scholarly journals Full particle-in-cell simulations of kinetic equilibria and the role of the initial current sheet on steady asymmetric magnetic reconnection

2016 ◽  
Vol 82 (3) ◽  
Author(s):  
J. Dargent ◽  
N. Aunai ◽  
G. Belmont ◽  
N. Dorville ◽  
B. Lavraud ◽  
...  
Keyword(s):  
Distribution Function ◽  
Magnetic Reconnection ◽  
Current Sheet ◽  
Initial Conditions ◽  
Electron Flow ◽  
Electron Distribution Function ◽  
Ion Distribution ◽  
Initial Current ◽  
Particle In Cell ◽  
Kinetic Equilibrium

Tangential current sheets are ubiquitous in space plasmas and yet hard to describe with a kinetic equilibrium. In this paper, we use a semi-analytical model, the BAS model, which provides a steady ion distribution function for a tangential asymmetric current sheet and we prove that an ion kinetic equilibrium produced by this model remains steady in a fully kinetic particle-in-cell simulation even if the electron distribution function does not satisfy the time independent Vlasov equation. We then apply this equilibrium to look at the dependence of magnetic reconnection simulations on their initial conditions. We show that, as the current sheet evolves from a symmetric to an asymmetric upstream plasma, the reconnection rate is impacted and the X line and the electron flow stagnation point separate from one another and start to drift. For the simulated systems, we investigate the overall evolution of the reconnection process via the classical signatures discussed in the literature and searched in the Magnetospheric MultiScale data. We show that they seem robust and do not depend on the specific details of the internal structure of the initial current sheet.

scholarly journals Forced current sheet structure, formation and evolution: application to magnetic reconnection in the magnetosphere

2004 ◽  
Vol 22 (7) ◽  
pp. 2547-2553 ◽  
Author(s):  
V. I. Domrin ◽  
A. P. Kropotkin
Keyword(s):  
Magnetic Reconnection ◽  
Current Sheet ◽  
Particle Interaction ◽  
Nonlinear Process ◽  
Ion Distribution ◽  
Wave Type ◽  
Ion Distributions ◽  
Sheet Structure ◽  
Formation And Evolution ◽  
Nonlinear Kinetic

Abstract. By means of a simulation model, the earlier predicted nonlinear kinetic structure, a Forced Kinetic Current Sheet (FKCS), with extremely anisotropic ion distributions, is shown to appear as a result of a fast nonlinear process of transition from a previously existing equilibrium. This occurs under triggering action of a weak MHD disturbance that is applied at the boundary of the simulation box. In the FKCS, current is carried by initially cold ions which are brought into the CS by convection from both sides, and accelerated inside the CS. The process then appears to be spontaneously self-sustained, as a MHD disturbance of a rarefaction wave type propagates over the background plasma outside the CS. Comparable to the Alfvénic discontinuity in MHD, transformation of electromagnetic energy into the energy of plasma flows occurs at the FKCS. But unlike the MHD case, ``free" energy is produced here: dissipation should occur later, through particle interaction with turbulent waves generated by unstable ion distribution being formed by the FKCS action. In this way, an effect of magnetic field ``annihilation" appears, required for fast magnetic reconnection. Application of the theory to observations at the magnetopause and in the magnetotail is considered.

scholarly journals Effects on magnetic reconnection of a density asymmetry across the current sheet

2008 ◽  
Vol 26 (8) ◽  
pp. 2471-2483 ◽  
Author(s):  
K. G. Tanaka ◽  
A. Retinò ◽  
Y. Asano ◽  
M. Fujimoto ◽  
I. Shinohara ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Magnetic Reconnection ◽  
Current Sheet ◽  
Three Dimensional ◽  
Flow Region ◽  
Electron Acceleration ◽  
Particle In Cell ◽  
Line Structure ◽  
Simulation Results

Abstract. The magnetopause (MP) reconnection is characterized by a density asymmetry across the current sheet. The asymmetry is expected to produce characteristic features in the reconnection layer. Here we present a comparison between the Cluster MP crossing reported by Retinò et al. (2006) and virtual observations in two-dimensional particle-in-cell simulation results. The simulation, which includes the density asymmetry but has zero guide field in the initial condition, has reproduced well the observed features as follows: (1) The prominent density dip region is detected at the separatrix region (SR) on the magnetospheric (MSP) side of the MP. (2) The intense electric field normal to the MP is pointing to the center of the MP at the location where the density dip is detected. (3) The ion bulk outflow due to the magnetic reconnection is seen to be biased towards the MSP side. (4) The out-of-plane magnetic field (the Hall magnetic field) has bipolar rather than quadrupolar structure, the latter of which is seen for a density symmetric case. The simulation also showed rich electron dynamics (formation of field-aligned beams) in the proximity of the separatrices, which was not fully resolved in the observations. Stepping beyond the simulation-observation comparison, we have also analyzed the electron acceleration and the field line structure in the simulation results. It is found that the bipolar Hall magnetic field structure is produced by the substantial drift of the reconnected field lines at the MSP SR due to the enhanced normal electric field. The field-aligned electrons at the same MSP SR are identified as the gun smokes of the electron acceleration in the close proximity of the X-line. We have also analyzed the X-line structure obtained in the simulation to find that the density asymmetry leads to a steep density gradient in the in-flow region, which may lead to a non-stationary behavior of the X-line when three-dimensional freedom is taken into account.

scholarly journals Kinetic entropy-based measures of distribution function non-Maxwellianity: theory and simulations

2020 ◽  
Vol 86 (5) ◽  
Author(s):  
Haoming Liang ◽  
M. Hasan Barbhuiya ◽  
P. A. Cassak ◽  
O. Pezzi ◽  
S. Servidio ◽  
...  
Keyword(s):  
Distribution Function ◽  
Magnetic Reconnection ◽  
Entropy Density ◽  
Distribution Functions ◽  
Local Distribution ◽  
Particle In Cell ◽  
Maxwellian Plasma ◽  
Plasma Distribution ◽  
Maxwellian Distribution Function ◽  
Quantitative Understanding

We investigate kinetic entropy-based measures of the non-Maxwellianity of distribution functions in plasmas, i.e. entropy-based measures of the departure of a local distribution function from an associated Maxwellian distribution function with the same density, bulk flow and temperature as the local distribution. First, we consider a form previously employed by Kaufmann & Paterson (J. Geophys. Res., vol. 114, 2009, A00D04), assessing its properties and deriving equivalent forms. To provide a quantitative understanding of it, we derive analytical expressions for three common non-Maxwellian plasma distribution functions. We show that there are undesirable features of this non-Maxwellianity measure including that it can diverge in various physical limits and elucidate the reason for the divergence. We then introduce a new kinetic entropy-based non-Maxwellianity measure based on the velocity-space kinetic entropy density, which has a meaningful physical interpretation and does not diverge. We use collisionless particle-in-cell simulations of two-dimensional anti-parallel magnetic reconnection to assess the kinetic entropy-based non-Maxwellianity measures. We show that regions of non-zero non-Maxwellianity are linked to kinetic processes occurring during magnetic reconnection. We also show the simulated non-Maxwellianity agrees reasonably well with predictions for distributions resembling those calculated analytically. These results can be important for applications, as non-Maxwellianity can be used to identify regions of kinetic-scale physics or increased dissipation in plasmas.

The Evolution of Collisionless Magnetic Reconnection from Electron Scales to Ion Scales

2021 ◽  
Vol 922 (1) ◽  
pp. 51
Author(s):  
Dongkuan Liu ◽  
Kai Huang ◽  
Quanming Lu ◽  
San Lu ◽  
Rongsheng Wang ◽  
...  
Keyword(s):  
Magnetic Reconnection ◽  
Current Sheet ◽  
High Speed ◽  
Two Dimensional ◽  
Ion Outflow ◽  
Particle In Cell ◽  
Electron Kinetics ◽  
Guide Field ◽  
Cell Simulation ◽  
The Magnetic Field

Abstract It is generally accepted that collisionless magnetic reconnection is initiated on electron scales, which is mediated by electron kinetics. In this paper, by performing a two-dimensional particle-in-cell simulation, we investigate the transition of collisionless magnetic reconnection from electron scales to ion scales in a Harris current sheet with and without a guide field. The results show that after magnetic reconnection is triggered on electron scales, the electrons are first accelerated by the reconnection electric field around the X line, and then leave away along the outflow direction. In the Harris current sheet without a guide field, the electron outflow is symmetric and directed away from the X line along the center of the current sheet, while the existence of a guide field will distort the symmetry of the electron outflow. In both cases, the high-speed electron outflow is decelerated due to the existence of the magnetic field B z , then leading to the pileup of B z . With the increase of B z , the ions are accelerated by the Lorentz force in the outflow direction, and an ion outflow at about one Alfvén speed is at last formed. In this way, collisionless magnetic reconnection is transferred from the electron scales to the ion scales.

Fully kinetic PIC simulations of particle acceleration and non-Maxwellian distribution functions due to current sheets in solar wind turbulence

2021 ◽  
Author(s):  
Patricio A. Munoz ◽  
Jörg Büchner ◽  
Neeraj Jain
Keyword(s):  
Solar Wind ◽  
Particle Acceleration ◽  
Current Sheet ◽  
Plasma Turbulence ◽  
Distribution Functions ◽  
Heat Fluxes ◽  
Magnetic Islands ◽  
Ion Distribution ◽  
Current Sheets ◽  
Particle In Cell

<p>Turbulence is ubiquitous in solar system plasmas like those of the solar wind and Earth's magnetosheath. Current sheets can be formed out of this turbulence, and eventually magnetic reconnection can take place in them, a process that converts magnetic into particle kinetic energy. This interplay between turbulence and current sheet formation has been extensively analyzed with MHD and hybrid-kinetic models. Those models cover all the range between large Alfvénic scales down to ion-kinetic scales. The consequences of current sheet formation in plasma turbulence that includes electron dynamics has, however, received comparatively less attention. For this sake we carry out 2.5D fully kinetic Particle-in-Cell simulations of kinetic plasma turbulence including both ion and electron spectral ranges. In order to further assess the electron kinetic effects, we also compare our results with hybrid-kinetic simulations including electron inertia in the generalized Ohm's law. We analyze and discuss the electron and ion energization processes in the current sheets and magnetic islands formed in the turbulence. We focus on the electron and ion distribution functions formed in and around those current sheets and their stability properties that are relevant for the micro-instabilities feeding back into the turbulence cascade. We also compare pitch angle distributions and non-Maxwellian features such as heat fluxes with recent in-situ solar wind observations, which demonstrated local particle acceleration processes in reconnecting solar wind current sheets [Khabarova et al., ApJ, 2020].</p>

scholarly journals Double peak structure and diamagnetic wings of the magnetotail current sheet

2004 ◽  
Vol 22 (7) ◽  
pp. 2541-2546 ◽  
Author(s):  
G. Zimbardo ◽  
A. Greco ◽  
P. Veltri ◽  
A. L. Taktakishvili ◽  
L. M. Zelenyi
Keyword(s):  
Distribution Function ◽  
Current Sheet ◽  
Current Profile ◽  
Ion Distribution ◽  
Double Peak ◽  
Weak Anisotropy ◽  
Double Peak Structure ◽  
Magnetic Turbulence ◽  
Peak Structure ◽  
Simulation Parameters

Abstract. Recent Cluster observations in the magnetotail at about 20 Earth radii downtail have unambiguously shown that sometimes the current sheet is bifurcated, i.e. it is divided in two layers. We report numerical simulations of the ion dynamics in a quasi-neutral sheet in the presence of magnetic turbulence, which is often observed in the magnetotail, and for various anisotropies of the ion distribution function. Ions are injected at the boundary of the simulation box with a velocity distribution corresponding to a shifted Maxwellian. The simulation parameters, are adjusted to be similar to those of Cluster observations. We find that even for moderate fluctuation levels, the computed current density profile develops a double peak, in agreement with the observations. By varying the anisotropy of the injected distribution function, we are able to reproduce, for weak anisotropy, the magnetic field overshoots which are sometimes observed prior to magnetotail traversals. Therefore, we suggest an ion current profile with a double peak due to magnetic turbulence, and with possible diamagnetic current wings, present in the case of weak anisotropy of the ion distribution function.

scholarly journals Forced magnetic reconnection and plasmoid coalescence

2019 ◽  
Vol 623 ◽  
pp. A15 ◽  
Author(s):  
M. A. Potter ◽  
P. K. Browning ◽  
M. Gordovskyy
Keyword(s):  
Magnetic Reconnection ◽  
Current Sheet ◽  
Energy Release ◽  
External Perturbation ◽  
Mhd Equations ◽  
Magnetic Islands ◽  
Initial Current ◽  
Reconnection Region ◽  
Non Linear ◽  
Forced Magnetic Reconnection

Context. Forced magnetic reconnection, a reconnection event triggered by external perturbation, should be ubiquitous in the solar corona. Energy released during such cases can be much greater than that which was introduced by the perturbation. The exact dynamics of magnetic reconnection events are determined by the structure and complexity of the reconnection region: the thickness of reconnecting layers, the field curvature; the presence, shapes and sizes of magnetic islands. It is unclear how the properties of the external perturbation and the initial current sheet affect the reconnection region properties, and thereby the reconnection dynamics and energy release profile. Aims. We investigate the effect of the form of the external perturbation and initial current sheet on the evolution of the reconnection region and the energy release process. Chiefly we explore the non-linear interactions between multiple, simultaneous perturbations, which represent more realistic scenarios. Future work will use these results in test particle simulations to investigate particle acceleration over multiple reconnection events. Methods. Simulations are performed using Lare2d, a 2.5D Lagrangian-remap solver for the visco-resistive MHD equations. The model of forced reconnection is extended to include superpositions of sinusoidal driving disturbances, including localised Gaussian perturbations. A transient perturbation is applied to the boundaries of a region containing a force-free current sheet. The simulation domain is sufficiently wide to allow multiple magnetic islands to form and coalesce. Results. Island coalescence contributes significantly to energy release and involves rapid reconnection. Long wavelength modes in perturbations dominate the evolution, without the presence of which reconnection is either slow, as in the case of short wavelength modes, or the initial current sheet remains stable, as in the case of noise perturbations. Multiple perturbations combine in a highly non-linear manner: reconnection is typically faster than when either disturbance is applied individually, with multiple low-energy events contributing to the same total energy release.

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