Three-dimensional simulations of sheared current sheets: transition to turbulence?

Abstract

Systems of multiple current sheets arise in various situations in natural plasmas, such as at the heliospheric current sheet in the solar wind and in the outer heliosphere in the heliosheath. Previous three-dimensional simulations have shown that such systems can develop turbulent-like fluctuations resulting from a forward and inverse cascade in wave vector space. We present a study of the transition to turbulence of such multiple current sheet systems, including the effects of adding a magnetic guide field and velocity shears across the current sheets. Three-dimensional hybrid simulations are performed of systems with eight narrow current sheets in a triply periodic geometry. We carry out a number of different analyses of the evolution of the fluctuations as the initially highly ordered state relaxes to one which resembles turbulence. Despite the evidence of a forward and inverse cascade in the fluctuation power spectra, we find that none of the simulated cases have evidence of intermittency after the initial period of fast reconnection associated with the ion tearing instability at the current sheets. Cancellation analysis confirms that the simulations have not evolved to a state which can be identified as fully developed turbulence. The addition of velocity shears across the current sheets slows the evolution in the properties of the fluctuations, but by the end of the simulation they are broadly similar. However, if the simulation is constrained to be two-dimensional, differences are found, indicating that fully three-dimensional simulations are important when studying the evolution of an ordered equilibrium towards a turbulent-like state.

Observing the prevalence of thin current sheets downstream of Earth's bow shock

“…Observing the prevalence of thin current sheets downstream of Earth's bow shock Together these trends reveal that the generation (and decay) of current sheets varies considerably in different regions of the magnetosheath. The influence of current sheets on the local plasma environment is known to be important across multiple scales: current sheets can serve to generate disordered or turbulent fluctuations at large scales 39 , and are also expected to play a role in the dissipation of energy at the smallest scales [7][8][9][10][11] . Furthermore, current sheets may convert a significant fraction of the incident energy flux from the solar wind 26 .…”

Sign Singularity of the Local Energy Transfer in Space Plasma Turbulence

“…fractal dimension, filling factor...) of sign-defined (smooth) coherent structures, such as the ones emerging in intermittent, turbulent flows. A standard technique to estimate sign singularity is provided by the cancellation analysis, previously used to describe the scaling properties of MHD, Hall-MHD and Vlasov-Maxwell turbulence in numerical simulations [44,[62][63][64][65] and in the current helicity in solar photospheric active regions [66][67][68][69].…”

Fast acceleration of transrelativistic electrons in astrophysical turbulence

“…This is closely related to first-order Fermi acceleration, and has been used to explain energetic particles observations near the heliospheric current sheet [30]. Systems of multiple current sheets evolve to become turbulent [31], so, in terms of particle acceleration, the relative role of magnetic structures in turbulence, compared with the closed magnetic islands found in reconnection geometries, becomes an interesting topic.…”