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

A Hamiltonian two-field gyrofluid model for kinetic Alfv\'en waves (KAWs) in a magnetized electron-proton plasma, retaining ion finite-Larmor-radius corrections and parallel magnetic field fluctuations, is used to study the inverse cascades that develop when turbulence is randomly driven at sub-ion scales. In the directions perpendicular to the ambient field, the dynamics of the cascade turns out to be nonlocal and the ratio $\chi_f$ of the wave period to the characteristic nonlinear time at the driving scale affect some of its properties. For example, at small values of $\chi_f$, parametric decay instability of the modes driven by the forcing can develop, enhancing for a while inverse transfers. The balanced state, obtained at early time when the two counter-propagating waves are equally driven, also becomes unstable at small $\chi_f$, leading to an inverse cascade. For $\beta_e$ smaller than a few units, the cascade slows down when reaching the low-dispersion spectral range. For higher $\beta_e$, the ratio of the KAW to the Alfv\'en frequencies displays a local minimum. At the corresponding transverse wavenumber, a condensate is formed, and the cascade towards larger scales is then inhibited. Depending on the parameters, a parallel inverse cascade can develop, enhancing the elongation of the ion-scale magnetic vortices that generically form., Comment: 35 pages, 14 figures, 1 table

A Hamiltonian two-field gyrofluid model for kinetic Alfv\'en waves (KAWs) in a magnetized electron-proton plasma, retaining ion finite-Larmor-radius corrections and parallel magnetic field fluctuations, is used to study the inverse cascades that develop when turbulence is randomly driven at sub-ion scales. In the directions perpendicular to the ambient field, the dynamics of the cascade turns out to be nonlocal and the ratio $\chi_f$ of the wave period to the characteristic nonlinear time at the driving scale affect some of its properties. For example, at small values of $\chi_f$, parametric decay instability of the modes driven by the forcing can develop, enhancing for a while inverse transfers. The balanced state, obtained at early time when the two counter-propagating waves are equally driven, also becomes unstable at small $\chi_f$, leading to an inverse cascade. For $\beta_e$ smaller than a few units, the cascade slows down when reaching the low-dispersion spectral range. For higher $\beta_e$, the ratio of the KAW to the Alfv\'en frequencies displays a local minimum. At the corresponding transverse wavenumber, a condensate is formed, and the cascade towards larger scales is then inhibited. Depending on the parameters, a parallel inverse cascade can develop, enhancing the elongation of the ion-scale magnetic vortices that generically form., Comment: 35 pages, 14 figures, 1 table

An example of wave collapse arising in dispersive magnetohydrodynamics is analyzed by means of both direct numerical simulations and envelope formalism. It is shown that in spite of the presence of various types of waves and of purely hydrodynamic effects, the evolution of a longitudinally homogeneous Alfven-wave beam propagating along an ambient magnetic field is accurately described by a cubic nonlinear Schrodinger equation with an external potential proportional to the initial wave intensity. In this description, an axisymmetric beam can only collapse on its axis when its transverse extension significantly exceeds the typical scale of the modulational instability of the carrying wave. An axisymmetric configuration is however unstable with respect to azimuthal perturbations, leading to off-center collapse, even in situations that are smooth when axisymmetry is preserved. In the case of a wave packet with a finite longitudinal extension, a minimum size is required for blowup, associated with the formation of strongly anisotropic magnetic structures. (C) 2003 Elsevier B.V. All rights reserved.

An example of wave collapse arising in dispersive magnetohydrodynamics is analyzed by means of both direct numerical simulations and envelope formalism. It is shown that in spite of the presence of various types of waves and of purely hydrodynamic effects, the evolution of a longitudinally homogeneous Alfven-wave beam propagating along an ambient magnetic field is accurately described by a cubic nonlinear Schrodinger equation with an external potential proportional to the initial wave intensity. In this description, an axisymmetric beam can only collapse on its axis when its transverse extension significantly exceeds the typical scale of the modulational instability of the carrying wave. An axisymmetric configuration is however unstable with respect to azimuthal perturbations, leading to off-center collapse, even in situations that are smooth when axisymmetry is preserved. In the case of a wave packet with a finite longitudinal extension, a minimum size is required for blowup, associated with the formation of strongly anisotropic magnetic structures. (C) 2003 Elsevier B.V. All rights reserved.