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

Silvio Sergio Cerri, Thierry Passot, Dimitri Laveder, Pierre-Louis Sulem, Matthew W. Kunz

Submitted on 9 July 2022


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 χ0 (the ratio between AW linear propagation time and nonlinear turnover time), different regimes are observed. For strong nonlinearities (χ01), turbulence is consistent with a dynamically aligned, critically balanced cascade -- fluctuations exhibit a scale-dependent alignment sinθkk1/4, a k3/2 spectrum and kk1/2 spectral anisotropy. At weaker nonlinearities (small χ0), a spectral break marking the transition between a large-scale weak regime and a small-scale k11/5 tearing-mediated range emerges, implying that dynamic alignment occurs also for weak nonlinearities. At χ0<1 the alignment angle θk shows a stronger scale dependence than in the χ01 regime, i.e. sinθkk1/2 at χ00.5, and sinθkk1 at χ00.1. Dynamic alignment in the weak regime also modifies the large-scale spectrum, scaling roughly as k3/2 for χ00.5 and as k1 for χ00.1. 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 χ0, 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


Comment: 12 pages, 4 figures, 1 table; submitted to The Astrophysical Journal

Subjects: Astrophysics - Solar and Stellar Astrophysics; Physics - Fluid Dynamics; Physics - Plasma Physics; Physics - Space Physics