Drift instabilities in current sheets with guide field

Abstract. We study the kinetic structure of intense ion-scale current sheets with strong electron currents and the guide field having a bell-shape profile. We consider four crossings of the Earth magnetotail current sheet by the Cluster mission in 2003. The thickness of these current sheets is about the ion inertial length and significantly smaller than the characteristic ion gyroradius. We analyze the asymmetry of the electron velocity distribution functions and show that the electron current is provided by the small electron subpopulation interpreted as an electron beam or two counter-streaming electron beams. The beam (counter-streaming beams) has a bulk velocity of the order of the electron thermal velocity and a density (difference of beam densities) of about 1–5% of the plasma density. To describe the observed current sheets we develop a kinetic model with particle beams. The model predicts different thickness of the current sheet for different types of current carriers (one electron beam or two counter-streaming electron beams). The observed ion-scale current sheets can be explained assuming that the current is carried by one electron beam and a co-streaming ion beam. Although the ion beam does not carry a significant current, this beam is required to balance the electron current perpendicular to the current sheet neutral plane. The developed model explains the dominance of the electron current and the ion scales of the current sheets.

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Bifurcated Current Sheet Observed on the Boundary of Kelvin-Helmholtz Vortices

Frontiers in Astronomy and Space Sciences ◽

10.3389/fspas.2021.782924 ◽

2021 ◽

Vol 8 ◽

Author(s):

K-J. Hwang ◽

K. Dokgo ◽

E. Choi ◽

J. L. Burch ◽

D. G. Sibeck ◽

...

Keyword(s):

Current Sheet ◽

Layer Structure ◽

Velocity Shear ◽

Current Layer ◽

Combined Effects ◽

Slow Mode ◽

Flow Shear ◽

Current Sheets ◽

Guide Field ◽

Line Region

On May 5, 2017 MMS observed a bifurcated current sheet at the boundary of Kelvin-Helmholtz vortices (KHVs) developed on the dawnside tailward magnetopause. We use the event to enhance our understanding of the formation and structure of asymmetric current sheets in the presence of density asymmetry, flow shear, and guide field, which have been rarely studied. The entire current layer comprises three separate current sheets, each corresponding to magnetosphere-side sunward separatrix region, central near-X-line region, and magnetosheath-side tailward separatrix region. Two off-center structures are identified as slow-mode discontinuities. All three current sheets have a thickness of ∼0.2 ion inertial length, demonstrating the sub-ion-scale current layer, where electrons mainly carry the current. We find that both the diamagnetic and electron anisotropy currents substantially support the bifurcated currents in the presence of density asymmetry and weak velocity shear. The combined effects of strong guide field, low density asymmetry, and weak flow shear appear to lead to asymmetries in the streamlines and the current-layer structure of the quadrupolar reconnection geometry. We also investigate intense electrostatics waves observed on the magnetosheath side of the KHV boundary. These waves may pre-heat a magnetosheath population that is to participate into the reconnection process, leading to two-step energization of the magnetosheath plasma entering into the magnetosphere via KHV-driven reconnection.

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Dynamic Alignment and Plasmoid Formation in Relativistic Magnetohydrodynamic Turbulence

The Astrophysical Journal ◽

10.3847/2041-8213/ac3afa ◽

2021 ◽

Vol 923 (1) ◽

pp. L13

Author(s):

Alexander Chernoglazov ◽

Bart Ripperda ◽

Alexander Philippov

Keyword(s):

Large Scale ◽

Magnetic Energy ◽

Magnetohydrodynamic Turbulence ◽

Current Sheets ◽

Guide Field ◽

Tearing Instability ◽

Decaying Turbulence ◽

2D And 3D ◽

Dynamic Alignment ◽

Dynamic Formation

Abstract We present high-resolution 2D and 3D simulations of magnetized decaying turbulence in relativistic, resistive magnetohydrodynamics. The simulations show dynamic formation of large-scale intermittent long-lived current sheets being disrupted into plasmoid chains by the tearing instability. These current sheets are locations of enhanced magnetic-field dissipation and heating of the plasma. We find magnetic energy spectra ∝k −3/2, together with strongly pronounced dynamic alignment of Elsässer fields and of velocity and magnetic fields, for strong guide-field turbulence, whereas we retrieve spectra ∝k −5/3 for the case of a weak guide-field.

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Distinctive features of the structure of current sheets formed in plasma in three-dimensional magnetic configurations with an X line (a review)

ADVANCES IN APPLIED PHYSICS ◽

10.51368/2307-4469-2021-9-6-464-478 ◽

2021 ◽

Vol 9 (6) ◽

pp. 464-478

Author(s):

Anna Frank

Keyword(s):

Longitudinal Magnetic Field ◽

Initial Conditions ◽

Three Dimensional ◽

Limited Range ◽

Transverse Plane ◽

Plasma Current ◽

Initial Value ◽

Distinctive Features ◽

Current Sheets ◽

Guide Field

A review is presented on experimental results related to investigation of distinctive features of the structure and evolution of plasma current sheets formed in three dimensional (3D) magnetic configurations with an X line, in the presence of a longitudinal magnetic field component (guide field) directed along the X line. It is shown that formation of a plasma current sheet results in enhancement of the guide field within the sheet. The excessive guide field is maintained by plasma currents that flow in the transverse plane relative to the main current in the sheet. As a result, the structure of the currents becomes three-dimensional. Increasing the initial value of the guide field brings about a decrease of compression into the sheet of both the electric current and plasma. This effect is caused by changing the pres- sure balance in the sheet when an excessive guide field appears in it. Deformation of plasma current sheets in 3D magnetic configurations, namely, an appearance of asymmetric and tilted sheets, results from excitation of the Hall currents and their interaction with the guide field. It is shown that the formation of current sheets in 3D magnetic configurations with an X line is possible in a relatively wide, but limited range of initial conditions

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Effect of guide field on three-dimensional electron shear flow instabilities in electron current sheets

Journal of Plasma Physics ◽

10.1017/s0022377815001257 ◽

2015 ◽

Vol 81 (6) ◽

Cited By ~ 7

Author(s):

Neeraj Jain ◽

Jörg Büchner

Keyword(s):

Shear Flow ◽

Current Sheet ◽

Three Dimensional ◽

Sheet Thickness ◽

Electron Current ◽

Tearing Mode ◽

Current Sheets ◽

Tearing Modes ◽

Guide Field ◽

Half Thickness

We examine, in the limit of electron plasma ${\it\beta}_{e}\ll 1$, the effect of an external guide field and current sheet thickness on the growth rates and nature of three-dimensional (3-D) unstable modes of an electron current sheet driven by electron shear flow. The growth rate of the fastest growing mode drops rapidly with current sheet thickness but increases slowly with the strength of the guide field. The fastest growing mode is tearing type only for thin current sheets (half-thickness ${\approx}d_{e}$, where $d_{e}=c/{\it\omega}_{pe}$ is the electron inertial length) and zero guide field. For finite guide field or thicker current sheets, the fastest growing mode is a non-tearing type. However, growth rates of the fastest 2-D tearing and 3-D non-tearing modes are comparable for thin current sheets ($d_{e}<\text{half thickness}<2\,d_{e}$) and small guide field (of the order of the asymptotic value of the component of magnetic field supporting the electron current sheet). It is shown that the general mode resonance conditions for tearing modes depend on the effective dissipation mechanism. The usual tearing mode resonance condition ($\boldsymbol{k}\boldsymbol{\cdot }\boldsymbol{B}_{0}=0$, $\boldsymbol{k}$ is the wavevector and $\boldsymbol{B}_{0}$ is the equilibrium magnetic field) can be recovered from the general resonance conditions in the limit of weak dissipation. The conditions (relating current sheet thickness, strength of the guide field and wavenumbers) for the non-existence of tearing mode are obtained from the general mode resonance conditions. We discuss the role of electron shear flow instabilities in magnetic reconnection.

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Formation and Reconnection of Three-dimensional Current Sheets with a Guide Field in the Solar Corona

The Astrophysical Journal ◽

10.3847/1538-4357/aa83ba ◽

2017 ◽

Vol 849 (1) ◽

pp. 28 ◽

Cited By ~ 6

Author(s):

J. K. Edmondson ◽

B. J. Lynch

Keyword(s):

Solar Corona ◽

Three Dimensional ◽

Current Sheets ◽

Guide Field

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Magnetic guide field generation in collisionless current sheets

Annales Geophysicae ◽

10.5194/angeo-28-789-2010 ◽

2010 ◽

Vol 28 (3) ◽

pp. 789-793 ◽

Cited By ~ 9

Author(s):

W. Baumjohann ◽

R. Nakamura ◽

R. A. Treumann

Keyword(s):

Magnetic Fields ◽

Thermal Fluctuation ◽

Space Plasma ◽

Current Layer ◽

Ion Diffusion ◽

Current Sheets ◽

Guide Field ◽

Magnetospheric Tail ◽

Current Electron ◽

Stream Instability

Abstract. In thin (Δ< few λi) collisionless current sheets in a space plasma like the magnetospheric tail or magnetopause current layer, magnetic fields can grow from thermal fluctuation level by the action of the non-magnetic Weibel instability (Weibel, 1959). The instability is driven by the counter-streaming electron inflow from the "ion diffusion" (ion inertial Hall) region into the inner current (electron inertial) region after thermalisation by the two-stream instability. Under magnetospheric tail conditions it takes ~50 e-folding times (~100 s) for the Weibel field to reach observable amplitudes |bW|~1 nT. In counter-streaming inflows these fields are of guide field type.

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