The onset of turbulence in collisionless magnetic reconnection

Two dimensional simulations of collisionless magnetic reconnection produce strong gradients in the current and plasma profiles near the magnetic X-line and along the separatrix during the nonlinear phase. Here we present 3D two-fluid MHD simulations of reconnection that show these sharp gradients break up due to the onset of secondary instabilities, leading to a strongly turbulent configuration in the full 3D system. Two main secondary modes are observed, an electron shear flow instability and the lower-hybrid drift instability. The fluctuations driven by the former mode produce an effective anomalous viscosity (but no resistivity) near the X-line that breaks the frozen-in condition and broadens the main current profile. The lower-hybrid drift fluctuations cause a substantial relaxation of the gradients near the separatrix through a new mechanism, based not on direct particle transport (which is negligible) but rather on the turbulent diffusion of the strong electron flows near the separatrix.