Structure of current sheets formed in collisionless plasma turbulence

2021 ◽  
Author(s):  
Neeraj Jain ◽  
Joerg Buechner ◽  
Patricio Munoz ◽  
Lev M. Zelenyi
Keyword(s):  
Solar Wind ◽  
Free Energy ◽  
Collisionless Plasma ◽  
Solar Wind Plasma ◽  
Plasma Turbulence ◽  
Equilibrium Structure ◽  
Energy Sources ◽  
Spatial Gradient ◽  
Current Sheets ◽  
Hybrid Simulations

<p>Plasma turbulence is ubiquitous in space and astrophysical environments and believed to play important role in a variety of space and astrophysical phenomena ranging from the entry of  energetic particles in Earth's magnetic environment and non-adiabatic heating of the solar wind plasma to star formation in inter stellar medium. Space and astrophysical plasmas are usually magnetized and collisionless. An unsolved problem in turbulent collisionless plasmas, e.g., the solar wind, is the mechanism of dissipation of macroscopic energy into heat without collisional dissipation. A number of observational and simulation studies show that kinetic sale current sheets formed self-consistently in collisionless plasma turbulence are the sites of the dissipation. Mechanisms of dissipation in current sheets are, however,  not well understood. Free energy sources in and equilibrium structure of current sheets are important factors in the determination of the dissipation mechanism. Recent PIC hybrid simulations (with mass-less electrons) of collisionless plasma turbulence show that current sheets thin down to below ion inertial length with current carried mainly by electrons. This can lead  to embedded current sheet structure which was recently studied analytically.  We carry out 2-D PIC-hybrid simulations (with finite-mass electrons) using a recently developed code CHIEF to study the free energy sources and structure of current sheets formed in turbulence. In this paper, we focus on  the spatial gradient driven free energy sources and embedded structure of current sheets.  The results are compared to the results obtained from hybrid simulations with mass-less electrons. </p>

Free energy sources in kinetic scale current sheets formed in collisionless plasma turbulence

2020 ◽  
Author(s):  
Neeraj Jain ◽  
Joerg Buechner
Keyword(s):  
Solar Wind ◽  
Free Energy ◽  
Collisionless Plasma ◽  
Plasma Turbulence ◽  
Energy Sources ◽  
Ion Temperature ◽  
Temperature Anisotropy ◽  
Current Sheets ◽  
Adiabatic Cooling ◽  
Collisionless Dissipation

<p>Spacecraft observations show the radial dependence of the solar wind temperature to be slower than what is expected from the adiabatic cooling of the solar wind expanding radially outwards from the sun. The most viable process considered to explain the observed slower-than-adiabatic cooling is the heating of the solar wind plasma by dissipation of the turbulent fluctuations. In solar wind which is  a collisionless plasma in turbulent state, macroscopic energy is cascaded down to kinetic scales where kinetic plasma processes can finally dissipate the energy into heat. The kinetic scale plasma processes responsible  for the dissipation of energy are, however, not well understood. A number of observational and simulation studies have shown that the heating is concentrated in and around current sheets self-consistently formed at kinetic scales. The current sheets contain free energy sources for the growth of plasma instabilities which can serve as the mechanism of the collisionless dissipation. A detailed information on the free energy sources contained in these current sheets of plasma turbulence is lacking but essential to understand the role of  plasma instabilities in collisionless dissipation.</p><p>We carry out 2-D hybrid simulations of kinetic plasma turbulence to study in detail free energy sources available in the current sheets formed in the turbulence. We focus on three free energy sources, namely, plasma density gradient, velocity gradients for both ions and electrons and ion temperature anisotropy. Our simulations show formation of current sheets in which electric current parallel to the externally applied magnetic field flows in a thickness of the order of an ion inertial length. Inside a current sheet, electron flow velocity dominates ion flow velocity in the parallel direction resulting in a larger cross-gradient of the former. The perpendicular electron velocity inside a current sheet also has variations sharper than the corresponding ion velocity. Cross gradients in plasma density are weak (under 10 % variation inside current sheets). Ion temperature is anisotropic in current sheets. Thus the current in the sheets is primarily due to electron shear flow. A theoretical model to explain the difference between electron and ion velocities in current sheets is developed. Spacecraft observations of electron shear flow in space plasma turbulence will be pointed out.   </p><p>These results suggest that the current sheets formed in kinetic plasma turbulence are close to the force free equilibrium rather than the often assumed Harris equilibrium.  This demands investigations of the linear stability properties and nonlinear evolution of force free current sheets with temperature anisotropy. Such studies can provide effective dissipation coefficients to be included in macroscopic model of the solar wind evolution.   </p>

scholarly journals Free Energy Sources in Current Sheets Formed in Collisionless Plasma Turbulence

2021 ◽  
Vol 919 (2) ◽  
pp. 103
Author(s):  
Neeraj Jain ◽  
Jörg Büchner ◽  
Horia Comişel ◽  
Uwe Motschmann
Keyword(s):  
Free Energy ◽  
Collisionless Plasma ◽  
Plasma Turbulence ◽  
Energy Sources ◽  
Current Sheets

scholarly journals A dynamical model of plasma turbulence in the solar wind

2015 ◽  
Vol 373 (2041) ◽  
pp. 20140145 ◽  
Author(s):  
G. G. Howes
Keyword(s):  
Solar Wind ◽  
Solar Wind Plasma ◽  
Plasma Turbulence ◽  
Alfven Waves ◽  
Dynamical Model ◽  
Alfvén Waves ◽  
Small Scale ◽  
Current Sheets ◽  
Turbulent Fluctuations ◽  
Physical Mechanisms

A dynamical approach, rather than the usual statistical approach, is taken to explore the physical mechanisms underlying the nonlinear transfer of energy, the damping of the turbulent fluctuations, and the development of coherent structures in kinetic plasma turbulence. It is argued that the linear and nonlinear dynamics of Alfvén waves are responsible, at a very fundamental level, for some of the key qualitative features of plasma turbulence that distinguish it from hydrodynamic turbulence, including the anisotropic cascade of energy and the development of current sheets at small scales. The first dynamical model of kinetic turbulence in the weakly collisional solar wind plasma that combines self-consistently the physics of Alfvén waves with the development of small-scale current sheets is presented and its physical implications are discussed. This model leads to a simplified perspective on the nature of turbulence in a weakly collisional plasma: the nonlinear interactions responsible for the turbulent cascade of energy and the formation of current sheets are essentially fluid in nature, while the collisionless damping of the turbulent fluctuations and the energy injection by kinetic instabilities are essentially kinetic in nature.

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.

scholarly journals Solar-Wind Structures That Are Not Destroyed by the Action of Solar-Wind Turbulence

2021 ◽  
Vol 8 ◽  
Author(s):  
Joseph E. Borovsky
Keyword(s):  
Solar Wind ◽  
Magnetic Structure ◽  
Solar Wind Plasma ◽  
Ion Composition ◽  
Mhd Turbulence ◽  
Current Sheets ◽  
Wind Structure ◽  
Solar Wind Structure ◽  
Wind Measurements ◽  
Dominant Process

If MHD turbulence is a dominant process acting in the solar wind between the Sun and 1 AU, then the destruction and regeneration of structure in the solar-wind plasma is expected. Six types of solar-wind structure at 1 AU that are not destroyed by turbulence are examined: 1) corotating-interaction-region stream interfaces, 2) periodic density structures, 3) magnetic structure anisotropy, 4) ion-composition boundaries and their co-located current sheets, 5) strahl-intensity boundaries and their co-located current sheets, and 6) non-evolving Alfvénic magnetic structure. Implications for the solar wind and for turbulence in the solar wind are highlighted and a call for critical future solar-wind measurements is given.

scholarly journals Identification and characterization of current sheets in collisionless plasma turbulence

2021 ◽  
Vol 28 (5) ◽  
pp. 052904
Author(s):  
Amirhassan Chatraee Azizabadi ◽  
Neeraj Jain ◽  
Jörg Büchner
Keyword(s):  
Collisionless Plasma ◽  
Plasma Turbulence ◽  
Current Sheets ◽  
Identification And Characterization

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 Fluidization of collisionless plasma turbulence

2019 ◽  
Vol 116 (4) ◽  
pp. 1185-1194 ◽  
Author(s):  
Romain Meyrand ◽  
Anjor Kanekar ◽  
William Dorland ◽  
Alexander A. Schekochihin
Keyword(s):  
Solar Wind ◽  
Power Law ◽  
Collisionless Plasma ◽  
Plasma Turbulence ◽  
Magnetized Plasma ◽  
Landau Damping ◽  
Astrophysical Plasmas ◽  
Phase Mixing ◽  
Fluid Turbulence ◽  
Collisionless Plasmas

In a collisionless, magnetized plasma, particles may stream freely along magnetic field lines, leading to “phase mixing” of their distribution function and consequently, to smoothing out of any “compressive” fluctuations (of density, pressure, etc.). This rapid mixing underlies Landau damping of these fluctuations in a quiescent plasma—one of the most fundamental physical phenomena that makes plasma different from a conventional fluid. Nevertheless, broad power law spectra of compressive fluctuations are observed in turbulent astrophysical plasmas (most vividly, in the solar wind) under conditions conducive to strong Landau damping. Elsewhere in nature, such spectra are normally associated with fluid turbulence, where energy cannot be dissipated in the inertial-scale range and is, therefore, cascaded from large scales to small. By direct numerical simulations and theoretical arguments, it is shown here that turbulence of compressive fluctuations in collisionless plasmas strongly resembles one in a collisional fluid and does have broad power law spectra. This “fluidization” of collisionless plasmas occurs, because phase mixing is strongly suppressed on average by “stochastic echoes,” arising due to nonlinear advection of the particle distribution by turbulent motions. Other than resolving the long-standing puzzle of observed compressive fluctuations in the solar wind, our results suggest a conceptual shift for understanding kinetic plasma turbulence generally: rather than being a system where Landau damping plays the role of dissipation, a collisionless plasma is effectively dissipationless, except at very small scales. The universality of “fluid” turbulence physics is thus reaffirmed even for a kinetic, collisionless system.

Sign in / Sign up

Export Citation Format

Share Document