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

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.

scholarly journals Thin current sheets with strong bell-shape guide field: Cluster observations and models with beams

2014 ◽  
Vol 32 (10) ◽  
pp. 1349-1360 ◽  
Author(s):  
I. Y. Vasko ◽  
A. V. Artemyev ◽  
A. A. Petrukovich ◽  
H. V. Malova
Keyword(s):  
Electron Beam ◽  
Current Sheet ◽  
Electron Beams ◽  
Ion Beam ◽  
Distribution Functions ◽  
Electron Velocity ◽  
Electron Current ◽  
Electron Velocity Distribution ◽  
Current Sheets ◽  
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.

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

2015 ◽  
Vol 81 (6) ◽  
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.

scholarly journals Magnetic guide field generation in collisionless current sheets

2010 ◽  
Vol 28 (3) ◽  
pp. 789-793 ◽  
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.

Drift instabilities in current sheets with guide field

2008 ◽  
Vol 15 (7) ◽  
pp. 072101 ◽  
Author(s):  
P. H. Yoon ◽  
A. T. Y. Lui
Keyword(s):  
Current Sheets ◽  
Guide Field

Propagation of magnetodynamic waves into a collisionless current layer

1976 ◽  
Vol 15 (3) ◽  
pp. 389-394 ◽  
Author(s):  
A. Hruška
Keyword(s):  
Magnetic Field ◽  
Current Layer ◽  
Fast Mode ◽  
Slow Mode ◽  
Field Aligned Current ◽  
The Waves

In a layer of magnetic field aligned current, waves corresonding to the slow mode in the limit of no current are absorbed and/or reflected as soon as they enter the layer, while, under certain conditions, the waves corresponding to the fast mode do propagate through the layer.

scholarly journals Localized fast flow disturbance observed in the plasma sheet and in the ionosphere

2005 ◽  
Vol 23 (2) ◽  
pp. 553-566 ◽  
Author(s):  
R. Nakamura ◽  
O. Amm ◽  
H. Laakso ◽  
N. C. Draper ◽  
M. Lester ◽  
...  
Keyword(s):  
Spatial Scale ◽  
Current Sheet ◽  
Plasma Sheet ◽  
Time Scale ◽  
Ionospheric Disturbance ◽  
Flow Shear ◽  
Current Location ◽  
Field Aligned Current ◽  
Temporal And Spatial ◽  
Fast Flow

Abstract. An isolated plasma sheet flow burst took place at 22:02 UT, 1 September 2002, when the Cluster footpoint was located within the area covered by the Magnetometers-Ionospheric Radars-All-sky Cameras Large Experiment (MIRACLE). The event was associated with a clear but weak ionospheric disturbance and took place during a steady southward IMF interval, about 1h preceding a major substorm onset. Multipoint observations, both in space and from the ground, allow us to discuss the temporal and spatial scale of the disturbance both in the magnetosphere and ionosphere. Based on measurements from four Cluster spacecraft it is inferred that Cluster observed the dusk side part of a localized flow channel in the plasma sheet with a flow shear at the front, suggesting a field-aligned current out from the ionosphere. In the ionosphere the equivalent current pattern and possible field-aligned current location show a pattern similar to the auroral streamers previously obtained during an active period, except for its spatial scale and amplitude. It is inferred that the footpoint of Cluster was located in the region of an upward field-aligned current, consistent with the magnetospheric observations. The entire disturbance in the ionosphere lasted about 10min, consistent with the time scale of the current sheet disturbance in the magnetosphere. The plasma sheet bulk flow, on the other hand, had a time scale of about 2min, corresponding to the time scale of an equatorward excursion of the enhanced electrojet. These observations confirm that localized enhanced convection in the magnetosphere and associated changes in the current sheet structure produce a signature with consistent temporal and spatial scale at the conjugate ionosphere.

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

2004 ◽  
Vol 11 (5/6) ◽  
pp. 579-587 ◽  
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.

Effects of a guide field on the evolution of a current sheet

2006 ◽  
Vol 13 (10) ◽  
pp. 102902 ◽  
Author(s):  
C. L. Tsai ◽  
L. C. Lee ◽  
B. H. Wu
Keyword(s):  
Current Sheet ◽  
Guide Field ◽  
A Current

scholarly journals Sausage mode instability of thin current sheets as a cause of magnetospheric substorms

1999 ◽  
Vol 17 (5) ◽  
pp. 604-612 ◽  
Author(s):  
J. Büchner ◽  
J.-P. Kuska
Keyword(s):  
Current Sheet ◽  
Three Dimensional ◽  
Dispersion Analysis ◽  
Magnetospheric Substorms ◽  
Linear Dispersion ◽  
Current Sheets ◽  
Kinetic Effects ◽  
The Stability ◽  
Substorm Onsets ◽  
Pic Code

Abstract. Observations have shown that, prior to substorm explosions, thin current sheets are formed in the plasma sheet of the Earth's magnetotail. This provokes the question, to what extent current-sheet thinning and substorm onsets are physically, maybe even causally, related. To answer this question, one has to understand the plasma stability of thin current sheets. Kinetic effects must be taken into account since particle scales are reached in the course of tail current-sheet thinning. We present the results of theoretical investigations of the stability of thin current sheets and about the most unstable mode of their decay. Our conclusions are based upon a non-local linear dispersion analysis of a cross-magnetic field instability of Harris-type current sheets. We found that a sausage-mode bulk current instability starts after a sheet has thinned down to the ion inertial length. We also present the results of three-dimensional electromagnetic PIC-code simulations carried out for mass ratios up to Mi / me=64. They verify the linearly predicted properties of the sausage mode decay of thin current sheets in the parameter range of interest.Key words. Magnetospheric physics (plasma waves and instabilities; storms and substorms) · Space plasma physics (magnetic reconnection)

scholarly journals Current sheets from Ulysses observation

2011 ◽  
Vol 29 (2) ◽  
pp. 237-249 ◽  
Author(s):  
B. Miao ◽  
B. Peng ◽  
G. Li
Keyword(s):  
Solar Wind ◽  
Current Sheet ◽  
Solar Wind Plasma ◽  
Mhd Turbulence ◽  
Automatic Data ◽  
Flux Tubes ◽  
Current Sheets ◽  
Solar Origin ◽  
Turbulence Intermittency ◽  
Solar Wind Magnetic Field

Abstract. Current sheet is a significant source of solar wind MHD turbulence intermittency. It has long been recognized that these structures can arise from non-linear interactions of MHD turbulence. Alternatively, they may also be relic structures in the solar wind that have a solar origin, e.g., magnetic walls of flux tubes that separate solar wind plasma into distinct parcels. Identifying these structures in the solar wind is crucial to understanding the properties of the solar wind MHD turbulence. Using Ulysses observations we examine 3-year worth of solar wind magnetic field data when the Ulysses is at low latitude during solar minimum. Extending the previous work of Li (2007, 2008), we develop an automatic data analysis method of current sheet identification. Using this method, we identify more than 28000 current sheets. Various properties of the current sheet are obtained. These include the distributions of the deflection angle across the current sheet, the thickness of the current sheet and the waiting time statistics between current sheets.

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