Instabilities in a thin current sheet and their consequences

Abstract. Using a fully 3-D particle in-cell simulation, we studied the electrodynamics of a thin current sheet (CS). Starting with a uniform plasma and anti-parallel magnetic field, Harris equilibrium is achieved during the early stage of the simulation. In the processes of reaching the equilibrium, both electrons and ions in the newly formed CS are energized and develop pitch-angle anisotropies. We find two distinct stages of primarily electrostatic instabilities; in the first stage the relative drift between electrons and ions drives the instability in the central regions of the CS. The electrostatic fluctuations scatter electrons causing current disruption in the central region. The associated reduction in the average drift velocity of the current-carrying electrons generates sheared flow. The second stage of the instability begins when the drift velocity develops a minimum in the central plane. Then the shear and the growing electrostatic fluctuations under the condition of the maintained anti-parallel driving magnetic field configuration feed each other making the instability explosive. The growing fluctuations create plasma clumps as the electrons and ions are progressively trapped in the large-amplitude waves. The density clumping also generates clumps in the current. The non-uniform current distribution causes magnetic reconnection, accompanied by heating of electrons and ion at a fast rate and nearly complete bifurcation of the current sheet. Anomalous resistivity during different stages of the evolution of the CS is calculated and compared against theory.

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Electrodynamics in a very thin current sheet leading to magnetic reconnection

Nonlinear Processes in Geophysics ◽

10.5194/npg-13-509-2006 ◽

2006 ◽

Vol 13 (5) ◽

pp. 509-523 ◽

Cited By ~ 8

Author(s):

N. Singh ◽

C. Deverapalli ◽

G. Khazanov

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Electron Temperature ◽

Magnetic Reconnection ◽

Current Sheet ◽

Electron Mass ◽

Angle Distribution ◽

Current Disruption ◽

Thin Current Sheet ◽

The Magnetic Field

Abstract. We study the formation of a very thin current sheet (CS) and associated plasma electrodynamics using three-dimensional (3-D) particle-in-cell simulations with ion to electron mass ratio M/m=1836. The CS is driven by imposed anti-parallel magnetic fields. The noteworthy features of the temporal evolution of the CS are the following: (i) Steepening of the magnetic field profile Bx(z) in the central part of the CS, (ii) Generation of three-peak current distribution with the largest peak in the CS center as Bx(z) steepens, (iii) Generation of converging electric fields forming a potential well in the CS center in which ions are accelerated. (iv) Electron and ion heating in the central part of the CS by current-driven instabilities (CDI). (v) Re-broadening of the CS due to increased kinetic plasma pressure in the CS center. (vi) Generation of electron temperature anisotropy with temperature perpendicular to the magnetic field being larger than the parallel one. (vii) Current disruption by electron trapping in an explosively growing electrostatic instability (EGEI) and electron tearing instability (ETI). (viii)The onset of EGEI coincides with an increase in the electron temperature above the temperature of the initially hot ions as well as the appearance of new shear in the electron drift velocity. (ix) Bifurcation of the central CS by the current disruption. (x) Magnetic reconnection (MR) beginning near the null in Bx and spreading outward. (xi) Generation of highly energized electrons reaching relativistic speeds and having isotropic pitch-angle distribution in the region of reconnected magnetic fields. We compare some of these features of the current sheet with results from laboratory and space experiments.

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Nonlinear equilibrium structure of thin currents sheets: influence of electron pressure anisotropy

Nonlinear Processes in Geophysics ◽

10.5194/npg-11-579-2004 ◽

2004 ◽

Vol 11 (5/6) ◽

pp. 579-587 ◽

Cited By ~ 59

Author(s):

L. M. Zelenyi ◽

H. V. Malova ◽

V. Yu. Popov ◽

D. Delcourt ◽

A. S. Sharma

Keyword(s):

Magnetic Field ◽

Current Sheet ◽

Magnetic Energy ◽

Plasma Pressure ◽

Quasi Equilibrium ◽

Electrostatic Effects ◽

Current Sheets ◽

Thin Current Sheet ◽

Field Lines ◽

The Magnetic Field

Abstract. Thin current sheets represent important and puzzling sites of magnetic energy storage and subsequent fast release. Such structures are observed in planetary magnetospheres, solar atmosphere and are expected to be widespread in nature. The thin current sheet structure resembles a collapsing MHD solution with a plane singularity. Being potential sites of effective energy accumulation, these structures have received a good deal of attention during the last decade, especially after the launch of the multiprobe CLUSTER mission which is capable of resolving their 3D features. Many theoretical models of thin current sheet dynamics, including the well-known current sheet bifurcation, have been developed recently. A self-consistent 1D analytical model of thin current sheets in which the tension of the magnetic field lines is balanced by the ion inertia rather than by the plasma pressure gradients was developed earlier. The influence of the anisotropic electron population and of the corresponding electrostatic field that acts to restore quasi-neutrality of the plasma is taken into account. It is assumed that the electron motion is fluid-like in the direction perpendicular to the magnetic field and fast enough to support quasi-equilibrium Boltzmann distribution along the field lines. Electrostatic effects lead to an interesting feature of the current density profile inside the current sheet, i.e. a narrow sharp peak of electron current in the very center of the sheet due to fast curvature drift of the particles in this region. The corresponding magnetic field profile becomes much steeper near the neutral plane although the total cross-tail current is in all cases dominated by the ion contribution. The dependence of electrostatic effects on the ion to electron temperature ratio, the curvature of the magnetic field lines, and the average electron magnetic moment is also analyzed. The implications of these effects on the fine structure of thin current sheets and their potential impact on substorm dynamics are presented.

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Electron holes in the Earth's magnetotail current sheet: role of magnetic field gradients and electron anisotropy

10.5194/egusphere-egu2020-11967 ◽

2020 ◽

Author(s):

Pavel Shustov ◽

Ilya Kuzichev ◽

Ivan Vasko ◽

Anton Artemyev ◽

Anatoliy Petrukovich

Keyword(s):

Magnetic Field ◽

Current Sheet ◽

Magnetic Field Gradient ◽

Electron Distribution Function ◽

Inhomogeneous Magnetic Field ◽

Field Configuration ◽

Slow Electron ◽

Electron Hole ◽

Field Lines ◽

Electron Holes

<p>Electron holes are nonlinear electrostatic structures that are often observed in the vicinity of the magnetotail energy release regions, e.g. magnetic reconnection. In this work we develop 1.5D Vlasov code simulations of the electron hole dynamics in the magnetic field configuration typical of the current sheet of the Earth's magnetotail. We consider the propagation of electron holes along magnetic field lines in the inhomogeneous magnetic field of the current sheet with realistically anisotropic electron distribution function. We demonstrate that electron holes generated near the equatorial plane of the current sheet brake as they propagate toward the boundaries of the current sheets. This effect is stronger for higher magnetic field gradient and larger electron field-aligned anisotropy. These simulations demonstrate that slow electron holes observed in the plasma sheet boundary layer may appear due to that effect of electron hole braking.</p>

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Resistive tearing-mode instability in a magnetic-field-reversing current sheet with coplanar viscous stagnation-point flow

Journal of Plasma Physics ◽

10.1017/s0022377800019267 ◽

1996 ◽

Vol 56 (2) ◽

pp. 265-284 ◽

Cited By ~ 18

Author(s):

Justin T. C. Ip ◽

Bengt U. Ö. Sonnerup

Keyword(s):

Magnetic Field ◽

Current Sheet ◽

Stagnation Point ◽

Linear Growth ◽

Field Configuration ◽

Stagnation Point Flow ◽

Magnetic Islands ◽

Tearing Mode ◽

Mode Instability ◽

Simulation Results

The tearing-mode instability of a magnetic-field-reversing current sheet in the presence of coplanar incompressible stagnation-point flow is examined. The unperturbed equilibrium state is an exact solution of the steady-state, dissipative, incompressible magnetohydrodynamic equations; thus the analysis is valid even for small viscous and resistive Lundquist numbers Sν and Sη. The instability problem has no known analytical solution; for this reason, it is studied numerically by use of a finite-element method. Simulation results indicate stability for sufficiently small values of Sν or Sη and instability for large values. The boundary separating stable and unstable regions in the (Sν, Sη) plane is located. In the unstable regime, the simulation results show formation and subsequent convection of magnetic islands along the current sheet at about 80% of the unperturbed outflow flow speed, on average. Stretching and pinching of convecting magnetic islands are also observed. The results show the occurrence of multiple X-line reconnection at the centre of the current sheet (x = 0). Small-scale structures of vorticity and current density near the X-point reconnection sites are found to be qualitatively consistent with results obtained by Matthaeus. Normalized global linear growth rates are found to obey the approximate power law, within the ranges 20 ≦ Sν ≦ 70 and 200 ≦ Sη 1000. At least for Sν ≦ 1000, the number of magnetic islands is found to be nearly independent of Sν indicating the existence of a narrow band of dominant wavelengths in this range. The stretching of magnetic islands, which is present in this coplanar flow and field configuration, but not in the perpendicular flow and field configuration examined by Phan and Sonnerup, causes a substantial decrease in linear growth rate relative to that obtained by those authors. The stability curves obtained are qualitatively similar in both analyses, but the stable region is much larger for coplanar flow and field. Unlike most simulations of the tearing mode, no symmetry conditions are imposed on the perturbations; nevertheless, they develop in a symmetric manner.

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Disruption of magnetospheric current sheet by quasi-electrostatic field

Annales Geophysicae ◽

10.5194/angeo-27-1941-2009 ◽

2009 ◽

Vol 27 (5) ◽

pp. 1941-1950 ◽

Cited By ~ 12

Author(s):

W. W. Liu ◽

J. Liang

Keyword(s):

Current Sheet ◽

Drift Velocity ◽

Electrostatic Field ◽

Numerical Solutions ◽

External Perturbation ◽

Ion Drift ◽

Current Disruption ◽

Local Current ◽

Fluid Theory ◽

Two Fluid

Abstract. Recent observational evidence has indicated that local current sheet disruptions are excited by an external perturbation likely associated with the kinetic ballooning (KB) instability initiating at the transition region separating the dipole- and tail-like geometries. Specifically a quasi-electrostatic field pointing to the neutral sheet was identified in the interval between the arrival of KB perturbation and local current disruption. How can such a field drive the local current sheet unstable? This question is considered through a fluid treatment of thin current sheet (TCS) where the generalized Ohm's law replaces the frozen-in-flux condition. A perturbation with the wavevector along the current is applied, and eigenmodes with frequency much below the ion gyrofrequency are sought. We show that the second-order derivative of ion drift velocity along the thickness of the current sheet is a critical stability parameter. In an E-field-free Harris sheet in which the drift velocity is constant, the current sheet is stable against this particular mode. As the electrostatic field grows, however, potential for instability arises. The threshold of instability is identified through an approximate analysis of the theory. For a nominal current sheet half-thickness of 1000 km, the estimated instability threshold is E~4 mV/m. Numerical solutions indicate that the two-fluid theory gives growth rate and wave period consistent with observations.

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Asymmetric configurations of a thin current sheet with a constant normal magnetic field component

Plasma Physics Reports ◽

10.1134/s1063780x09010097 ◽

2009 ◽

Vol 35 (1) ◽

pp. 76-83 ◽

Cited By ~ 15

Author(s):

O. V. Mingalev ◽

I. V. Mingalev ◽

Kh. V. Malova ◽

L. M. Zelenyi ◽

A. V. Artem’ev

Keyword(s):

Magnetic Field ◽

Current Sheet ◽

Field Component ◽

Magnetic Field Component ◽

Thin Current Sheet ◽

Normal Magnetic Field

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Evolution of thin current sheet with a southward interplanetary magnetic field studied by a three-dimensional electromagnetic particle code

Journal of Geophysical Research Atmospheres ◽

10.1029/1999ja000215 ◽

2000 ◽

Vol 105 (A6) ◽

pp. 13017-13028 ◽

Cited By ~ 8

Author(s):

K.-I. Nishikawa ◽

S. Ohtani

Keyword(s):

Magnetic Field ◽

Interplanetary Magnetic Field ◽

Current Sheet ◽

Three Dimensional ◽

Thin Current Sheet

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Origin of quasi-harmonic emission bands with fine structure in the dynamic spectra of high-frequency interpulses of the Crab pulsar

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1424 ◽

2020 ◽

Vol 495 (4) ◽

pp. 3715-3721

Author(s):

V V Zheleznyakov ◽

V E Shaposhnikov

Keyword(s):

Magnetic Field ◽

Fine Structure ◽

Current Sheet ◽

High Frequency ◽

Neutral Current ◽

Transverse Magnetic ◽

Central Plane ◽

Harmonic Emission ◽

Radiation Spectra ◽

Suprathermal Electrons

ABSTRACT We study the origin of quasi-harmonic emission bands with fine structure observed in the dynamic radiation spectra of high-frequency interpulses. The possible explanation of observed structure is based on the effect of double plasma resonance (DPR) at electron cyclotron harmonics realized in the magnetosphere of pulsar in a local radio emission source filled with non-relativistic plasma. The model of the source consists of neutral current sheet with a transverse magnetic field where plasma waves are generated due to DPR effect. It is shown that the emergence of emission bands and their frequency spacing are due to the inhomogeneity of the plasma and magnetic field along the current sheet, and their fine structure is due to the inhomogeneity of the current sheet in the direction orthogonal to it. Each quasi-harmonic emission band represents a system of elements of fine features of radiation that is generated by suprathermal electrons under DPR conditions. The observed upward drift of quasi-harmonic emission bands is due to the displacement of suprathermal electrons across the current sheet and an increase in the DPR frequencies with distance from the central plane of the layer.

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