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Current sheet bifurcation and collapse in electron magnetohydrodynamics

Inertial effects in nonlinear magnetic reconnection are studied within the context of 2D electron magnetohydrodynamics (EMHD) with resistive and viscous dissipation. Families of nonlinear solutions for relevant current sheet parameters are predicted and confirmed numerically in all regimes of interest. Electron inertia becomes important for current sheet thicknesses $δ$ below the inertial length $d_{e}$. In this case, in the absence of electron viscosity, the sheet thickness experiences a nonlinear collapse. Viscosity regularizes solutions at small scales. Transition from resistive to viscous regimes shows a nontrivial dependence on resistivity and viscosity, featuring a hysteresis bifurcation. In all accessible regimes, the nonlinear reconnection rate is found to be explicitly independent of the electron inertia and dissipation coefficients.