Tearing-initiated reconnection in partially ionized magnetohydrodynamic plasmas*

In high-Lundquist-number plasmas, reconnection proceeds via onset of tearing, followed by a nonlinear phase when plasmoids continuously form, merge, and are ejected from the current sheet. 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. We study tearing-initiated reconnection in such partially ionized systems analytically and numerically. In partially ionized plasmas, ambipolar diffusion (ion-neutral drift) dominates the late stages of current sheet formation, causing nonlinear sharpening of the magnetic-field profile. This sharpening occurs at an increasing rate, such that the tearing growth rate does not become larger than the current sheet formation rate, preventing tearing onset. Once the current sheet becomes thin enough, however, ions and neutrals decouple and thinning of the current sheet ceases, allowing tearing onset in a time of order ν_ni ^-1, with  ν_ni  the neutral-ion collision frequency.  The wavelength of the mode that first disrupts the sheet decreases with ionization fraction. After onset, the system displays a stochastic plasmoid chain in which plasmoids are characterized by enhanced ionization fraction.