Turbulent regimes in collisions of 3D Alfv\'en-wave packets

TL;DR

This study uses 3D gyrofluid simulations to explore how different levels of nonlinearity affect Alfvén wave collisions, revealing regimes of turbulence, spectral behaviors, and the role of magnetic reconnection in energy cascade processes.

Contribution

It introduces a comprehensive analysis of turbulence regimes in Alfvén-wave collisions across varying nonlinearity levels, highlighting the transition to tearing-mediated turbulence and providing a phenomenological theory.

Findings

Strong nonlinearity leads to critically balanced turbulence with specific spectral scalings.

Weak nonlinearity regimes exhibit spectral breaks and enhanced dynamic alignment.

Tearing-mediated turbulence can occur at larger scales than previously predicted, affecting the turbulence transition.

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 ($\chi_0\sim1$), 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 $k_\|\propto k_\perp^{1/2}$ 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 $\chi_0\sim1$ regime, i.e. $\sin\theta_k\propto k_\perp^{-1/2}$ at $\chi_0\sim0.5$, and $\sin\theta_k\propto k_\perp^{-1}$ at $\chi_0\sim0.1$. Dynamic alignment in the weak regime also modifies the large-scale spectrum, scaling roughly as $k_\perp^{-3/2}$ for $\chi_0\sim0.5$ and as $k_\perp^{-1}$ for $\chi_0\sim0.1$. 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.