Thin Current Sheet Research Articles

Generalized Harris Sheet Equilibrium in Regularized Kappa Distributed Plasmas

Harris sheet models with antiparallel magnetic field are the equilibrium solutions to the steady Vlasov–Maxwell equations, which have been widely adopted for the study of plasma instabilities and magnetic reconnection in thin current sheets. The original Harris equilibrium model assumes the Maxwellian velocity distribution, which necessarily gives rise to the isothermal current sheets. Many observations have shown that the suprathermal populations are indispensable features of particle distribution profiles, which may well be fitted by the Kappa distribution (KD) functions with the free parameter κ. The Harris equilibrium based on the standard Kappa distribution (SKD), however, may become unphysical for . Alternatively, the regularized KD (RKD) with two adjustable parameters, κ and α 2, has been proposed to replace the SKD in various studies. The RKD may recover the SKD for α 2 = 0, and Maxwellian model for α 2 = 0 and κ → ∞ . This paper presents the first results of adopting the nonthermal RKDs in the Harris equilibrium models. It is shown that the new RKD–Harris equilibrium is applicable for all κ values, and exhibits a stronger magnetic field associated with higher temperature in the asymptotic region. The proposed formulations for the generalized Harris sheet equilibrium may be applied to multiple component plasmas (e.g., electrons, protons) with different RKDs and temperatures.

How Magnetic Reconnection May Affect the Coherence of Interplanetary Coronal Mass Ejections

On 2020 April 19–20, a solar ejection was seen by spacecraft in a radial alignment that included Solar Orbiter and Wind. The ejection contained a magnetic flux rope where magnetic field and plasma parameters were well correlated between spacecraft. This structure is called an “unperturbed magnetic flux rope” (UMFR). Ahead of the UMFR is a portion of the ejection (not sheath) that is referred to as “upstream” (US). We focus on the US and inquire why the correlation is so much weaker there. Specifically, we analyze data collected by Solar Orbiter at 0.81 au and Wind at L1. We show that a plausible cause for the lack of coherence in the US is a combination of front erosion and internal reconnection occurring there. Front erosion is inferred from an analysis of azimuthal magnetic flux balance in the UMFR. In the present case, we contend that the US, rather than the UMFR, is the source of the eroded field lines. The presence of erosion is supported further by a direct comparison of the magnetic field data at both spacecraft that shows, in particular, a massive shrinkage of the front portion of the US. Internal reconnection is also happening at thin current sheets inside the US. Strong nonradial flows are reconfiguring the structure. As a result of these reconnection processes, a whole section of the US is disrupted and field lines move down the flanks of the ejection and out of view of Wind.

Investigation of the collisionless plasmoid instability based on gyrofluid and gyrokinetic integrated approach

In this work, the development of two-dimensional current sheets with respect to tearing modes, in collisionless plasmas with a strong guide field, is analysed. During their nonlinear evolution, these thin current sheets can become unstable to the formation of plasmoids, which allows the magnetic reconnection process to reach high reconnection rates. We carry out a detailed study of the effect of a finite $\beta _e$ , which also implies finite electron Larmor radius effects, on the collisionless plasmoid instability. This study is conducted through a comparison of gyrofluid and gyrokinetic simulations. The comparison shows in general a good capability of the gyrofluid models in predicting the plasmoid instability observed with gyrokinetic simulations. We show that the effects of $\beta _e$ promotes the plasmoid growth. The effect of the closure applied during the derivation of the gyrofluid model is also studied through the comparison among the variations of the different contributions to the total energy.

The Influence of Oxygen Ions on the Formation of a Thin Current Sheet in the Magnetotail

The Influence of Oxygen Ions on the Formation of a Thin Current Sheet in the Magnetotail

The Influence of Oxygen Ions on the Formation of a Thin Current Sheet in the Magnetotail

A thin current sheet in the Earth’s magnetotail with characteristic thickness of one to several proton gyroradii is often observed during magnetospheric disturbances named substorms, when a relatively thick current configuration in the magnetotail is narrowed to an extremely small thickness and then can spontaneously be destroyed. The current-sheet destruction is usually accompanied by such active processes as plasma acceleration and heating, as well as generation of an induced electric field and magnetohydrodynamic waves. In this paper, we developed and investigated a model of formation of a thin current sheet in which, along with protons, we have taken into account single-charged oxygen ions coming from the ionosphere into the magnetotail current sheet during magnetically active periods. The aim of this simulation is to study the peculiarities of thin current-sheet formation in two-component plasma and investigate its structure. It is shown that equilibrium configuration can have some characteristic properties. In particular, if the system consists only of protons or heavy ions, single-scale current equilibrium supported by quasi-adiabatic particles is formed. When a current sheet is formed in plasma that consists of a mixture of protons and oxygen ions in comparable concentrations, a current sheet can be formed with heavy ions as current carriers and chaotic proton trajectories that make a negative contribution to the current, due to which the current-density profile becomes bifurcated with the minimum at the center and maxima at the periphery of the sheet. The results may be useful for the interpretation of observational data in the Earth’s magnetotail.

Current Spreading in Thin Foils or Flat Current Sheets

To consider the evolution of current distribution in inhomogeneous thin conductive layers or foils, we apply an integrodifferential equation, which reduces the three-dimensional problem for the magnetic field to a two-dimensional problem, and, for the current distribution across the width of inhomogeneous conductive sheets or foils, this equation reduces the two-dimensional problem for the magnetic field to a one-dimensional problem. For homogeneous conductive layers with constant conductivity, the spatial scale of current distribution, initially concentrated in a limited area, increases proportionally to time at a rate of u = c2/4πσΔ, where σ is the conductivity of the layer material and Δ is its thickness. As an application to the problems of current transfer through electroexplosive opening switches, a current distribution across the width of a foil is considered for a periodic serpentine-type system of flat foils. It is shown that initially a current distribution corresponding to the perfect conductivity of foils is established in this system. Then, in a time on the order of s/u (2s is the width of a foil), the current distribution in the foil relaxes to a uniform distribution. Estimates show that if the foils are used as opening switches, then currents through the foils during the current transfer to the load are expected to have time to get uniformly distributed across their width; therefore, corrections for the nonuniformity of the current distribution in the switches should be small.

Current Spreading in Thin Foils or Flat Current Sheets

Current Spreading in Thin Foils or Flat Current Sheets

Applying Magnetic Curvature to MMS Data to Identify Thin Current Sheets Relative to Tail Reconnection.

Thin current sheets (TCSs) have been postulated to be a necessary precondition for reconnection onset. Magnetic reconnection X-lines in the magnetotail have been observed to be more common duskward of midnight. We take advantage of the MMS tetrahedral formation during the 2017-2020 MMS tail seasons to calculate the thickness of the cross-tail neutral sheet relative to ion gyroradius. While a similar technique was applied to Cluster data, current sheet thickness over a broader range of radial distances has not been robustly explored before this study. We compare our analysis to recent theories regarding mechanisms of tail current sheet thinning and to recent simulations. We find MMS spent more than twice as long in ion-scale TCSs in the pre-midnight sector than post-midnight, despite nearly even plasma sheet dwell time. The dawn-dusk asymmetry in the distribution of Ion Diffusion Regions, as previously reported in relation to regions of TCSs, is also analyzed.

Hybrid simulations of the decay of reconnected structures downstream of the bow shock

Observations by Magnetospheric Multiscale have demonstrated that magnetic reconnection occurs at Earth's bow shock, typically at thin current sheets arising from plasma instabilities and turbulence in the shock transition region. Observational surveys of both the shock transition and the magnetosheath downstream suggest that the number of current sheets in these regions may not be strongly dependent on the shock Mach number MA or the angle between the upstream magnetic field and shock normal (θBn). This result is somewhat surprising given that quasi-parallel and high Mach number shocks tend to have a more disordered and non-stationary structure. In order to investigate how shock reconnection manifests across different parameters, we perform a series of hybrid (fluid electron, kinetic ion) particle-in-cell simulations across a range of Mach numbers and orientations. Given that hybrid simulations cannot resolve electron-scale current sheets and reconnection, these simulations isolate an ion-scale mechanism for shock reconnection driven by an ion–ion beam instability in the foot. We find that this mechanism is strongly constrained to quasi-parallel shocks across all simulated Mach numbers. By quantifying reconnection using the area occupied by plasma on closed magnetic field lines, we find the number of reconnecting structures and closed field area increase with MA and decrease with θBn in the upstream and ramp regions. Downstream of the shock, however, we find a similar result to observational surveys: within the subset of quasi-parallel shocks, the decay rate of the closed field area (and hence thin current sheets) is not strongly dependent on upstream shock parameters.

Current Sheet Thinning in the Wake of a Bubble Injection

AbstractA crucial property of the substorm growth phase is the current sheet thinning, which is often attributed to adiabatic convection. Injecting low‐entropy bubbles reduce pressure‐balance inconsistencies and restore current sheet thickness to its initial value. Recent observations from Cluster and MMS showed additional thinning of the current sheet in the wake of a transitory bubble injection compared with the configuration before the injection. We employ the Rice Convection Model‐MHD coupled code to investigate how the transport of bubbles causes the fast thinning. The simulation results reconstructed the observations' most prominent characteristics. We find more earthward transport of the magnetic flux in the bubble than its wake, therefore depleting magnetic flux and stretching the magnetic field lines there. Interestingly, additional R0‐sense field‐aligned currents close the enhanced dawn‐to‐dusk current behind the bubble.

Equilibrium Kinetic Theory of Weakly Anisotropic Embedded Thin Current Sheets

AbstractStatistical and case studies, as well as data‐mining reconstructions suggest that the magnetotail current in the substorm growth phase has a multiscale structure with a thin ion‐scale current sheet embedded into a much thicker sheet. This multiscale structure may be critically important for the tail stability and onset conditions for magnetospheric substorms. The observed thin current sheets are found to be too long to be explained by the models with isotropic plasmas. At the same time, plasma observations reveal only weak field‐aligned anisotropy of the ion species, whereas the anisotropic electron contribution is insufficient to explain the force balance discrepancy. Here we elaborate a self‐consistent equilibrium theory of multiscale current sheets, which differs from conventional isotropic models by weak ion anisotropy outside the sheet and agyrotropy caused by quasi‐adiabatic ion orbits inside the sheet. It is shown that, in spite of weak anisotropy, the current density perturbation may be quite strong and localized on the scale of the figure‐of‐eight ion orbits. The magnetic field, current and plasma density in the limit of weak field‐aligned ion anisotropy and strong current sheet embedding, when the ion scale thin current sheet is nested in a much thicker Harris‐like current sheet, are investigated and presented in an analytical form making it possible to describe the multiscale equilibrium in sharply stretched 2D magnetic field configurations and to use it in kinetic simulations and stability analysis.

Equilibrium Configurations of Super‐Thin Current Sheets in Space Plasma: Characteristic Scaling of Multilayer Structures

AbstractThin current sheets (TCSs) are the key structures in space and laboratory plasmas responsible for the accumulation and subsequent release of the stored magnetic energy. Spacecraft measurements in the near Earth's space revealed the complicated multilayer TCSs structure consisting from extremely thin and strong electron current embedded inside the thicker ion current layer. In this paper we present for the first time the analytical self‐consistent model of 1D super thin electron current sheet (STCS) supported by magnetized electrons within a much wider (but still very thin) proton structure carried by nonmagnetized (quasi‐adiabatic) particles. We take into account the dependence of electron STCS on anisotropy of electron distribution and magnetic field shear often existing in realistic TCS configurations. It is shown that the structure of the embedded STCS can be described by the simple nonlinear equation, which allows to estimate the scaling of STCS thickness due to coupling of electron and ion domains, although electron dynamics described by guiding center drift approximation is scale free. The theoretical results are in a good agreement with MMS spacecraft observations in the Earth's magnetotail.

Tripolar guide magnetic fields due to island coalescence in solar wind current sheets: Simulation and theory

Reconnecting current sheets (CSs) in the solar wind near 1 AU have been previously identified that exhibit a modified Hall magnetic field perturbation of the nominal guide field characterized by a “tripolar” structure with depressions on both sides of a central maximum [Eriksson et al., Astrophys. J. 805, 43 (2015)]. Such CSs were inferred to contain multiple interacting islands based on measurements from all four Cluster satellites. A new set of 2D particle-in-cell simulations have been performed providing the foundation for a theoretical model for the origin of the tripolar guide-field perturbation. The simulations are initialized with a thin CS unstable to the formation of many small islands that undergo pairwise coalescence and growth. The guide field depressions develop as the result of a balance between the electrostatic (curl-free or irrotational) and electromagnetic (divergence-free or solenoidal) components of the parallel electric field. Field-line and flow-line tracing provide additional support for the model by demonstrating how a level of charge separation sufficient to support a large electrostatic potential can be maintained following island coalescence. A parameter study reveals that the plasma beta is the primary quantity controlling the evolution of the tripolar field. Dependence on the initial guide-field strength is also investigated.

Thinning of the Magnetotail Current Sheet Inferred From Low‐Altitude Observations of Energetic Electrons

AbstractThe magnetospheric substorm is a key mode of flux and energy transport throughout the magnetosphere associated with distinct and repeatable magnetotail dynamical processes and plasma injections. The substorm growth phase is characterized by current sheet thinning and magnetic field reconfiguration around the equatorial plane. The global characteristics of current sheet thinning are important for understanding of magnetotail state right before the onset of magnetic reconnection and of the key substorm expansion phase. In this paper, we investigate this thinning at different radial distances using plasma sheet (PS) energetic (>50 keV) electrons that reach from the equator to low altitudes during their fast (∼1 s) travel along magnetic field lines. We perform a multi‐case study and a statistical analysis of 34 events with near‐equatorial observations of the current sheet thinning by equatorial missions and concurrent, latitudinal crossings of the ionospheric projection of the magnetotail by the low‐altitude Electron Losses and Fields Investigation (ELFIN) CubeSats at approximately the same local time sector. Energetic electron fluxes thus collected by ELFIN provide near‐instantaneous (<5 min duration) radial snapshots of magnetotail fluxes. Main findings of this study confirm the previously proposed concepts with low‐altitude energetic electron measurements: (a) Energy distributions of low‐altitude fluxes are quantitatively close to the near‐equatorial distributions, which justifies the investigation of the magnetotail current sheet reconfiguration using low‐altitude measurements. (b) The magnetic field reconfiguration during the current sheet thinning (which lasts ≥ an hour) results in a rapid shrinking of the low‐altitude projection of the entire PS (from near‐Earth, ∼10RE, to the lunar orbit ∼60RE) to 1–2° of magnetic latitude in the ionosphere. (c) The current sheet dipolarization, common during the substorm onset, is associated with a very quick (∼10 min) change of the tail magnetic field configuration to its dipolar state, as implied by a poleward expansion of the PSPS at low altitudes.

Interplay of turbulence and proton-microinstability growth in space plasmas

Numerous prior studies have shown that as proton beta increases, a narrower range of proton temperature anisotropy values is observed. This effect has often been ascribed to the actions of kinetic microinstabilities because the distribution of observational data aligns with contours of constant instability growth rates in the beta-anisotropy plane. However, the linear Vlasov theory of instabilities assumes a uniform background in which perturbations grow. The established success of linear-microinstability theories suggests that the conditions in regions of extreme temperature anisotropy may remain uniform for a long enough time so that the instabilities have the chance to grow to sufficient amplitude. Turbulence, on the other hand, is intrinsically nonuniform and nonlinear. Thin current sheets and other coherent structures generated in a turbulent plasma may quickly destroy the uniformity. It is, therefore, not a-priori obvious whether the presence of intermittency and coherent structures favors or disfavors instabilities. To address this question, we examined the statistical distribution of growth rates associated with proton temperature-anisotropy driven microinstabilities and local nonlinear time scales in turbulent plasmas. Linear growth rates are, on average, substantially less than the local nonlinear rates. However, at the regions of extreme values of temperature anisotropy, near the “edges” of the populated part of the proton temperature anisotropy-parallel beta plane, the instability growth rates are comparable or faster than the turbulence time scales. These results provide a possible answer to the question as to why the linear theory appears to work in limiting plasma excursions in anisotropy and plasma beta.

Radial and Local Time Variations in the Thickness of Jupiter's Magnetospheric Current Sheet

AbstractThe magnetic field observations from the Galileo, Ulysses, Voyager 1 and 2, and Pioneer 10 and 11 spacecraft are analyzed to understand the structures and thickness of the Jovian magnetospheric current sheet. Using a new technique that can determine the motion of the spacecraft relative to the current sheet, we have built the first global map of the thickness of the Jovian magnetospheric current sheet. We show that the current sheet thickness in Jupiter's magnetosphere is not equal everywhere but changes systematically with both radial distance and local time. The extremely thin dawnside current sheet appears to be linked to the stresses imposed by the solar wind on the open magnetic flux housed in the lobes of the magnetosphere in that sector. We also show evidence of a mostly closed magnetosphere in the dusk sector. We show that the current sheet thickness asymmetries likely arise from a field and plasma convection system set up by the reconnection of the interplanetary magnetic field with that of Jupiter.

MMS Observations of a Compressed Current Sheet: Importance of the Ambipolar Electric Field.

Spacecraft data reveal a nonuniform ambipolar electric field transverse to the magnetic field in a thin current sheet in Earth's magnetotail that leads to intense E×B velocity shear and nongyrotropic particle distributions. The E×B drift far exceeds the diamagnetic drift and thus drives observed lower hybrid waves. The shear-driven waves are localized to the magnetic field reversal region and are therefore ideally suited for the anomalous dissipation necessary for reconnection. It also reveals substructures embedded in the current density, indicating a compressed current sheet.

Energized Oxygen in the Magnetotail: Onset and Evolution of Magnetic Reconnection.

Oxygen ions are a major constituent of magnetospheric plasma, yet the role of oxygen in processes such as magnetic reconnection continues to be poorly understood. Observations show that significant amounts of energized O+ can be present in a magnetotail current sheet (CS). A population of thermal O+ only has a relatively minor effect on magnetic reconnection. Despite this, published studies have so far only concentrated on the role of the low-energy thermal O+. We present a study of magnetic reconnection in a thinning CS with energized O+ present. Well-established, three-species, 2.5D particle-in-cell (PIC) kinetic simulations are used. Simulations of thermal H+ and thermal O+ validate our setup against published results. We then energize a thermal background O+ based on published in situ measurements. A range of energization is applied to the background O+. We discuss the effects of energized O+ on CS thinning and the onset and evolution of magnetic reconnection. The presence of energized O+ causes a two-regime onset response in a thinning CS. As energization increases in the lower-regime, reconnection develops at a single primary X-line, increases time-to-onset, and suppresses the rate of evolution. As energization continues to increase in the higher-regime, reconnection develops at multiple X-lines, forming a stochastic plasmoid chain; decreases time-to-onset; and enhances evolution via a plasmoid instability. Energized O+ drives a depletion of the background H+ around the central CS. As the energization increases, the CS thinning begins to slow and eventually reverses.

Suppression of reconnection in polarized, thin magnetotail current sheets: 2D simulations and implications

Many in situ spacecraft observations have demonstrated that magnetic reconnection in the Earth's magnetotail is largely controlled by the pre-reconnection current sheet configuration. One of the most important thin current sheet characteristics is the preponderance of electron currents driven by strong polarized electric fields, which are commonly observed in the Earth's magnetotail well before the reconnection. We use particle-in-cell simulations to investigate magnetic reconnection in the 2D magnetotail current sheet with a finite magnetic field component normal to the current sheet and with the current sheet polarization. Under the same external driving conditions, reconnection in a polarized current sheet is shown to occur at a lower rate than in a nonpolarized current sheet. The reconnection rate in a polarized current sheet decreases linearly as the electron current's contribution to the cross-tail current increases. In simulations with lower background temperature, the reconnection electric field is higher. We demonstrate that after reconnection in such a polarized current sheet, the outflow energy flux is mostly in the form of ion enthalpy flux, followed by electron enthalpy flux, Poynting flux, ion kinetic energy flux, and electron kinetic energy flux. These findings are consistent with spacecraft observations. Because current sheet polarization is not uniform along the magnetotail, our results suggest that it may slow down reconnection in the most polarized near-Earth magnetotail and thereby move the location of reconnection onset downtail.

Thin Current Sheet Formation and Reconnection at X ∼ −10 RE During the Main Phase of a Magnetic Storm

AbstractThe main question addressed in this study is whether particles with relativistic energy can be injected directly into the inner magnetosphere by very near‐Earth reconnection during magnetic storms. We study a sequence of events observed in the solar wind, in the magnetotail, at geosynchronous orbit (GEO), in the inner magnetosphere, and in the ionosphere during the main phase of a geomagnetic storm on 16 June 2012–17 June 2012. The storm was caused by a magnetic cloud with a high dynamic pressure and a strong southward IMF lasting about 10 hr. These conditions caused an extreme compression of the magnetosphere (SymH reached ∼150 nT) and an enhancement of the lobe magnetic field strength to ∼90 nT at R ∼ 10 Earth radii (RE). We focus on an hour‐long interval between 1050 and 1150 UT on 17 June 2012 when the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites were at apogee at geocentric distances R ≈ 12 RE near midnight. At that time a thin current sheet formed between cis‐GEO distances and THEMIS. This thin current sheet reconnected between GEO and THEMIS. A strong lobe magnetic field enabled ion and electron energization to energies E ≈ 600 keV. Fluxes of high energy (up to relativistic) particles at GEO increased within ≈20 s after reconnection onset detected by THEMIS. Fluxes of relativistic electrons at L ∼ 4 in the morning sector increased within about 600 s after reconnection onset. We interpret these observations as signatures of direct injection of reconnected particles into the inner magnetosphere.