Structure Of Current Sheet Research Articles

Current Sheets with Multicomponent Plasma in Magnetospheres of Planets of the Solar System

A self-consistent hybrid model of a thin current sheet (TCS) with a thickness of the order of several ion gyroradii is proposed that takes into account the multicomponent nature of collisionless space plasma. Several plasma components can be present in the tails of the magnetospheres of terrestrial planets (for example, the Earth, Mercury, Mars, and Venus). Variations in the current sheet (CS) structure in magnetospheric plasma in the presence of heavy oxygen ions with different characteristics are analyzed. It is shown that high relative concentrations of oxygen ions, as well as their relatively high temperatures and drift velocities, lead to significant thickening of CS and the formation of an additional embedded scale. In this case, on the profiles of the main characteristics—current density and magnetic field—symmetric breaks appear, which correspond to a sharp change in the gradients of variation in values. A comparison is performed and a qualitative agreement is shown between the simulation results and observational data in the tail of the Martian magnetosphere.

Comparison of the Flank Magnetopause at Near‐Earth and Lunar Distances: MMS and ARTEMIS Observations

AbstractOne of the main sources of magnetospheric particles is solar wind penetration through the magnetopause—a current sheet separating the cold, dense magnetosheath from the hot, rarified magnetospheric plasmas. The mechanism responsible for magnetosheath particle transport across the magnetopause has been better investigated for the near‐Earth dayside magnetopause (contrary to the nightside), where it was found that such a transport is controlled by the current sheet thickness, structure, and dynamics. Because plasma properties and magnetic field intensity in the magnetosheath significantly change as a function of radial distance from the Earth, the current sheet characteristics are different near Earth and at distant nightside magnetopause. Comparative investigations between near‐Earth and distant magnetopauses, however, require statistical observations at the two locations during the same time interval, that enable control for the same upstream solar wind driving conditions. In this paper, we perform such a study using the four Magnetosheric Multiscale (MMS) and two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes to compare the characteristics of magnetic field and plasma populations during magnetopause crossings, which are separated by about 50 RE. We find that the current sheet profiles is similar at two locations. We also show that the magnetopause current sheet thickness scales with the local magnetosheath ion gyroradius. A weaker magnetic field on both sides of the current sheet is correlated with smaller current density at lunar distances.

Determination of the Structure of Current Sheets and Detection of Sheet Disruption Based on Measurements by External Magnetic Probes

A simple macroscopic method for detection of current sheet disruptions and determination of the width of metastable sheets is proposed based on the measurements of magnetic fields at the external surface of the vacuum chamber. This contactless diagnostic method does not affect the plasma processes in the current sheet and allows one to increase the signal-to-noise ratio. Such changes will allow us to relatively quickly determine the conditions of current sheet development under which sheet disruptions are most probable.

Ionospheric Outflow During the Substorm Growth Phase: THEMIS Observations of Oxygen Ions at the Plasma Sheet Boundary

AbstractIonospheric outflow is an important plasma source that feeds the near‐Earth magnetotail with heavy oxygen ions. Because these ions can significantly alter the structure and stability of the magnetotail current sheet, the characteristics of this outflow are important for accurate magnetosphere modeling, including modeling of substorms—a key element of magnetosphere dynamics. Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measurements in the magnetotail (around the plasma sheet boundary), we investigate characteristics oxygen outflows observed during substorm growth phases. The observed oxygen ion temperature and flow energy indicate that the outflow is marginally stable to ion acoustic wave generation: The oxygen temperature is slightly lower than the electron temperature and slightly higher than the oxygen flow energy. Moreover, the observed outflows are accompanied by low frequency electrostatic waves that may contribute to outflow thermalization. The oxygen bulk velocity has a significant component directed toward the equatorial plane, originating from the cross‐field drift in the convection electric field. The estimated radial distances at which oxygen ions reach the plasma sheet are ∼20–40RE downtail, that is, outflows during substorm growth phases can alter current sheet characteristics around the potential magnetic reconnection region.

Modeling Observable Differences in Flare Loop Evolution due to Reconnection Location and Current Sheet Structure

Abstract Flare reconnection is expected to occur at some point within a large-scale coronal current sheet. The structure of the magnetic field outside this sheet is almost certain to affect the flare, especially its energy release. Different models for reconnection have invoked different structures for the current sheet’s magnetic field and different locations for the reconnection electric field within it. Models invoking Petschek-type reconnection often use a uniform field. Others invoke a field bounded by two Y-points with a field strength maximum between them and propose this maximum as the site of the reconnection electric field. Still other models, such as the collapsing trap model, require that the field strength peak at or near the edge of the current sheet and propose that reconnection occurs above this peak. At present there is no agreement as to where reconnection might occur within a global current sheet. We study the post-reconnection dynamics under all these scenarios, seeking potentially observable differences between them. We find that reconnection occurring above the point of strongest field leads to the highest density and the highest emission measure of the hottest material. This scenario offers a possible explanation of superhot coronal sources seen in some flares.

Formation of Post-CME Blobs Observed by LASCO-C2 and K-Cor on 2017 September 10

Abstract Understanding the formation of post-CME blobs, we investigate 2 blobs in the outer corona observed by LASCO-C2 and 34 blobs in the inner corona by K-Cor on 2017 September 10 from 17:11 to 18:58 UT. By visual inspection of the structure of a post-CME current sheet (CS) and its associated blobs, we find that the CS is well identified in the K-Cor and its radial lengths are nine times longer than lateral widths, indicating the CS is unstable to the linear tearing mode. The inner corona blobs can be classified into two groups: 27 blobs generated in the middle of the CS (Group 1) and 7 blobs occurred above the tips of it (Group 2). Their lateral widths are and , which is smaller than, or similar to, those of the CS. They have elongated shapes: ratios of lateral to radial widths are and , respectively. In the first group, only three blobs propagate above the tip of the CS while the others are located in the CS. In the second group, only two blobs have associations with those of outer corona in their temporal and spatial relationship and their initial heights are 1.81 and 1.95 R ☉, measured from the center of the Sun. The others cannot be identified in the outer corona. Our results first demonstrate that LASCO-C2 blobs could be generated by the tearing mode instability near the tips of post-CME CSs, similar to the magnetic reconnection process in the tail CS of Earth's magnetosphere.

Influence of Oxygen Ions on the Structure of the Thin Current Sheet in the Earth’s Magnetotail

During geomagnetic substorms, the current sheet in the Earth’s magnetotail can transversely reduce in thickness from a few radii of the Earth (RE) to one to several proton gyroradii, 250–2000 km. It is the key structure in which the energy of the magnetic field is stored and later released due to the development of instability and magnetic reconnection during the substorm period. Despite its small thickness, the thin current sheet has a complex multiscale structure with a hierarchy of embedded layers that determines its properties. During substorms, single-charged oxygen ions enter the Earth’s magnetotail from the ionosphere and their concentrations can be comparable to those of protons. The interaction of oxygen ions with the current sheet, which results in changes in its structure and properties, is not well studied. The self-consistent profiles of the magnetic field, current densities, and plasma in the multicomponent magnetotail plasma are analyzed in a wide range of system parameters within the hybrid model of the quasi-equilibrium current sheet. It is shown that the current sheet is a multiscale structure embedded in a wide plasma layer. The increase in the concentration of oxygen ions in the current sheet leads to its thickening and formation of additional embedded scale. At the same time, breaks characterizing the transition from the oxygen-ion dominated region in the current sheet to the proton- and electron-dominated region appear on the profiles of the magnetic field and current density. The amplitude of the current density of such an embedded layer decreases in proportion to the concentration of oxygen ions. The dependence of the embedding parameter on relative concentrations of heavy ions, as well as their thermal and drift velocities, is studied.

Contribution of Anisotropic Electron Current to the Magnetotail Current Sheet as a Function of Location and Plasma Conditions

AbstractThe magnetotail current sheet carries the current responsible for the largest fraction of the energy storage in the magnetotail, the magnetic energy in the lobes. It is thus inextricably linked with the dynamics and evolution of many magnetospheric phenomena, such as substorms. The magnetotail current sheet structure and stability depend mostly on the kinetic properties of the plasma populating the magnetotail. One of the most underinvestigated properties of this plasma is electron temperature anisotropy, which may contribute a large fraction of the total current. Using observations from five missions in the magnetotail, we examine the electron temperature anisotropy, Te‖/Te⊥, and its potential contribution to the current density, quantified by the firehose parameter (βe‖−βe⊥)/2, across y∈[−20,20]RE and x∈[−100,−10]RE. We find that a significant fraction (>30%) of all current sheets have an anisotropic electron current density >10% of the total current. These current sheets form two distinct groups: (1) near‐Earth (<30 RE) accompanied by weak plasma flows (<100 km/s) and enhanced equatorial magnetic field (>3 nT) and (2) middle tail (>40 RE) accompanied by fast plasma flows (>300 km/s) and small equatorial magnetic field (≤1 nT). For a significant number of near‐Earth current sheets, the anisotropic electron current can be >25% of the total current density. Our findings suggest that electron temperature anisotropy should be included in current sheet models describing realistic magnetotail structure and dynamics.

Vertical Current Density Structure of Saturn's Equatorial Current Sheet

AbstractRoutine spacecraft encounters with the Saturn current sheet due to the passage of aperiodic waves provide the opportunity to analyze the current sheet structure. The current density is expected to peak where the field strength reaches a minimum if approximated as a Harris current sheet. However, in Earth's magnetotail this is not always the case as the sheet is sometimes bifurcated (having two or more maxima in the current density). We utilize measurements of Saturn's magnetic field to estimate the current density during crossings of the current sheet by time differentiating the Ba component of the field in a current sheet coordinate system, where Ba is perpendicular to both the current and current sheet normal. This is then averaged and organized by the magnitude of Ba. Using this method, we can identify a classical Harris‐style or bifurcated current sheet as a peak at the center or two distinct maxima on either side of Ba=0, respectively. We find that around 10% of current sheet profiles exhibit a bifurcated current sheet signature, which is substantially lower than an ∼25% occurrence rate at Earth.

Modeling Star–Planet Interactions in Far-out Planetary and Exoplanetary Systems

Abstract The magnetized wind from a host star plays a vital role in shaping the magnetospheric configuration of the planets it harbors. We carry out three-dimensional (3D) compressible magnetohydrodynamic simulations of the interactions between magnetized stellar winds and planetary magnetospheres corresponding to a far-out star–planet system, with and without planetary dipole obliquity. We identify the pathways that lead to the formation of a dynamical steady-state magnetosphere and find that magnetic reconnection plays a fundamental role in the process. The magnetic energy density is found to be greater on the nightside than on the dayside, and the magnetotail is comparatively more dynamic. It is found that stellar wind plasma injection into the inner magnetosphere is possible through the magnetotail. We further study magnetospheres with extreme tilt angles, keeping in perspective the examples of Uranus and Neptune. High dipole obliquities may also manifest due to polarity excursions during planetary field reversals. We find that global magnetospheric reconnection sites change for large planetary dipole obliquity, and more complex current sheet structures are generated. We discuss the implications of these findings for atmospheric erosion, the introduction of stellar and interplanetary species that modify the composition of the atmosphere, auroral activity, and magnetospheric radio emission. This study is relevant for exploring star–planet interactions and its consequence on atmospheric dynamics and habitability in solar system planets and exoplanets.

Ion Behaviors in the Reconnection Diffusion Region of a Corrugated Magnetotail Current Sheet

AbstractThe current sheet structure and ion behaviors in a magnetotail reconnection diffusion region are investigated. The multispacecraft analysis suggests a corrugated current sheet structure, interpreted as due to a flapping motion that propagates along geocentric solar magnetospheric along the +y direction in the Geocentric Solar Magnetospheric (GSM) coordinate. The electric field (E) and ion distributions have similarities with those in a planar current sheet. Energetic ions move along the current direction, suggesting the acceleration by the observed reconnection E during the meandering motion. Counterstreaming ions along the current sheet normal suggest the acceleration by the Hall E that is observed to be the dominant component. However, at certain locations, E and counterstreaming ions significantly deviate from the local normal direction, and more than one pair of counterstreaming populations exist, possibly because the corrugated current sheet enables ions entering the current sheet at different locations with different velocities to mix together.

Current sheets in planetary magnetospheres

In this article we aim to highlight the problems related to the structure and stability of the comparatively thin current sheets that were relatively recently discovered by space missions in the magnetospheres of the Earth and planets, as well as in the solar wind. These magnetoplasma structures are universal in collisionless cosmic plasmas and can play a key role in the processes of storage and release of energy in the space environment. The development of a self-consistent theory for these sheets in the Earth’s magnetosphere, where they were first discovered, has a long and dramatic history. Solution of the problem of the thin current sheet structure and stability become possible in the framework of a kinetic quasi-adiabatic approach required to explain their embedding and metastability properties. It was found that the structure and stability of current structures are completely determined by the nonlinear dynamics of plasma particles. Theoretical models have been developed to predict many properties of these structures and interpret many experimental observations in planetary magnetospheres and the heliosphere.

Global View of Current Sheet Thinning: Plasma Pressure Gradients and Large‐Scale Currents

AbstractThe formation of a thin magnetotail current sheet is a key process in magnetic energy accumulation before magnetic reconnection during substorms. Although the 3‐D configuration of a thinning current sheet has been well reproduced in many numerical simulations, observational details of this configuration have been less thoroughly studied. To investigate the global and local 3‐D structure of a thinning current sheet, we use a data set collected by three spacecraft of the Time History of Events and Macroscale Interactions during Substorms mission, two spacecraft of the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun mission, and the Geostationary Operational Environmental Satellite 15. We demonstrate that near‐Earth current sheet thinning is accompanied by the formation of equatorial plasma pressure gradients directed from the flanks toward midnight. These gradients are associated with an intensification of the dawn‐dusk current (during current sheet thinning) and field‐aligned currents of opposite polarity at the dawn and dusk flanks. Simultaneous Time History of Events and Macroscale Interactions during Substorms and Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun observations show that current sheet thinning occupies a large portion of the tail, from the near‐Earth region to lunar orbit. An increase in the equatorial plasma pressure (and the lobe magnetic field pressure) is provided by a cold plasma density increase in the near‐Earth tail and likely by a plasma temperature increase in the distant tail. We discuss plasma convection patterns that are consistent with observed properties of the current sheet thinning.

Wavelet methods for studying the onset of strong plasma turbulence

Recent simulations have demonstrated that coherent current sheets dominate the kinetic-scale energy dissipation in strong turbulence of magnetized plasma. Wavelet basis functions are a natural tool for analyzing turbulent flows containing localized coherent structures of different spatial scales. Here, wavelets are used to study the onset and subsequent transition to fully developed turbulence from a laminar state. Originally applied to neutral fluid turbulence, an iterative wavelet technique decomposes the field into coherent and incoherent contributions. In contrast to Fourier power spectra, finite time Lyapunov exponents, and simple measures of intermittency such as non-Gaussian statistics of field increments, the wavelet technique is found to provide a quantitative measure for the onset of turbulence and to track the transition to fully developed turbulence. The wavelet method makes no assumptions about the structure of the coherent current sheets or the underlying plasma model. Temporal evolution of the coherent and incoherent wavelet fluctuations is found to be highly correlated (a Pearson correlation coefficient of >0.9) with the magnetic field energy and plasma thermal energy, respectively. The onset of turbulence is identified with the rapid growth of a background of incoherent fluctuations spreading across a range of scales and a corresponding drop in the coherent components. This is suggestive of the interpretation of the coherent and incoherent wavelet fluctuations as measures of coherent structures (e.g., current sheets) and dissipation, respectively. The ratio of the incoherent to coherent fluctuations Ric is found to be fairly uniform in the turbulent state across different plasma models and provides an empirical threshold of ∼0.1 for turbulence onset. The utility of this technique is illustrated through examples. First, it is applied to the Kelvin–Helmholtz instability from different simulation models including fully kinetic, hybrid (kinetic ion/fluid electron), and Hall MHD simulations. Second, the wavelet diagnostic is applied to the development of turbulence downstream of the bowshock in a global magnetosphere simulation. Finally, the wavelet technique is also shown to be useful as a de-noising method for particle simulations.

Structure of Current Sheets with Quasi-Adiabatic Dynamics of Particles in the Solar Wind

Within the self-consistent hybrid model based on the quasi-adiabatic approximation of the proton dynamics, a fine structure of strong current sheets (SCSs) in the solar wind has been investigated, including the heliospheric current sheet. The motion of electrons is fast and considered in the Boltzmann approximation. The simulation results have been shown that the SCS profiles have a multiscale enclosed structure with a narrow central current sheet that is enclosed in a wider sheet, similar to the heliospheric current sheet surrounded by the plasma sheet. The features of the SCS structure are determined by the relative contributions of the current of demagnetized protons in serpentine orbits and drift currents of electrons. The model predicts and describes the properties of SCSs observed by spacecraft. It has been shown that the multiscale structure of current sheets is an inherent intrinsic property of current sheets in the solar wind.

Role of the Plasmoid Instability in Magnetohydrodynamic Turbulence.

The plasmoid instability in evolving current sheets has been widely studied due to its effects on the disruption of current sheets, the formation of plasmoids, and the resultant fast magnetic reconnection. In this Letter, we study the role of the plasmoid instability in two-dimensional magnetohydrodynamic (MHD) turbulence by means of high-resolution direct numerical simulations. At a sufficiently large magnetic Reynolds number (R_{m}=10^{6}), the combined effects of dynamic alignment and turbulent intermittency lead to a copious formation of plasmoids in a multitude of intense current sheets. The disruption of current sheet structures facilitates the energy cascade towards small scales, leading to the breaking and steepening of the energy spectrum. In the plasmoid-mediated regime, the energy spectrum displays a scaling that is close to the spectral index -2.2 as proposed by recent analytic theories. We also demonstrate that the scale-dependent dynamic alignment exists in 2D MHD turbulence and the corresponding slope of the alignment angle is close to 0.25.

Study of magnetic reconnection in large-scale magnetic island coalescence via spatially coupled MHD and PIC simulations

We study the process of magnetic reconnection in a coalescing magnetic island setup by means of numerical simulation. This process mimics flux tube merging which can take place in the solar corona, laboratory, and astrophysical objects. Simulations are performed with magnetohydrodynamics (MHD), Hall-MHD, and a newly developed Coupled MHD and Particle-In-Cell (PIC) model (CMAP). This model treats the global simulation domain with MHD, while the region around the reconnection zone is treated with PIC. This CMAP code allows us to simulate larger-scale domains with lesser computing power compared to fully PIC simulations. CMAP reproduces the dynamics of fully kinetic simulations which Hall-MHD does not capture, as seen in the Hall magnetic field and the reconnecting current sheet structure. For large islands in kinetic simulations, the current sheet does not form smoothly and shows chaotic behavior, and the magnetic islands also bounce and slosh. The current sheet thickness, length, and aspect ratios are calculated. They show that in the CMAP model, the thickness remains close to the ion skin depth, while the length changes weakly with the system size, giving a steady aspect ratio for the two largest system size simulations. The pressure tensor also shows large deviations from isotropy and gyrotropy near the current sheet. The CMAP simulations for smaller system sizes are compared to fully kinetic simulations, and we find that a minimum fraction of area has to be provided PIC feedback in the CMAP simulations in order to produce reconnection rates and dynamics similar to fully kinetic simulations. The reconnection rate reduces with the increasing island size. For the CMAP model, this reduction is steeper compared to MHD and Hall-MHD initially, but for larger system sizes, the reconnection rates in CMAP simulations show a steady behavior.

Field‐Aligned Currents in Saturn's Magnetosphere: Observations From the F‐Ring Orbits

AbstractWe investigate the azimuthal magnetic field signatures associated with high‐latitude field‐aligned currents observed during Cassini's F‐ring orbits (October 2016–April 2017). The overall ionospheric meridional current profiles in the northern and southern hemispheres, that is, the regions poleward and equatorward of the field‐aligned currents, differ most from the 2008 observations. We discuss these differences in terms of the seasonal change between data sets and local time (LT) differences, as the 2008 data cover the nightside while the F‐ring data cover the post‐dawn and dusk sectors in the northern and southern hemispheres, respectively. The F‐ring field‐aligned currents typically have a similar four current sheet structure to those in 2008. We investigate the properties of the current sheets and show that the field‐aligned currents in a hemisphere are modulated by that hemisphere's “planetary period oscillation” (PPO) systems. We separate the PPO‐independent and PPO‐related currents in both hemispheres using their opposite symmetry. The average PPO‐independent currents peak at ~1.5 MA/rad just equatorward of the open closed field line boundary, similar to the 2008 observations. However, the PPO‐related currents in both hemispheres are reduced by ~50% to ~0.4 MA/rad. This may be evidence of reduced PPO amplitudes, similar to the previously observed weaker equatorial oscillations at similar dayside LTs. We do not detect the PPO current systems' interhemispheric component, likely a result of the weaker PPO‐related currents and their closure within the magnetosphere. We also do not detect previously proposed lower latitude discrete field‐aligned currents that act to “turn off” the PPOs.

Time Evolution of the Macroscopic Characteristics of a Thin Current Sheet in the Course of Its Formation in the Earth’s Magnetotail

A numerical model is developed that allows tracing the time evolution of a current sheet from a relatively thick current configuration with isotropic distributions of the pressure and temperature in an extremely thin current sheet, which plays a key role in geomagnetic processes. Such a configuration is observed in the Earth’s magnetotail in the stage preceding a large-scale geomagnetic disturbance (substorm). Thin current sheets are reservoirs of the free energy released during geomagnetic disturbances. The time evolution of the components of the pressure tensor caused by changes in the structure of the current sheet is investigated. It is shown that the pressure tensor in the current sheet evolves in two stages. In the first stage, a current sheet with a thickness of eight to ten proton Larmor radii forms. This stage is characterized by the plasma drift toward the current sheet and the Earth and can be described in terms of the Chu–Goldberger–Low approximation. In the second stage, an extremely thin current sheet with an anisotropic plasma pressure tensor forms, due to which the system is maintained in an equilibrium state. Estimates of the characteristic time of the system evolution agree with available experimental data.

Physics of the saturation of particle acceleration in relativistic magnetic reconnection

We investigate the saturation of particle acceleration in relativistic reconnection using two-dimensional particle-in-cell simulations at various magnetizations \sigma. We find that the particle energy spectrum produced in reconnection quickly saturates as a hard power law that cuts off at \gamma~4\sigma, confirming previous work. Using particle tracing, we find that particle acceleration by the reconnection electric field in X-points determines the shape of the particle energy spectrum. By analyzing the current sheet structure, we show that physical cause of saturation is the spontaneous formation of secondary magnetic islands that can disrupt particle acceleration. By comparing the size of acceleration regions to the typical distance between disruptive islands, we show that the maximum Lorentz factor produced in reconnection is \gamma ~ 5 \sigma, which is very close to what we find in our particle energy spectra. We also show that the dynamic range in Lorentz factor of the power law spectrum in reconnection is < 40. The hardness of the power law combined with its narrow dynamic range implies that relativistic reconnection is capable of producing the hard narrowband flares observed in the Crab Nebula but has difficulty producing the softer broadband prompt GRB emission.