetic skeleton and quasi-separatrix structures than the potential field, leading to different sub-structure in the solar-wind speed proxies. It contains twisted magnetic structures that can perturb the separatrix surfaces traced down from the base of the heliospheric current sheet. A significant difference between the models is the greater amount of open magnetic flux in the non-potential model. Using existing empirical formulae this leads to higher predicted wind speeds for two reasons: partly because magnetic flux tubes expand less rapidly with height, but more importantly because more open field lines are further from coronal-hole boundaries.

Two-dimensional MHD model of the Jovian magnetodisk

A self-consistent stationary axially symmetric MHD model of the Jovian magnetodisk is constructed. This model is a generalization of the models of plane current sheets that have been proposed earlier in order to describe the structure of the current sheet in the magnetotail of the Earth [1, 2]. The model takes centrifugal force, which is induced by the corotation electric field, and the azimuthal magnetic field into account. The configurations of the magnetic field lines for the isothermic (plasma temperature assumed to be constant) and the isentropic (plasma entropy assumed to be constant) models of the magnetodisk are determined. The dependence of the thickness of the magnetodisk on the distance to Jupiter is obtained. The thickness of the magnetodisk and the magnetic field distribution in the isothermic and isentropic models are similar. The inclusion of a low background plasma pressure results in a considerable reduction in the thickness of the magnetodisk. This effect may be attributed to the fact that centrifugal force prevails over the pressure gradient at large distances from the planet. The mechanism of unipolar induction and the related large-scale current system are analyzed. The direct and return Birkeland currents are determined in the approximation of a weak azimuthal magnetic field. The modeling results agree with theoretical estimates from other studies and experimental data.

Current sheets with inhomogeneous plasma temperature: Effects of polarization electric field and 2D solutions

We develop current sheet models which allow to regulate the level of plasma temperature and density inhomogeneities across the sheet. These models generalize the classical Harris model via including two current-carrying plasma populations with different temperature and the background plasma not contributing to the current density. The parameters of these plasma populations allow regulating contributions of plasma density and temperature to the pressure balance. A brief comparison with spacecraft observations demonstrates the model applicability for describing the Earth magnetotail current sheet. We also develop a two dimensional (2D) generalization of the proposed model. The interesting effect found for 2D models is the nonmonotonous profile (along the current sheet) of the magnetic field component perpendicular to the current sheet. Possible applications of the model are discussed.

Formation of self‐organized shear structures in thin current sheets

AbstractSelf‐consistent kinetic (particle‐in‐cell) model of magnetotail thin current sheet (TCS) is used to understand the formation of self‐consistent sheared magnetic structures. It is shown that shear configurations appear in TCS as a result of self‐consistent evolution of some initial magnetic perturbation at the current sheet center. Two general shapes of shear TCS components are found as a function of the transverse coordinate: symmetric and antisymmetric. We show that TCS formation goes together with the emergence of field‐aligned currents in the center of the current sheet, as a result of north‐south asymmetry of quasi‐adiabatic ion motions. Ion drift currents can also contribute to the magnetic shear evolution, but their role is much less significant, their contribution depending upon the normal component Bz and the amplitude of the initial perturbation in TCS. Parametric maps illustrating different types of TCS equilibria are presented that show a higher probability of formation of symmetric shear TCS configuration at lower values of the normal magnetic component.

Two‐dimensional configuration of the magnetotail current sheet: THEMIS observations

AbstractIn this paper we investigate 2‐D configuration of the magnetotail current sheet. We use measurements of magnetic field and plasma parameters by Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission in 2008–2009 to restore for the first time the current sheet configuration at three distances downtail (i.e., in the corresponding locations of THEMIS spacecraft: x <− 25RE, x ∼− 17RE, and x ∼− 10RE). The vertical velocity of current sheets (flapping motion) allows estimating the current sheet thickness L(x) and the current density j0(x)≈(c/4π)B0/L, where B0(x) is the amplitude of Bx field. We have shown that the current sheet configuration is more or less stable (spacecraft observe the same current sheet thickness and magnetic field amplitudes for successive crossings of the neutral plane) for time intervals around 1–3 h. The typical 2‐D configuration of the magnetotail current sheet assumes the increase of L(x) and magnetic fields B0(x), Bz(x) toward the Earth, while the ratio Bz/B0 is almost constant along the magnetotail. Observed current sheets are very stretched with the typical scale of Bz(x) inhomogeneity Lx≫LB0/2Bz. We compare obtained results with existing current sheet models.

3-D FEM Analysis of a Novel Magnetic Levitation System

This paper deals with the 3-D finite element method (FEM) analysis of a novel magnetic levitation (maglev) train driven by an air-cored tubular linear induction motor. This new maglev system originates from electromagnetic launcher (EML) technology, especially linear induction launcher (LIL) type. Some 2-D magnetic analyses based on current sheet model or transmission line approach for the LIL type launchers are available in the literature, but this paper provides an alternative way to analyze LIL type of EMLs using 3-D FEM. The analysis examines the variation of propulsion, levitation, and guidance forces induced in the moving part. It is expected that the possible results of this magnetic analysis will lead to some new and important design considerations for the novel maglev system.

Inductance extraction of superconductor structures with internal current sources

The sheet current model underlying the 3D-MLSI software package for calculation of inductances of multilayer superconducting circuits has been further elaborated. The developed approach permits us to overcome serious limitations on the shape of the circuit layout and opens the way for simulation of internal contacts or vias between layers. Two models for internal contacts have been considered. They are a hole as a current terminal and a distributed current source. Advantages of the developed approach are illustrated by calculating the spatial distribution of the superconducting current in several typical layouts of superconducting circuits. A new meshing procedure now permits us to implement triangulation for joint projection of all nets, thus improving the discrete physical model for inductance calculations of circuits made in both planarized and non-planarized fabrication processes. To speed up triangulation and build a mesh of better quality, we adopt the known program Triangle.