Free energy sources in current sheets formed in collisionless plasma turbulence

Abstract

Collisionless dissipation of macroscopic energy into heat is an unsolved problem of space and astrophysical plasmas, e.g., solar wind and Earth's magnetosheath. The most viable process under consideration is the turbulent-cascade of macroscopic energy to kinetic-scales where collisionless-plasma-processes dissipate the energy. Space observations and numerical simulations show the formation of kinetic scale current sheets in turbulent plasmas. Instabilities in these CS can provide collisionless dissipation and influence the turbulence. Spatial gradients of physical quantities and non-Maxwellian velocity distribution functions provide the free-energy-sources for CS plasma instabilities. To determine the free-energy-sources provided by the spatial gradients of plasma density and electron/ion bulk velocities in CS formed in collisionless turbulent plasmas with an external magnetic field $\mathbf{B}_0$, we carried out two-dimensional PIC-hybrid simulations and interpret the results within the limitations of the simulation model. We found that ion-scale CS in a collisionless turbulent plasma are formed primarily by electron shear flows, i.e., electron bulk velocity inside CS is much larger than ion bulk velocity while the density variations through the CS are relatively small ($<$ 10\%). The electron-bulk-velocity and, thus, the current density inside the sheets are directed mainly parallel to $\mathbf{B}_0$. The shear in the perpendicular electron- and ion-bulk-velocities generates parallel electron- and ion-flow-vorticities. Inside CS, parallel electron-flow-vorticity exceeds the parallel ion-flow-vorticity, changes sign around the CS centers and peaks near the CS edges. An ion temperature anisotropy develops near CS during the CS formation. It has positive correlation with the parallel ion- and electron-flow-vorticities. Theoretical estimates support the simulation results.