Ion-scale current sheet instabilities in kinetic plasma turbulence
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Final Report Abstract
Dissipation of macroscopic magnetic energy in usually turbulent and collisionless space and astrophysical plasmas is an unsolved problem. Studies so far have suggested that the dissipation occurs in and around kinetic scale current sheets self-consistently formed in these collisionless turbulent plasmas. The dissipation mechanism and current sheet characteristics controlling the dissipation are, however, not well understood, yet. We studied the characteristics of kinetic scale current sheets formed in collisionless plasma turbulence by hybrid-kinetic simulations (ions treated as particles and electrons as inertia-less fluid) using A.I.K.E.F. code. The simulations show that electric current in kinetic scale current sheets formed in the turbulence is almost entirely contributed by the electron bulk velocity parallel to the mean magnetic field. The resulting electron shear flow in current sheets can provide the free energy sources for the growth of the plasma instabilities and consequently the dissipation of the turbulent energy. Our simulations and theoretical estimates show that the ratio of the parallel bulk velocities of electrons and ions in kinetic scale current sheets formed in the turbulence is much larger than unity and increases with the thinning of current sheets below ion inertial length. This conclusion is confirmed by 3-D simulations of kinetic plasma turbulence for plasma beta of the order of unity and 2-D simulations for plasma beta in the range 0.1-10. Although the ratio is larger than unity for plasma beta much larger than one, its value is found to be smaller in comparison to the case of plasma beta less than or of the order of one. We used the jump in the value of the ratio from order of unity (outside current sheets) to a value much larger than unity (inside current sheets) as a condition to detect current sheets in space observations by WIND spacecraft. Identification of current sheets via the jump condition and an already known three-parameter method show a clear clustering of current sheets in the same locations, supporting the idea that the electron to ion velocity ratio can be considered as one of the key parameters to detect kinetic scale current sheets in space observations. We developed a PYTHON based computer program for automated detection and characterization of current sheets in numerical simulations of plasma turbulence and applied it to our 2-D simulations to obtain the distributions of peak current density, the associated peak parallel electron velocity, half thickness and length of current sheets. The distributions of the peak current density and the peak parallel electron velocity are similar consistent with the simulation results that the current in the sheets is almost entirely due to the electron bulk velocity. The distribution of half-thicknesses always peaks at the grid resolution of the simulations implying that current sheets formed in hybrid-kinetic simulations of kinetic plasma turbulence with inertia-less electrons continue to thin below ion inertial length until the thinning is stopped by the numerical effects at the grid scales. In a physical scenario, current sheets will ultimately thin down to electron scales where effects of electron inertia become important and can not be ignored. The length of the majority of current sheets, on the other hand, lies in the range of 5-25 ion inertial lengths. In order to study the thinning of current sheets down to electron scales, we parallelized the serial hybrid-kinetic code CHIEF which accurately treats the electron inertial effects. The code shows a good scaling up to several thousand compute cores on large computational grids and large number of particles per cell. The 2-D simulations of kinetic plasma turbulence carried out using the newly parallelized code CHIEF show that the electron inertia is indeed important at electron scales and its accurate consideration in hybrid-kinetic model, as is implemented in CHIEF code, is required to study the kinetic plasma turbulence from ion to electron scales. In our follow up research, we plan to study kinetic plasma turbulence from ion to electron scales and characteristics of current sheets formed therein by hybrid-kinetic simulations with electron inertia using CHIEF code.
Publications
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Free Energy Sources in Current Sheets Formed in Collisionless Plasma Turbulence. The Astrophysical Journal, 919(2), 103.
Jain, Neeraj; Büchner, Jörg; Comişel, Horia & Motschmann, Uwe
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Identification and characterization of current sheets in collisionless plasma turbulence. Physics of Plasmas, 28(5).
Azizabadi, Amirhassan Chatraee; Jain, Neeraj & Büchner, Jörg
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Electron-to-ion Bulk Speed Ratio as a Parameter Reflecting the Occurrence of Strong Electron-dominated Current Sheets in the Solar Wind. The Astrophysical Journal, 933(1), 97.
Khabarova, Olga; Büchner, Jörg; Jain, Neeraj; Sagitov, Timothy; Malova, Helmi & Kislov, Roman
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Hybrid-Kinetic Approach: Inertial Electrons. Space and Astrophysical Plasma Simulation (2022, 8, 8), 283-311. American Geophysical Union (AGU).
Jain, Neeraj; Muñoz, Patricio A. & Büchner, Jörg
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Importance of accurate consideration of the electron inertia in hybrid-kinetic simulations of collisionless plasma turbulence: The 2D limit. Physics of Plasmas, 29(5).
Jain, Neeraj; Muñoz, Patricio A.; Farzalipour, Tabriz Meisam; Rampp, Markus & Büchner, Jörg
