Structure Of Current Sheet Research Articles

On the scattering of charged particles in complex current configurations

The scattering of charged particles in the magnetic fields of current sheets with bell-shaped and double-humped current density profiles has been studied analytically and numerically. The particle scattering processes are shown to depend significantly on the structure of the current sheet. Thus, for example, when particles are scattered in double current sheets, a regime under which the compensation of two successive jumps in particle magnetic moments can lead to a quasi-regular type of motion, instead of a chaotic one, can be maintained in the system.

A Flux Emergence Model for Solar Eruptions

We have simulated the 3D emergence and interaction of two twisted flux tubes, which rise from the interior into the outer atmosphere of the Sun. We present evidence for the multiple formation and eruption of flux ropes inside the emerging flux systems and hot arcade-like structures in between them. Their formation is due to internal reconnection, occurring between oppositely directed, highly stretched and sheared fieldlines at photospheric heights. Most of the eruptions escape into the corona, but some are confined and fade away without leaving the low atmosphere. As these flux ropes erupt, new reconnected fieldlines accumulate around the main axis of the initial magnetic flux systems. We also show the complex 3D fieldline geometry and the structure of the multiple current sheets, which form as a result of the reconnection between the emerging flux systems.

A 3D kinetic-fluid numerical code for stationary equilibrium states in magnetized plasmas

In this paper we present a detailed description of a new stationary three-dimensional hybrid code with ions represented by particles and electrons by a massless fluid. We solve the ion equations of motion and ion distribution function moments are obtained. Conversely, we use a fluid approximation for the electrons. We determine the electron bulk velocity perpendicular to the magnetic field from the electron momentum equation with m e → 0 and without collisions. The parallel component of the electron bulk velocity is derived from the electron continuity equation. Once we have calculated the total current density, we determine the magnetic field from the Ampere's law, applying FFT algorithms, and from Faraday's law, we are able to calculate the components of the electric field. We use a recursive technique to obtain a self-consistent solution of our set of equations. We use this code to study how electrons influence the structure of the magnetotail current sheet in many physical processes as, for example, the formation of a double peak in the current density, which is observed very often by the CLUSTER spacecraft crossing of the neutral sheet.

Identifying Current-Sheet-like Structures in the Solar Wind

In past years, measurements of the solar wind plasma have advanced our understanding of MHD turbulence tremendously. At small scales, the solar wind is believed to be ve`xry multifractal with nonlinear interactions causing an intermittent energy dissipation, leading to possible current-sheet structures. In this Letter, we propose a systematic data analysis procedure to examine the existence of current sheets in the solar wind. We show that by studying the integrated distribution function F(θ , ζ) of the angle between two unit magnetic fields and , as well as its ζ-dependence, one can unambiguously identify the existence of current-sheet-like structures in the solar wind. Using this procedure, we analyze magnetic field data from the VHM/FGM instrument on board the spacecraft Ulysses for two periods, one in solar maximum and the other in solar minimum. In both cases, current sheets are clearly inferred. Furthermore, we also outline a procedure that allows the identification of the actual locations of these current sheets. Results from our analysis and the implications of the existence of current sheets in the solar wind are discussed.

Holographic interferometry study of two-fluid properties of the plasma in current sheets formed in heavy noble gases

Two-exposure holographic interferometry was used to study the structure of current sheets formed in three-dimensional magnetic configurations with a singular X line in heavy noble gases (Ar, Kr, and Xe). It is found that, in the presence of a longitudinal magnetic field BZ directed along the X line, plasma sheets take on an unusual shape: they are titled and asymmetric. Their asymmetry becomes more pronounced as the mass of a plasma ion increases—a manifestation of the two-fluid properties of the plasma. The observed effects can be attributed to additional forces arising due to the interaction of the longitudinal magnetic field BZ with Hall currents excited in a plane perpendicular to the X line. A qualitative model describing plasma dynamics with allowance for the Hall effect and accounting for most of the experimentally observed effects is proposed.

X - and Y-line equilibria

In this Brief Communication, it is shown that an analytical solution for an exact two-dimensional X- or Y-line equilibrium may be constructed quite easily on the basis of the fundamental theoretical equation. The equilibria discussed in the present report show the potential for a rich variety of current sheet structures that satellites may encounter in space.

Thinning and stretching of the plasma sheet

With Cluster observations in the magnetotail we study dynamics of plasma sheet thinning and stretching during 39 intervals associated with substorm growth phases. The cross‐tail current density and normal magnetic field generally scale as Bn ∼ TpNp1/2/J0, but with frequent transient variations. Typical pre‐onset values are Bz ∼ 1–2 nT, J0 ∼ 4–8 nA/m2, thickness (Harris estimate) >3000 km. A current density increase in each particular event is not accompanied with a corresponding number density increase. About 30% of the events are characterized by a large (>5 nT) field component parallel to the current (in most of cases equal to By), implying adiabatic particle dynamics even with small Bz. Most local onsets, associated with the ends of thin sheet intervals, were accompanied with tailward plasma flows. In some cases embedded current sheet structure was detected and, therefore, estimation of thickness requires caution.

Cluster Observation and Analysis of Bifurcation Current Sheet Structure in a Quiet Time

AbstractWe analyse the vertical structure of the magnetotail current sheet during 2011~2030 UT on 19 August 2001, during which the Cluster spacecrafts crossed the current sheet in a quiet time. In the intervals, the current sheet moved slowly, and the value of the AE index was relatively small, about 40~130 nT. In the quiet time, without fast plasma flow and without significant flapping motion, we also directly observed the bifurcated current sheet. Ion density, velocity and temperature, measured by Cluster spacecrafts in the current sheet during 2011~2030 UT 19 August 2001 are discussed. It is shown that ion density was comparatively bigger near the neutral line, presented non‐symmetry distribution, but the distribution of temperature was uniform. And it is observed that in the y direction, the proton bulk velocity is opposite on both sides of the neutral line. Finally we discuss a possible mechanism for the bifurcated current sheet in a quiet time, which is nonlinear evolution of the lower‐hybrid drift instability in the current sheet. These may be important for perfecting the formation mechanism of current sheet bifurcation.

A three‐dimensional kinetic‐fluid numerical code to study the equilibrium structure of the magnetotail: The role of electrons in the formation of the bifurcated current sheet

In this work we present a new stationary three‐dimensional kinetic‐fluid code with ions represented by particles and electrons by a massless fluid. The magnetotail current sheet is modeled as a magnetic field reversal with a normal magnetic field component Bn. A cross tail electric field Ey is included. An ion test particle simulation is performed in these fields assuming an anisotropic plasma source at the magnetospheric lobes, and ion distribution functions with their moments, such as density, current, and temperature are obtained. The plasma is assumed to be quasineutral and we use the electron momentum equation (with me → 0 and without collisions) and the continuity equation to derive the electron current density. We set up an iterative technique to obtain a self‐consistent solution of our set of equilibrium model equations. We use this code to study how electrons influence the structure of the magnetotail current sheet and how they contribute to the formation of a double peak in the cross‐tail current density, which is observed very often by CLUSTER when crossing the neutral sheet. We find that the electron finite Larmor radius term and the electron drift term, are responsible for the formation of a double peak in the total current density even in those case where the ion current density does not display any bifurcated structure. We also obtained an electric field component normal to the current sheet, which is often observed. Another interesting and physically relevant result which we obtained, is the formation of counterstreaming beamlets in the plasma sheet at some distance from the current sheet, which were observed by Galileo spacecraft in 1990. Finally, we compare the simulation results with the most recent CLUSTER observations.

Magnetic structure of current sheets in magnetic fields with a singular X line

Direct measurements of magnetic fields in a plasma show that current sheets can form in magnetic configurations with an X line in the presence of a longitudinal magnetic field. It is found that, in a plane perpendicular to the X line and to the direction of the main current, the current sheet has two very different dimensions. The tangential and normal components of the magnetic field and current density in the sheet are determined. The influence of the initial conditions (such as the strength of the longitudinal magnetic field, the gradient of the transverse field, and the plasma ion mass) on the current sheet parameters is investigated.

Study of the structure and dynamics of current sheets in three-dimensional magnetic configurations with an X line by holographic interferometry

Results are presented from studies of the structure and dynamics of current sheets in three-dimensional magnetic configurations with an X line by means of holographic interferometry. It is found that the efficiency of plasma compression into the sheet is reduced as the longitudinal magnetic field B z , directed along the X line, increases. This effect is attributed to the enhancement of the longitudinal component of the magnetic field within the sheet and to the corresponding increase in the magnetic pressure. It is shown that the formation of a plasma sheet lags behind the beginning of the plasma current pulse, the delay time being close to the characteristic Alfven time.

Parameter dependence of quick magnetic reconnection triggering: A survey study using two-dimensional simulations

[1] The current sheet structure modification by lower-hybrid drift instability (LHDI) has been shown to lead to quick magnetic reconnection triggering (QMRT) even in a thick current sheet (Tanaka et al., 2005). The three-dimensional simulations, however, have been limited to certain sets of parameters. Having learned which effects of LHDI are crucial for QMRT in these three-dimensional results, we perform a series of two-dimensional simulations of LHDI (reconnection itself not included) with various current sheet thickness D, guide field intensity B0y, and the frequency ratio τ (τ = ωpe/Ωe, where ωpe is the electron plasma frequency and Ωe is the electron gyrofrequency) and inspect how these parameters affect the QMRT process. While QMRT looked promising in the three-dimensional studies with B0y = 0 and τ = 1, the two realistic factors, nonzero guide field and τ larger than 1, requires rather small current sheet thickness for QMRT to be operative in the magnetospheric situations.

Dynamics of thin current sheets associated with magnetotail reconnection

We have examined the detailed structure of thin current sheets and their evolution during a substorm interval on 24 August 2003, when Cluster experienced several rapid current sheet crossings within a couple of ion gyrotimes. These crossings took place during an interval of high‐speed ion flows with BZ reversals and signatures of accelerated electrons, suggesting crossing of the reconnection region. On the basis of four‐point observations with a tetrahedron scale of ∼200 km, we could quantify for the first time the thickness of the current sheet, which was comparable to or less than one ion inertia length, and resolve some internal structures such as multiple peaks within these thin current sheets. Different patterns in jX and in electron anisotropy were identified during the current sheet crossings: two crossings during tailward flow interval exhibited a quadrupole‐type Hall current in the ion diffusion region without a guide field, while one crossing during earthward flow showed a current system as predicted in the ion diffusion region under the presence of a guide field. Multiple flux rope type signatures or transient skewed structures are observed in the thin current sheets, particularly in regions where signatures of electron acceleration are observed. These observations suggest that three‐dimensional localized/transient structures could play an essential role in the dynamics of the thin current sheets, while a gross X‐line picture can be established only in an average sense.

Bifurcated Current Sheet Structure in a Quiet Time by ClusterSpacecrafts

We analyse the vertical structure of the magnetotail current sheetfor two time intervals during which the Cluster spacecraftscrossed the neutral sheet in a quiet time. In the intervals, thecurrent sheet moved slowly, and the value of the AE index wasrelatively small, about 40–130 nT. We find two examples of currentsheets, with the current density maximum at the magnetic equator (Bx = 0), as well as an example of off-centre or bifurcated current sheets.In the quiet time, without any fast plasma flow and withoutsignificant flapping motion, we also directly observed the bifurcatedcurrent sheet. The bifurcated current sheet is probably associated withinstabilities in the current sheet. These may be important forresearching the mechanism of current sheet bifurcation.

Local structure of the magnetotail current sheet: 2001 Cluster observations

Abstract. Thirty rapid crossings of the magnetotail current sheet by the Cluster spacecraft during July-October 2001 at a geocentric distance of 19 RE are examined in detail to address the structure of the current sheet. We use four-point magnetic field measurements to estimate electric current density; the current sheet spatial scale is estimated by integration of the translation velocity calculated from the magnetic field temporal and spatial derivatives. The local normal-related coordinate system for each case is defined by the combining Minimum Variance Analysis (MVA) and the curlometer technique. Numerical parameters characterizing the plasma sheet conditions for these crossings are provided to facilitate future comparisons with theoretical models. Three types of current sheet distributions are distinguished: center-peaked (type I), bifurcated (type II) and asymmetric (type III) sheets. Comparison to plasma parameter distributions show that practically all cases display non-Harris-type behavior, i.e. interior current peaks are embedded into a thicker plasma sheet. The asymmetric sheets with an off-equatorial current density peak most likely have a transient nature. The ion contribution to the electric current rarely agrees with the current computed using the curlometer technique, indicating that either the electron contribution to the current is strong and variable, or the current density is spatially or temporally structured.

Current Sheet Evolution in the Aftermath of a CME Event

We report on SOHO UVCS observations of the coronal restructuring following a coronal mass ejection (CME) on 2002 November 26, at the time of a SOHO-Ulysses quadrature campaign. Starting about 1.5 hr after a CME in the northwest quadrant, UVCS began taking spectra at 1.7 R, covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 A line of [Fe XVIII], indicating temperatures above 6 x 10(exp 6) K, was observed throughout the campaign in a spatially limited location. Comparison with EIT images shows the [Fe XVIII] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fe XVIII] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of our observations: in particular, we give the temperature versus time in the current sheet and estimate its density. At the time of the quadrature, Ulysses was directly above the location of the CME and intercepted the ejecta. High ionization state Fe was detected by the Ulysses SWICS throughout the magnetic cloud associated with the CME, although its rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. The SOHO-Ulysses data set provided us with the unique opportunity of analyzing a current sheet structure from its lowest coronal levels out to its in situ properties. Both the remote and in situ observations are compared with predictions of theoretical CME models.

Correction to “Signatures of nonlinear charged particle dynamics in Geotail comprehensive plasma instrument observations”

[1] During the preparation of an article in which we exploit the methodology presented in the paper ‘‘Signatures of nonlinear charged particle dynamics in Geotail comprehensive plasma instrument observations’’ by D. L. Holland, W. R. Paterson, L. A. Frank, S. Kokubun, and Y. Yamamoto (Journal of Geophysical Research, 104, 2479, 1999), we came across two errors. Neither of them undermines the basic conclusion that the ion energy resonance signature allows one to infer a lot of information about the mesoscale current sheet structure. It is, however, important to correct the errors since they directly affect the application of the technique. [2] The first error concerns the derivation of equation used for inferring current sheet topology from the resonance peak. The proper derivation is as follows. In their original paper describing the energy resonance phenomena, Burkhart and Chen [1991] followed an source distribution of particles through the modified Harris magnetic field

Determination of the quiet time current sheet thickness using Geotail CPI data and nonlinear dynamics modeling

[1] In this paper we present a survey of the quiet (Kp < 1+) current sheet thickness for X GSE between -20 R E and -80 R E as determined from an ion distribution function signature of nonlinear particle dynamics in current sheet-like magnetic fields. The signature manifests itself as a series of peaks in the ion distribution function whose separation depend on the fourth root of the energy and parameters that describe the current sheet structure. We have found clear evidence of the distribution function signature throughout the entire region of interest. Analysis of the data shows that the current sheet thickness is remarkably uniform in the region under study with an average thickness of 0.76 ± 0.56 R E . This result is consistent with measurements of the quiet time current sheet thickness made using other techniques.

Self‐consistent Magnetohydrodynamic Modeling of Current Sheet Structure and Heating Using Realistic Descriptions of Transport Processes

A magnetohydrodynamic (MHD) model of an electron-ion, collision-dominated plasma that includes the electrical conductivity and thermoelectric tensors in Ohm's law is used to generate current sheet solutions in parameter ranges that correspond to those of the solar transition region and lower corona. The model contains a prescribed sheared magnetic field with a characteristic length scale L. The characteristic sheet width is 2L, but it is found that the temperature has transition region or coronal values only within a diffusion region (DR) with a width several orders of magnitude smaller than 2L. The heating rate per unit mass and flow speed in the DR are orders of magnitude larger, and the density is orders of magnitude smaller than in the surrounding plasma. The heating rate per unit volume is a maximum in the DR and falls off steadily outside the DR. The Joule heating rate and current density each consist of a conduction component driven by the center-of-mass electric field and a thermoelectric component driven by the temperature gradient. It is found that these components largely cancel, leading to a total heating rate and current density orders of magnitude smaller than either of their components. This suggests that thermoelectric current drive is important in determining current sheet structure. The center-of-mass electric field that provides the energy to maintain the plasma in a steady state is almost entirely the convection electric field. The electron magnetization Me is the product of the electron cyclotron frequency and the electron-ion collision time. Nonzero values of Me cause the conductivity and thermoelectric tensors to be anisotropic. It is found that the large values of Me that occur in the DR increase the heating rates per unit volume and mass by several orders of magnitude and can change the sign of the heating rate per unit mass from negative to positive, corresponding to a change from a cooling process to a heating process. This suggests that electron magnetization, and hence anisotropic transport, is a major factor in current sheet heating.

Global structure of Jupiter's magnetospheric current sheet

Jupiter's magnetosphere contains a gigantic sheet‐like structure located near its dipole magnetic equator that contains most of the plasma and energetic particles swirling in Jupiter's magnetosphere. Called the “current sheet,” it behaves like a rigid structure inside a radial distance of ∼50 RJ where the periodic reversals of the Br component are highly predictable. Beyond a radial distance of ∼25 RJ, the tilted current sheet lags behind the dipole magnetic equator in proportion to the radial distance of the observer. On the nightside, at radial distances &gt;50 RJ, the current sheet is seen to become parallel to the solar wind flow direction. In this work, we analyze magnetic field observations from all six spacecraft that have explored Jupiter's magnetosphere (Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, and Galileo) to determine the global structure of Jupiter's current sheet. We have assembled a database of 6328 current sheet crossings by using an automated procedure which utilizes reversals in the radial component of the magnetic field to identify current sheet crossings. The assembled database of current sheet crossings spans all local times in Jupiter's magnetosphere under differing solar wind conditions. The new model is based on a further generalization of the hinged‐magnetodisc models of Behannon et al. (1981) and Khurana (1992). Four new features of the improved model are that (1) close to Jupiter, the prime meridian of the current sheet (the azimuthal direction in which it attains its highest inclination) is found to be shifted by 2.2° from the VIP4 model current sheet (Connerney et al., 1998). (2) In addition to the delay caused by the wave travel time, the location of the current sheet is further delayed because of the sweep‐back of the field lines. (3) The signal delay associated with wave propagation is seen to vary both with radial distance and local time, and (4) the current sheet is allowed to become parallel to the solar wind direction at large distances in the magnetotail in agreement with the observations. The new model is much superior at predicting current sheet crossing times than previously published models (especially in the midnight and dusk sectors). The RMS error of fit between the modeled and observed current sheet crossing longitudes is 19.3°. A comparison of the new model with previous models is presented.