Turbulent regimes in collisions of 3D Alfvén-wave packets

Abstract

Using 3D gyrofluid simulations, we revisit the problem of Alfven-wave (AW) collisions as building blocks of the Alfvenic cascade and their interplay with magnetic reconnection at magnetohydrodynamic (MHD) scales. Depending on the large-scale nonlinearity parameter ${\chi }_0$ (the ratio between AW linear propagation time and nonlinear turnover time), different regimes are observed. For strong nonlinearities (${\chi }_0\sim 1$), turbulence is consistent with a dynamically aligned, critically balanced cascade--fluctuations exhibit a scale-dependent alignment $\sin {\theta }_k\propto k_{\perp }^{-1/4}$, a $k_{\perp }^{-3/2}$ spectrum and $k_{\Vert }\propto k_{\perp }^{1/2}$ spectral anisotropy. At weaker nonlinearities (small ${\chi }_0$), a spectral break marking the transition between a large-scale weak regime and a small-scale $k_{\perp }^{-11/5}$ tearing-mediated range emerges, implying that dynamic alignment occurs also for weak nonlinearities. At ${\chi }_0<1$ the alignment angle ${\theta }_{k_{\perp }}$ shows a stronger scale dependence than in the ${\chi }_0\sim 1$ regime, i.e. $\sin {\theta }_k\propto k_{\perp }^{-1/2}$ at ${\chi }_0\sim 0.5$, and $\sin {\theta }_k\propto k_{\perp }^{-1}$ at ${\chi }_0\sim 0.1$. Dynamic alignment in the weak regime also modifies the large-scale spectrum, scaling roughly as $k_{\perp }^{-3/2}$ for ${\chi }_0\sim 0.5$ and as $k_{\perp }^{-1}$ for ${\chi }_0\sim 0.1$. A phenomenological theory of dynamically aligned turbulence at weak nonlinearities that can explain these spectra and the transition to the tearing-mediated regime is provided; at small ${\chi }_0$, the strong scale dependence of the alignment angle combines with the increased lifetime of turbulent eddies to allow tearing to onset and mediate the cascade at scales that can be larger than those predicted for a critically balanced cascade by several orders of magnitude. Such a transition to tearing-mediated turbulence may even supplant the usual weak-to-strong transition.

13 pages, 5 figures, 1 table; accepted for publication in The Astrophysical Journal

References in corpus (22)

• The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere
• Recent progress in astrophysical plasma turbulence from solar wind observations
• Cosmic Ray Propagation: Nonlinear Diffusion Parallel and Perpendicular to Mean Magnetic Field
• Dynamic Alignment in Driven Magnetohydrodynamic Turbulence
• Magnetic reconnection as a driver for a sub-ion scale cascade in plasma turbulence
• Rapid directional alignment of velocity and magnetic field in magnetohydrodynamic turbulence
• Multiscale nature of the dissipation range in gyrokinetic simulations of Alfvénic turbulence
• Collisionless Reconnection in Magnetohydrodynamic and Kinetic Turbulence
• Hybrid-Kinetic Simulations of Ion Heating in Alfvénic Turbulence
• Magnetohydrodynamic turbulence mediated by reconnection
• Kinetic cascade in solar-wind turbulence: 3D3V hybrid-kinetic simulations with electron inertia
• Fully kinetic versus reduced-kinetic modelling of collisionless plasma turbulence
• Critical Balance and the Physics of MHD Turbulence
• Reconciling cosmic-ray transport theory with phenomenological models motivated by Milky-Way data
• Kinetic plasma turbulence: recent insights and open questions from 3D3V simulations
• The theory of cosmic-ray scattering on pre-existing MHD modes meets data
• Three-dimensional magnetic reconnection in particle-in-cell simulations of anisotropic plasma turbulence
• Imbalanced kinetic Alfvén wave turbulence: from weak turbulence theory to nonlinear diffusion models for the strong regime
• Weak Alfvénic turbulence in relativistic plasmas II: Current sheets and dissipation
• Spectrum of kinetic plasma turbulence at 0.3-0.9 astronomical units from the Sun
• Characterizing current structures in 3D hybrid-kinetic simulations of plasma turbulence
• Direct kinetic Alfvén wave energy cascade in the presence of imbalance

Cited by in corpus (3)

• Internal gravity waves in stratified flows with and without vortical modes
• Revisiting the role of cosmic-ray driven Alfvén waves in pre-existing magnetohydrodynamic turbulence. I. Turbulent damping rates and feedback on background fluctuations
• Gyrofluid simulations of turbulence and reconnection in space plasmas