The Dynamical Generation of Current Sheets in Astrophysical Plasma Turbulence

Abstract

Turbulence profoundly affects particle transport and plasma heating in many astrophysical plasma environments, from galaxy clusters to the solar corona and solar wind to Earthʼs magnetosphere. Both fluid and kinetic simulations of plasma turbulence ubiquitously generate coherent structures, in the form of current sheets, at small scales, and the locations of these current sheets appear to be associated with enhanced rates of dissipation of the turbulent energy. Therefore, illuminating the origin and nature of these current sheets is critical to identifying the dominant physical mechanisms of dissipation, a primary aim at the forefront of plasma turbulence research. Here, we present evidence from nonlinear gyrokinetic simulations that strong nonlinear interactions between counterpropagating Alfvén waves, or strong Alfvén wave collisions, are a natural mechanism for the generation of current sheets in plasma turbulence. Furthermore, we conceptually explain this current sheet development in terms of the nonlinear dynamics of Alfvén wave collisions, showing that these current sheets arise through constructive interference among the initial Alfvén waves and nonlinearly generated modes. The properties of current sheets generated by strong Alfvén wave collisions are compared to published observations of current sheets in the Earthʼs magnetosheath and the solar wind, and the nature of these current sheets leads to the expectation that Landau damping of the constituent Alfvén waves plays a dominant role in the damping of turbulently generated current sheets.

Diagnosing collisionless energy transfer using field–particle correlations: gyrokinetic turbulence

“…Therefore, this important result shows definitively that Landau resonant collisionless energy transfer can occur in a spatially non-uniform manner, in contrast to naive expectations of Landau damping based on the plane wave decomposition usually used to derive linear Landau damping. Ongoing work using field-particle correlations will determine whether Landau damping can indeed be responsible for particle energization that is highly intermittent in space, such as that occurring in the vicinity of current sheets, as has been previously suggested Howes 2015Howes , 2016.…”

The multi-scale nature of the solar wind

“…Intermittency is associated with localized large gradients in the magnetic field, which, according to Ampère's law in Equation (23), corresponds to current sheets: localized regions of enhanced current j, which are a type of coherent structure as introduced in Sect. 5.4.1 Howes, 2016). Current sheets are candidate regions for magnetic reconnection, which demonstrates the direct link between turbulence and reconnection (Matthaeus et al, 1984;Servidio et al, 2009Servidio et al, , 2010Osman et al, 2014b), and reconnection acts as a dissipation channel for the turbulent fluctuations Sundkvist et al, 2007;Cerri and Califano, 2017;Shay et al, 2018).…”

Role of reconnection in inertial kinetic-Alfvén turbulence

“…Large-scale low-frequency fluctuations in astrophysical systems such as the interstellar medium, the solar wind, and others, are associated with nearly incompressible magnetohydrodynamic (Alfvén) turbulence [e.g., [1][2][3]. The nonlinear Alfvén wave packets that compose such turbulence are three-dimensionally anisotropic: elongated along the strong background magnetic field and resembling current sheets in the perpendicular plane [1,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. At scales smaller than the plasma microscales, such as the ion gyroscale or ion inertial scale, the shear-Alfvén modes transform into kinetic Alfvén modes.…”

“…Balancing this equation with the nonlinear evolution rate γ * at scale a ∼ 1/k ⊥ (the critical balance condition), we find using Eqs. (19,20) the fieldparallel size of the eddy:…”