Fundamental role of ion viscosity on fast magnetic reconnection in large-guide-field regimes

Nonlinear analytical theory of magnetic reconnection with a large guide field is presented for the first time. We confirm that two distinct steady-state reconnection regimes are possible depending on the relative size of the diffusion region thickness δ versus the sound gyroradius ρs. The reconnection is slow (Sweet-Parker-like) for δ > or approx. ρs, and fast otherwise. However, unlike earlier work, we find that ion viscosity μ plays a fundamental role in the fast regime. In particular, for δ<ρs we obtain δ∝Ha-1, with Ha∝1/√(ημ) as the Hartmann number, and the reconnection rate Ez∝Pr-1/2, with Pr=μ/η as the Prandtl number and η as the resistivity. If the perpendicular ion viscosity is employed for μ, the reconnection rate becomes independent of plasma β and collision frequencies, and therefore potentially fast.