Invited Talk - Splinter Magnetic

Thursday, 24 September 2020, 14:00   (virtual room H)

Free energy sources in current sheets formed in collisionless plasma turbulence

Neeraj Jain and Joerg Buechner
Technical University Berlin, Berlin

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. In usually magnetized and collisionless space and astrophysical plasmas, e.g., the solar wind, mechanism of dissipation of macroscopic energy into heat without the normal channels of collisional viscosity and electrical resistivity is an unsolved problem. 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. Tearing instability leading to magnetic reconnection in the turbulent current sheets is often invoked as the mechanism of the collisionless dissipation. It is, however, not clear if free energy sources and conducive environment required for the growth of tearing and/or other plasma instabilities are available in the turbulent current sheets. Free energy sources for plasma instabilities in a current sheet can come from spatial gradients of physical variables or the non-Maxwellian features of the distribution function of plasma particles. In this paper, we carry out 2-D PIC-hybrid simulations of collisionless plasma turbulence to study the free energy sources provided by the spatial gradients of plasma number density and electron/ion bulk velocities in current sheets formed in the turbulence. We find that current sheets are formed primarily by electron shear flow, i.e., electron bulk velocity is much larger than ion bulk velocity and density variation is relatively small (<10\%) inside current sheets. Electron bulk velocity and thus current inside sheets are directed mainly parallel to the external magnetic field. Shear flow in perpendicular bulk velocities of electrons and ions generates parallel components of electrons and ions vorticity, the former of which is larger than the later inside current sheets, changes sign around the center and peaks near the edges of current sheets. Ion temperature anisotropy develops near current sheets during the formation of current sheets and correlates well with both the electron and ion vorticities. Theoretical estimates in the limit of un-magnetized ions support the simulation results.