Energy dynamics and current sheet structure in fluid and kinetic simulations of decaying magnetohydrodynamic turbulence

Abstract

Simulations of decaying magnetohydrodynamic (MHD) turbulence are performed with a fluid and a kinetic code. The initial condition is an ensemble of long-wavelength, counterpropagating, shear-Alfvén waves, which interact and rapidly generate strong MHD turbulence. The total energy is conserved and the rate of turbulent energy decay is very similar in both codes, although the fluid code has numerical dissipation whereas the kinetic code has kinetic dissipation. The inertial range power spectrum index is similar in both the codes. The fluid code shows a perpendicular wavenumber spectral slope of k −1.3 ⊥ . The kinetic code shows a spectral slope of k −1.5 ⊥ for smaller simulation domain, and k −1.3 ⊥ for larger domain. We estimate that collisionless damping mechanisms in the kinetic code can account for the dissipation of the observed nonlinear energy cascade. Current sheets are geometrically characterized. Their lengths and widths are in good agreement between the two codes. The length scales linearly with the driving scale of the turbulence. In the fluid code, their thickness is determined by the grid resolution as there is no explicit diffusivity. In the kinetic code, their thickness is very close to the skin-depth, irrespective of the grid resolution. This work shows that kinetic codes can reproduce the MHD inertial range dynamics at large scales, while at the same time capturing important kinetic physics at small scales.

Dissipation and particle energization in moderate to low beta turbulent plasma via PIC simulations

“…For ω p,e t < 200 we can see an increase of magnetic energy and a decrease of kinetic energy. This is consistent with our earlier simulations [8]. The kinetic energy is smaller than magnetic energy in the lower β case because the relativistic Alfvén velocity becomes smaller compared to the background magnetic field.…”

“…The spectrum is multiplied by k 1.3 ⊥ in order to compare with previous results in Ref. [8]. In the previous simulations at β = 0.33 at this resolution the spectral slope was closer to k −1.5 ⊥ (Fig.…”

“…3f in Ref. [8]). Also in those simulations the spectral slope tended to k −1.3 ⊥ when the simulation box was made larger, matching the spectral slope in MHD simulations.…”

“…We have simulated decaying plasma turbulence at moderate plasma β (ratio of thermal to magnetic pressure) using both magnetohydrodynamic (MHD) and PIC simulations previously [8]. In that study we found that the simulations compare remarkably well.…”

“…In Ref. [8], all the simulations had β = 0.33. In this study we have two cases, case 1 with β = 0.092 and case 2 with β = 0.026.…”