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The impact of ion pressure anisotropy on the energy cascade rate of Hall-MHD turbulence with biadiabatic ions and isothermal electrons is evaluated in three-dimensional direct numerical simulations, using the exact (or third-order) law derived in \citet{simon_exact_2022}. It is shown that pressure anisotropy can enhance or reduce the cascade rate, depending on the scales, in comparison with the prediction of the exact law with isotropic pressure, by an amount that correlates well with pressure anisotropy $a_p=\frac{p_\perp}{p_\parallel}\neq1$ that develops in simulations initialized with an isotropic pressure (${a_p}_0=1$). A simulation with initial pressure anisotropy, ${a_p}_0=4$, confirms this trend, exhibiting a stronger impact on the cascade rate, both in the inertial range and at larger scales, close to the forcing scales. Furthermore, a Fourier-based numerical method, to compute exact laws in numerical simulations in the full $(\ell_\perp,\ell_\parallel)$ increment plane, is presented., Comment: 20 pages, 11 figures, submitted to PRE

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

Magnetic reconnection (MR) plays a fundamental role in plasma dynamics under many different conditions, from space and astrophysical environments to laboratory devices. High-resolution in-situ measurements from space missions allow to study naturally occurring MR processes in great detail. Alongside direct measurements, numerical simulations play a key role in investigating the fundamental physics underlying MR. The choice of an adequate plasma model to be employed in numerical simulations, while also compromising with their computational cost, is crucial to efficiently address the problem. We consider a new plasma model that includes a refined electron response within the hybrid-kinetic framework (kinetic ions, fluid electrons). The extent to which this new model can reproduce a full-kinetic description of 2D MR, with particular focus on its robustness during the non-linear stage, is evaluated. We perform 2D simulations of MR with moderate guide field by means of three different plasma models: a hybrid-Vlasov-Maxwell model with isotropic, isothermal electrons, a hybrid-Vlasov-Landau-fluid (HVLF) model where an anisotropic electron fluid is equipped with a Landau-fluid closure, and a full-kinetic one. When compared to the full-kinetic case, the HVLF model effectively reproduces the main features of MR, as well as several aspects of the associated electron micro-physics and its feedback onto proton dynamics. This includes the global evolution of MR and the local physics occurring within the so-called electron-diffusion region, as well as the evolution of species pressure anisotropy. In particular, anisotropy driven instabilities (such as firehose, mirror, and cyclotron instabilities) play a relevant role in regulating electrons anisotropy during the non-linear stage of MR. As expected, the HVLF model captures all these features, except for the electron-cyclotron instability., Comment: 15 pages, 7 figure

The nature of the turbulent energy transfer rate is studied using direct numerical simulations of weakly collisional space plasmas. This is done comparing results obtained from hybrid Vlasov-Maxwell simulations of colissionless plasmas, Hall-magnetohydrodynamics, and Landau fluid models reproducing low-frequency kinetic effects, such as the Landau damping. In this partially developed turbulent scenario, estimates of the local and global scaling properties of different energy channels are obtained using a proxy of the local energy transfer (LET). This approach provides information on the structure of energy fluxes, under the assumption that the turbulent cascade transfers most of the energy that is then dissipated at small scales by various kinetic processes in this kind of plasmas., Comment: 15 pages, 8 figures

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