The description of turbulence and reconnection in collisionless space plasma such as the solar wind or planetary magnetospheres is challenging, especially when a wide range of scales is to be retained, or parametric studies are to be performed. It is then of great interest to use a reduced description of the low-frequency dynamics, such as provided by gyrofluid models which capture some important kinetic effects. The aim of this talk is to present a short review of three recent results obtained by simulation of a two-field Hamiltonian gyrofluid model retaining ion finite Larmor radius corrections, parallel magnetic field fluctuations and electron inertia [1,2]. This model describes the quasi-perpendicular dynamics of Alfvén and kinetic Alfvén waves, as well as inertial kinetic Alfvén waves (such as detected by MMS in the Earth's magnetosheath) and is suitable for studies of collisionless reconnection. We first demonstrate the development of a reconnection mediated regime in the three-dimensional reduced MHD turbulence that develops from collisions of counter propagating Alfvén-wave packets at various levels of the nonlinearity parameter [3]. We then discuss Alfvenic turbulent cascades, from the MHD to the sub-ion scales [4], when there is a strong imbalance between the energies of the co- and contra-propagating waves (such as observed by Parker Solar Probe close to the Sun). In this regime, we will show that a so-called “helicity barrier” emerges between the MHD and the kinetic scales, which causes the development of a transition range in the magnetic-field spectrum across the ion scales. We also briefly discuss, as a function of the ratio of ion to electron temperatures, the development of turbulence that results from secondary instabilities in the nonlinear phase of 2D collisionless reconnection [5]. The nature of this turbulence, which transfers energy to sub-electron scales, is affected by the presence of electron finite Larmor radius effects. We conclude by presenting a new 4-field model that captures these effects, as well as the coupling between Alfvén- and slow-magnetosonic waves.

[1] Passot, T., Sulem, P.L. & Tassi, E. 2018 Gyrofluid modeling and phenomenology of low-βe Alfvén wave turbulence, Phys. Plasmas 25, 042107.
[2] Passot, T. & Sulem, P.L. 2019 Imbalanced kinetic Alfvén wave turbulence: from weak turbulence theory to nonlinear diffusion models for the strong regime, J. Plasma Phys. 85, 905850301.
[3] Cerri, SS., Passot, T., Laveder, D., Sulem, P.-L. & Kunz, M. W. 2022 Turbulent Regimes in Collisions of 3D Alfvén-wave Packets, Astrophys. J. 939, 36.
[4] Passot, T., Sulem, P.L. & Laveder, L. 2022 Direct kinetic Alfvén wave energy cascade in the presence of imbalance, J. Plasma Phys. 88, 905880312.
[5] Granier, C. et al. in preparation