Onset of magnetic reconnection in poorly ionized plasmas

In high-Lundquist-number plasmas, reconnection proceeds via onset of tearing, followed by a nonlinear phase when plasmoids form, merge, and are ejected from the current sheet (CS). This process is understood in fully ionized, magnetohydrodynamic plasmas. However, many plasma environments, such as star-forming molecular clouds and the solar chromosphere, are poorly ionized. One novel effect in such plasmas is the collisional drag force exerted on the ionized species by the neutral species. We use theory and computation to understand the effect of this drag on tearing-initiated reconnection. Ion-neutral drag causes a CS to sharpen at an increasing rate via ambipolar diffusion. This nonlinear steepening delays the onset of reconnection by decreasing the CS width at which the tearing growth rate becomes larger than the CS formation rate. Once the CS becomes thin enough, however, ions decouple from neutrals and thinning of the CS slows, allowing tearing onset. An eigenmode can be derived for the resulting CS profile, yielding growth rates and wavenumber predictions that compare well with simulation. Later, the system enters a nonlinear phase with a stochastic plasmoid chain and a reconnection rate slowed by ion-neutral drag. The plasmoids are sites of enhanced ionization fraction.