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>

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>

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 Effects of the Background Turbulence on the Relaxation of Ion Temperature Anisotropy in Space Plasmas

2021 ◽  
Vol 9 ◽  
Author(s):  
Pablo S. Moya ◽  
Roberto E. Navarro
Keyword(s):  
Solar Wind ◽  
Power Law ◽  
Large Scale ◽  
Wave Spectrum ◽  
Collisionless Plasma ◽  
Space Plasmas ◽  
Ion Temperature ◽  
Temperature Anisotropy ◽  
Background Spectrum ◽  
Background Magnetic Field

Turbulence in space plasmas usually exhibits two regimes separated by a spectral break that divides the so called inertial and kinetic ranges. Large scale magnetic fluctuations are dominated by non-linear MHD wave-wave interactions following a −5/3 or −2 slope power-law spectrum. After the break, at scales in which kinetic effects take place, the magnetic spectrum follows a steeper power-law k−α shape given by a spectral index α > 5/3. Despite its ubiquitousness, the possible effects of a turbulent background spectrum in the quasilinear relaxation of solar wind temperatures are usually not considered. In this work, a quasilinear kinetic theory is used to study the evolution of the proton temperatures in an initially turbulent collisionless plasma composed by cold electrons and bi-Maxwellian protons, in which electromagnetic waves propagate along a background magnetic field. Four wave spectrum shapes are compared with different levels of wave intensity. We show that a sufficient turbulent magnetic power can drive stable protons to transverse heating, resulting in an increase in the temperature anisotropy and the reduction of the parallel proton beta. Thus, stable proton velocity distribution can evolve in such a way as to develop kinetic instabilities. This may explain why the constituents of the solar wind can be observed far from thermodynamic equilibrium and near the instability thresholds.

scholarly journals Apparent temperature anisotropies due to wave activity in the solar wind

2011 ◽  
Vol 29 (5) ◽  
pp. 909-917 ◽  
Author(s):  
D. Verscharen ◽  
E. Marsch
Keyword(s):  
Solar Wind ◽  
Distribution Function ◽  
Collisionless Plasma ◽  
Velocity Distribution Function ◽  
Wave Activity ◽  
Apparent Temperature ◽  
Time Averaging ◽  
Ion Temperature ◽  
Temperature Anisotropy ◽  
Underlying Distribution

Abstract. The fast solar wind is a collisionless plasma permeated by plasma waves on many different scales. A plasma wave represents the natural interplay between the periodic changes of the electromagnetic field and the associated coherent motions of the plasma particles. In this paper, a model velocity distribution function is derived for a plasma in a single, coherent, large-amplitude wave. This model allows one to study the kinetic effects of wave motions on particle distributions. They are by in-situ spacecraft measured by counting, over a certain sampling time, the particles coming from various directions and having different energies. We compare our results with the measurements by the Helios spacecraft, and thus find that by assuming high wave activity we are able to explain key observed features of the measured distributions within the framework of our model. We also address the recent discussions on nonresonant wave–particle interactions and apparent heating. The applied time-averaging procedure leads to an apparent ion temperature anisotropy which is connected but not identical to the intrinsic temperature of the underlying distribution function.

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 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.

scholarly journals Hybrid models of solar wind plasma heating

2011 ◽  
Vol 29 (6) ◽  
pp. 1071-1079 ◽  
Author(s):  
L. Ofman ◽  
A.-F. Viñas ◽  
P. S. Moya
Keyword(s):  
Solar Wind ◽  
Heavy Ions ◽  
Plasma Heating ◽  
Wave Spectrum ◽  
Solar Wind Plasma ◽  
Ion Temperature ◽  
Temperature Anisotropy ◽  
Streaming Instability ◽  
Turbulent Spectrum

Abstract. Remote sensing and in-situ observations show that solar wind ions are often hotter than electrons, and the heavy ions flow faster than the protons by up to an Alfvén speed. Turbulent spectrum of Alfvénic fluctuations and shocks were detected in solar wind plasma. Cross-field inhomogeneities in the corona were observed to extend to several tens of solar radii from the Sun. The acceleration and heating of solar wind plasma is studied via 1-D and 2-D hybrid simulations. The models describe the kinetics of protons and heavy ions, and electrons are treated as neutralizing fluid.The expansion of the solar wind is considered in 1-D hybrid model. A spectrum of Alfvénic fluctuations is injected at the computational boundary, produced by differential streaming instability, or initial ion temperature anisotropy, and the parametric dependence of the perpendicular heating of H+-He++ solar wind plasma is studied. It is found that He++ ions are heated efficiently by the Alfvénic wave spectrum below the proton gyroperiod.

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