Fast reconnection in partially ionized plasmas

Magnetic reconnection is commonly studied in fully ionized plasmas, but neutrals are important in many astrophysical environments such as the solar chromosphere or interstellar medium, and comparatively little attention has been given towards understanding reconnection physics in partially ionized systems. In particular, the transition from slow, collisional reconnection to fast, collisionless reconnection is not well understood. Analytic models have predicted that, when neutrals are present, this transition occurs when the current sheet thickness reaches the total inertial length (defined with the total mass density), but multi-fluid simulations have not seen this effect. In this work, fully kinetic, particle-in-cell simulations of weakly-collisional, partially ionized magnetic reconnection are performed. The transition from collisional to collisionless reconnection is found to occur when the current sheet thins below the ion-inertial length (defined by only the ion mass density). Within the collisionless regime, and in sufficiently large systems, the peak reconnection rate scales with the total Alfvén speed in agreement with experimental results. To clarify the role of kinetic and collisionless effects, well-matched fluid simulations are compared with the particle-in-cell simulations and recent experimental measurements on the Magnetic Reconnection Experiment (MRX).