The onset of turbulence in collisionless magnetic reconnection

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Abstract
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.