Fluid simulations of ion scale plasmas with weakly distorted magnetic fields

Three-dimensional simulations of turbulence in collisionless plasmas are presented, using a fluid model that extends the anisotropic MHD to scales of the order of the ion gyroradius and below in directions perpendicular to the ambient magnetic field. This model, which includes linear Landau damping and finite Larmor radius corrections to all the retained moments, provides an efficient tool to describe Alfvenic turbulence in the absence of cyclotron resonance. When sufficiently small scales are retained, no artificial damping nor collisional effects is required. Simulations with large-scale Alfvenic driving show the development of perpendicular power-law spectra (taken at zero parallel wavenumber) with an exponent close to -2.8 for the perpendicular magnetic field at scales smaller than the ion inertial length. The electric field spectrum displays a break at intermediate scales, consistent with Solar Wind observations. These spectra appear in a quasi-stationary state after early-formed sheet-like density and current structures have evolved into filaments. In the presence of temperature anisotropy, the nonlinear development of the mirror instability leads to pressure-balanced magnetic structures surrounded by significant ion velocity fields perpendicular to the ambient field. At later time, the system becomes turbulent, with the disruption of the magnetic structures into parallel filaments.